User:Doggitydogs/GPS



The Global Positioning System (GPS) is a space-based global navigation satellite system (GNSS) that provides location and time information in all weather, anywhere on or near the Earth, where there is an unobstructed line of sight to four or more GPS satellites. It is maintained by the United States government and is freely accessible by anyone with a GPS receiver with some technical limitations which are only removed for military users.

The GPS program provides critical capabilities to military, civil and commercial users around the world. It is an engine of economic growth and jobs, and has generated billions of dollars of economic activity. It maintains future warfighter advantage over opponents and is one of the four core military capabilities. In addition, GPS is the backbone for modernizing the global air traffic system.

The GPS project was developed in 1973 to overcome the limitations of previous navigation systems, integrating ideas from several predecessors, including a number of classified engineering design studies from the 1960s. GPS was created and realized by the U.S. Department of Defense (DoD) and was originally run with 24 satellites. It became fully operational in 1994.

Advances in technology and new demands on the existing system have now led to efforts to modernize the GPS system and implement the next generation of GPS III satellites and Next Generation Operational Control System (OCX). Announcements from the Vice President and the White House in 1998 initiated these changes. In 2000, U.S. Congress authorized the modernization effort, referred to as GPS III.

In addition to GPS, other systems are in use or under development. The Russian GLObal NAvigation Satellite System (GLONASS) was in use by only the Russian military, until it was made fully available to civilians in 2007. There are also the planned European Union Galileo positioning system, Chinese Compass navigation system, and Indian Regional Navigational Satellite System.

History
The design of GPS is based partly on similar ground-based radio-navigation systems, such as LORAN and the Decca Navigator developed in the early 1940s, and used during World War II. In 1956, Friedwardt Winterberg proposed a test of general relativity (for time slowing in a strong gravitational field) using accurate atomic clocks placed in orbit inside artificial satellites. (To achieve accuracy requirements, GPS uses principles of general relativity to correct the satellites' atomic clocks. ) Additional inspiration for GPS came when the Soviet Union launched the first man-made satellite, Sputnik in 1957. Two American physicists, William Guier and George Weiffenbach, at Johns Hopkins's Applied Physics Laboratory (APL), decided on their own to monitor Sputnik's radio transmissions. They soon realized that, because of the Doppler effect, they could pinpoint where the satellite was along its orbit from the Doppler shift. The Director of the APL gave them access to their brand new UNIVAC II to do the heavy calculations required. When they released the orbit of Sputnik to the media the Russians were dumbfounded to learn how powerful American computers had become, as they would not have been able to calculate the orbit themselves. The following spring, Frank McClure, the deputy director of the APL, asked Guier and Weiffenbach to look at the inverse problem where you know the location of the satellite and you want to find your own location. (The Navy was developing the submarine launched Polaris missile, which required them to know the submarine's location.) This led them and APL to develop the Transit system.

The first satellite navigation system, Transit (satellite), used by the United States Navy, was first successfully tested in 1960. It used a constellation of five satellites and could provide a navigational fix approximately once per hour. In 1967, the U.S. Navy developed the Timation satellite that proved the ability to place accurate clocks in space, a technology required by GPS. In the 1970s, the ground-based Omega Navigation System, based on phase comparison of signal transmission from pairs of stations, became the first worldwide radio navigation system. Limitations of these systems drove the need for a more universal navigation solution with greater accuracy.

While there were wide needs for accurate navigation in military and civilian sectors, almost none of those were seen as justification for the billions of dollars it would cost in research, development, deployment, and operation for a constellation of navigation satellites. During the Cold War arms race, the nuclear threat to the existence of the United States was the one need that did justify this cost in the view of the United States Congress. This deterrent effect is why GPS was funded. It is also the reason for the ultra secrecy at that time. The nuclear triad consisted of the United States Navy's submarine-launched ballistic missiles (SLBMs) along with United States Air Force (USAF) strategic bombers and intercontinental ballistic missiles (ICBMs). Considered vital to the nuclear deterrence posture, accurate determination of the SLBM launch position was a force multiplier.

Precise navigation would enable United States submarines to get an accurate fix of their positions prior to launching their SLBMs. The USAF with two-thirds of the nuclear triad also had requirements for a more accurate and reliable navigation system. The Navy and Air Force were developing their own technologies in parallel to solve what was essentially the same problem. To increase the survivability of ICBMs, there was a proposal to use mobile launch platforms (such as Russian SS-24 and SS-25) and so the need to fix the launch position had similarity to the SLBM situation.

In 1960, the Air Force proposed a radio-navigation system called MOSAIC (MObile System for Accurate ICBM Control) that was essentially a 3-D LORAN. A follow-on study called Project 57 was worked in 1963 and it was "in this study that the GPS concept was born." That same year the concept was pursued as Project 621B, which had "many of the attributes that you now see in GPS" and promised increased accuracy for Air Force bombers as well as ICBMs. Updates from the Navy Transit system were too slow for the high speeds of Air Force operation. The Navy Research Laboratory continued advancements with their Timation (Time Navigation) satellites, first launched in 1967, and with the third one in 1974 carrying the first atomic clock into orbit.

With these parallel developments in the 1960s, it was realized that a superior system could be developed by synthesizing the best technologies from 621B, Transit, Timation, and SECOR in a multi-service program.

During Labor Day weekend in 1973, a meeting of about 12 military officers at the Pentagon discussed the creation of a Defense Navigation Satellite System (DNSS). It was at this meeting that "the real synthesis that became GPS was created." Later that year, the DNSS program was named Navstar. With the individual satellites being associated with the name Navstar (as with the predecessors Transit and Timation), a more fully encompassing name was used to identify the constellation of Navstar satellites, Navstar-GPS, which was later shortened simply to GPS.

After Korean Air Lines Flight 007, carrying 269 people, was shot down in 1983 after straying into the USSR's prohibited airspace, in the vicinity of Sakhalin and Moneron Islands, President Ronald Reagan issued a directive making GPS freely available for civilian use, once it was sufficiently developed, as a common good. The first satellite was launched in 1989, and the 24th satellite was launched in 1994.

Initially, the highest quality signal was reserved for military use, and the signal available for civilian use was intentionally degraded (Selective Availability). This changed with President Bill Clinton ordering Selective Availability to be turned off at midnight May 1, 2000, improving the precision of civilian GPS from 100 meters (about 300 feet) to 20 meters (about 65 feet). The executive order signed in 1996 to turn off Selective Availability in 2000 was proposed by the US Secretary of Defense, William Perry, because of the widespread growth of differential GPS services to improve civilian accuracy and eliminate the US military advantage. Moreover, the US military was actively developing technologies to deny GPS service to potential adversaries on a regional basis.

Over the last decade, the U.S. has implemented several improvements to the GPS service, including new signals for civil use and increased accuracy and integrity for all users, all while maintaining compatibility with existing GPS equipment.

GPS modernization has now become an ongoing initiative to upgrade the Global Positioning System with new capabilities to meet growing military, civil, and commercial needs. The program is being implemented through a series of satellite acquisitions, including GPS Block III and the Next Generation Operational Control System (OCX). The U.S. Government continues to improve the GPS space and ground segments to increase performance and accuracy.

GPS is owned and operated by the United States Government as a national resource. Department of Defense (DoD) is the steward of GPS. Interagency GPS Executive Board (IGEB) oversaw GPS policy matters from 1996 to 2004. After that the National Space-Based Positioning, Navigation and Timing Executive Committee was established by presidential directive in 2004 to advise and coordinate federal departments and agencies on matters concerning the GPS and related systems. The executive committee is chaired jointly by the deputy secretaries of defense and transportation. Its membership includes equivalent-level officials from the departments of state, commerce, and homeland security, the joint chiefs of staff, and NASA. Components of the executive office of the president participate as observers to the executive committee, and the FCC chairman participates as a liaison.

The DoD is required by law to "maintain a Standard Positioning Service (as defined in the federal radio navigation plan and the standard positioning service signal specification) that will be available on a continuous, worldwide basis," and "develop measures to prevent hostile use of GPS and its augmentations without unduly disrupting or degrading civilian uses."

Timeline and modernization

 * In 1972, the USAF Central Inertial Guidance Test Facility (Holloman AFB), conducted developmental flight tests of two prototype GPS receivers over White Sands Missile Range, using ground-based pseudo-satellites.
 * In 1978, the first experimental Block-I GPS satellite was launched.
 * In 1983, after Soviet interceptor aircraft shot down the civilian airliner KAL 007 that strayed into prohibited airspace because of navigational errors, killing all 269 people on board, U.S. President Ronald Reagan announced that GPS would be made available for civilian uses once it was completed., although it had been previously published [in Navigation magazine] that the CA code would be available to civilian users.
 * By 1985, ten more experimental Block-I satellites had been launched to validate the concept. Command & Control of these satellites had moved from Onizuka AFS, CA and turned over to the 2nd Satellite Control Squadron (2SCS) located at Falcon Air Force Station in Colorado Springs, Colorado.
 * On February 14, 1989, the first modern Block-II satellite was launched.
 * The Gulf War from 1990 to 1991, was the first conflict where GPS was widely used.
 * In 1992, the 2nd Space Wing, which originally managed the system, was de-activated and replaced by the 50th Space Wing.
 * By December 1993, GPS achieved initial operational capability (IOC), indicating a full constellation (24 satellites) was available and providing the Standard Positioning Service (SPS).
 * Full Operational Capability (FOC) was declared by Air Force Space Command (AFSPC) in April 1995, signifying full availability of the military's secure Precise Positioning Service (PPS).
 * In 1996, recognizing the importance of GPS to civilian users as well as military users, U.S. President Bill Clinton issued a policy directive declaring GPS to be a dual-use system and establishing an Interagency GPS Executive Board to manage it as a national asset.
 * In 1998, United States Vice President Al Gore announced plans to upgrade GPS with two new civilian signals for enhanced user accuracy and reliability, particularly with respect to aviation safety and in 2000 the United States Congress authorized the effort, referring to it as GPS III.
 * In 1998, GPS technology was inducted into the Space Foundation Space Technology Hall of Fame.
 * On May 2, 2000 "Selective Availability" was discontinued as a result of the 1996 executive order, allowing users to receive a non-degraded signal globally.
 * In 2004, the United States Government signed an agreement with the European Community establishing cooperation related to GPS and Europe's planned Galileo system.
 * In 2004, United States President George W. Bush updated the national policy and replaced the executive board with the National Executive Committee for Space-Based Positioning, Navigation, and Timing.
 * November 2004, QUALCOMM announced successful tests of assisted GPS for mobile phones.
 * In 2005, the first modernized GPS satellite was launched and began transmitting a second civilian signal (L2C) for enhanced user performance.
 * On September 14, 2007, the aging mainframe-based Ground Segment Control System was transferred to the new Architecture Evolution Plan.
 * On May 19, 2009, the United States Government Accountability Office issued a report warning that some GPS satellites could fail as soon as 2010.
 * On May 21, 2009, the Air Force Space Command allayed fears of GPS failure saying "There's only a small risk we will not continue to exceed our performance standard."
 * On January 11, 2010, an update of ground control systems caused a software incompatibility with 8000 to 10000 military receivers manufactured by a division of Trimble Navigation Limited of Sunnyvale, Calif.
 * On February 25, 2010, the U.S. Air Force awarded the contract to develop the GPS Next Generation Operational Control System (OCX) to improve accuracy and availability of GPS navigation signals, and serve as a critical part of GPS modernization.
 * A GPS satellite was launched on May 28, 2010. The oldest GPS satellite still in operation was launched on November 26, 1990, and became operational on December 10, 1990.
 * The GPS satellite, GPS IIF-2, was launched on July 16, 2011 at 06:41 GMT from Space Launch Complex 37B at the Cape Canaveral Air Force Station.

Awards
On February 10, 1993, the National Aeronautic Association selected the GPS Team as winners of the 1992 Robert J. Collier Trophy, the nation's most prestigious aviation award. This team combines researchers from the Naval Research Laboratory, the USAF, the Aerospace Corporation, Rockwell International Corporation, and IBM Federal Systems Company. The citation honors them "for the most significant development for safe and efficient navigation and surveillance of air and spacecraft since the introduction of radio navigation 50 years ago."

Two GPS developers received the National Academy of Engineering Charles Stark Draper Prize for 2003:
 * Ivan Getting, emeritus president of The Aerospace Corporation and an engineer at the Massachusetts Institute of Technology, established the basis for GPS, improving on the World War II land-based radio system called LORAN (Long-range Radio Aid to Navigation).
 * Bradford Parkinson, professor of aeronautics and astronautics at Stanford University, conceived the present satellite-based system in the early 1960s and developed it in conjunction with the U.S. Air Force. Parkinson served twenty-one years in the Air Force, from 1957 to 1978, and retired with the rank of colonel.

GPS developer Roger L. Easton received the National Medal of Technology on February 13, 2006.

Francis X. Kane (Col. USAF, ret.) was inducted into the U.S. Air Force Space and Missile Pioneers Hall of Fame at Lackland A.F.B., San Antonio, Texas, March 2, 2010 for his role in space technology development and the engineering design concept of GPS conducted as part of Project 621B.

On October 4, 2011, the International Astronautical Federation (IAF) awarded the Global Positioning System (GPS) its 60th Anniversary Award, nominated by IAF member, the American Institute for Aeronautics and Astronautics (AIAA). The IAF Honors and Awards Committee recognized the uniqueness of the GPS program and the exemplary role it has played in building international collaboration for the benefit of humanity.

Basic concept of GPS
A GPS receiver calculates its position by precisely timing the signals sent by GPS satellites high above the Earth. Each satellite continually transmits messages that include
 * the time the message was transmitted
 * precise orbital information (the ephemeris)
 * the general system health and rough orbits of all GPS satellites (the almanac).

The receiver uses the messages it receives to determine the transit time of each message and computes the distance to each satellite. These distances along with the satellites' locations are used with the possible aid of trilateration, depending on which algorithm is used, to compute the position of the receiver. This position is then displayed, perhaps with a moving map display or latitude and longitude; elevation information may be included. Many GPS units show derived information such as direction and speed, calculated from position changes.

Three satellites might seem enough to solve for position since space has three dimensions and a position near the Earth's surface can be assumed. However, even a very small clock error multiplied by the very large speed of light — the speed at which satellite signals propagate — results in a large positional error. Therefore receivers use four or more satellites to solve for the receiver's location and time. The very accurately computed time is effectively hidden by most GPS applications, which use only the location. A few specialized GPS applications do however use the time; these include time transfer, traffic signal timing, and synchronization of cell phone base stations.

Although four satellites are required for normal operation, fewer apply in special cases. If one variable is already known, a receiver can determine its position using only three satellites. For example, a ship or aircraft may have known elevation. Some GPS receivers may use additional clues or assumptions (such as reusing the last known altitude, dead reckoning, inertial navigation, or including information from the vehicle computer) to give a less accurate (degraded) position when fewer than four satellites are visible.

Position calculation introduction
To provide an introductory description of how a GPS receiver works, error effects are deferred to a later section. Using messages received from a minimum of four visible satellites, a GPS receiver is able to determine the times sent and then the satellite positions corresponding to these times sent. The x, y, and z components of position, and the time sent, are designated as $$\scriptstyle\left[x_i,\, y_i,\, z_i,\, t_i\right]$$ where the subscript i is the satellite number and has the value 1, 2, 3, or 4. Knowing the indicated time the message was received $$\scriptstyle\ \tilde{t}_\text{r}$$, the GPS receiver could compute the transit time of the message as $$\scriptstyle\left ( \tilde{t}_\text{r}-t_i\right ) $$, if $$\scriptstyle\  \tilde{t}_\text{r}$$ would be equal to correct reception time, $$\scriptstyle\  t_\text{r}$$. A pseudorange, $$\scriptstyle p_i \triangleq \left ( \tilde{t}_\text{r}-t_i\right )c$$, would be the traveling distance of the message, assuming it traveled at the speed of light, c.

A satellite's position and pseudorange define a sphere, centered on the satellite, with radius equal to the pseudorange. The position of the receiver is somewhere on the surface of this sphere. Thus with four satellites, the indicated position of the GPS receiver is at or near the intersection of the surfaces of four spheres. In the ideal case of no errors, the GPS receiver would be at a precise intersection of the four surfaces.

If the surfaces of two spheres intersect at more than one point, they intersect in a circle. The article trilateration shows this mathematically. A figure, Two Sphere Surfaces Intersecting in a Circle, is shown below. Two points where the surfaces of the spheres intersect are clearly shown in the figure. The distance between these two points is the diameter of the circle of intersection. The intersection of a third spherical surface with the first two will be its intersection with that circle; in most cases of practical interest, this means they intersect at two points. Another figure, Surface of Sphere Intersecting a Circle (not a solid disk) at Two Points, illustrates the intersection. The two intersections are marked with dots. Again the article trilateration clearly shows this mathematically.

For automobiles and other near-earth vehicles, the correct position of the GPS receiver is the intersection closest to the Earth's surface. For space vehicles, the intersection farthest from Earth may be the correct one.

The correct position for the GPS receiver is also the intersection closest to the surface of the sphere corresponding to the fourth satellite.

Correcting a GPS receiver's clock
One of the most significant error sources is the GPS receiver's clock. Because of the very large value of the speed of light, c, the estimated distances from the GPS receiver to the satellites, the pseudoranges, are very sensitive to errors in the GPS receiver clock; for example an error of one microsecond (0.000 001 second) corresponds to an error of 300 m. This suggests that an extremely accurate and expensive clock is required for the GPS receiver to work. Because manufacturers prefer to build inexpensive GPS receivers for mass markets, the solution for this dilemma is based on the way sphere surfaces intersect in the GPS problem.

It is likely that the surfaces of the three spheres intersect, because the circle of intersection of the first two spheres is normally quite large, and thus the third sphere surface is likely to intersect this large circle. It is very unlikely that the surface of the sphere corresponding to the fourth satellite will intersect either of the two points of intersection of the first three, because any clock error could cause it to miss intersecting a point. However, the distance from the valid estimate of GPS receiver position to the surface of the sphere corresponding to the fourth satellite can be used to compute a clock correction.

Let $$\scriptstyle r_4 = \left ( t_\text{r}-t_4\right )c$$, which is the distance from the valid estimate of GPS receiver position to the fourth satellite, and let $$\scriptstyle p_4$$ denote the pseudorange of the fourth satellite. Let $$\scriptstyle da \,=\, r_4 \,-\, p_4$$, which is the distance from the computed GPS receiver position to the surface of the sphere corresponding to the fourth satellite. Thus the quotient, $$\scriptstyle b \,=\, - da / c\ $$, provides an estimate of GPS receiver's clock bias: $$\scriptstyle b= \tilde{t}_\text{r} - t_\text{r}$$, where $$\scriptstyle\  \tilde{t}_\text{r}$$ is the time indicated by the receiver's on-board clock and $$\scriptstyle\  t_\text{r}$$ is the correct reception time. The GPS receiver clock can be advanced if $$\scriptstyle b $$ is positive or delayed if $$\scriptstyle b $$ is negative.

Structure
The current GPS consists of three major segments. These are the space segment (SS), a control segment (CS), and a user segment (US). The U.S. Air Force develops, maintains, and operates the space and control segments. GPS satellites broadcast signals from space, and each GPS receiver uses these signals to calculate its three-dimensional location (latitude, longitude, and altitude) and the current time.

The space segment is composed of 24 to 32 satellites in medium Earth orbit and also includes the payload adapters to the boosters required to launch them into orbit. The control segment is composed of a master control station, an alternate master control station, and a host of dedicated and shared ground antennas and monitor stations. The user segment is composed of hundreds of thousands of U.S. and allied military users of the secure GPS Precise Positioning Service, and tens of millions of civil, commercial, and scientific users of the Standard Positioning Service (see GPS navigation devices).

Space segment
The space segment (SS) is composed of the orbiting GPS satellites, or Space Vehicles (SV) in GPS parlance. The GPS design originally called for 24 SVs, eight each in three approximately circular orbits, but this was modified to six orbital planes with four satellites each. The orbits are centered on the Earth, not rotating with the Earth, but instead fixed with respect to the distant stars. The six orbit planes have approximately 55° inclination (tilt relative to Earth's equator) and are separated by 60° right ascension of the ascending node (angle along the equator from a reference point to the orbit's intersection). The orbital period is one-half a sidereal day, i.e. 11 hours and 58 minutes. The orbits are arranged so that at least six satellites are always within line of sight from almost everywhere on Earth's surface. The result of this objective is that the four satellites are not evenly spaced (90 degrees) apart within each orbit. In general terms, the angular difference between satellites in each orbit is 30, 105, 120, and 105 degrees apart which, of course, sum to 360 degrees.

Orbiting at an altitude of approximately 20200 km; orbital radius of approximately 26600 km, each SV makes two complete orbits each sidereal day, repeating the same ground track each day. This was very helpful during development because even with only four satellites, correct alignment means all four are visible from one spot for a few hours each day. For military operations, the ground track repeat can be used to ensure good coverage in combat zones.

, there are 31 actively broadcasting satellites in the GPS constellation, and two older, retired from active service satellites kept in the constellation as orbital spares. The additional satellites improve the precision of GPS receiver calculations by providing redundant measurements. With the increased number of satellites, the constellation was changed to a nonuniform arrangement. Such an arrangement was shown to improve reliability and availability of the system, relative to a uniform system, when multiple satellites fail. About nine satellites are visible from any point on the ground at any one time (see animation at right).

Satellites
A GPS satellite is a satellite used by the NAVSTAR Global Positioning System (GPS). The first satellite in the system, Navstar 1, was launched February 22, 1978. The GPS satellite constellation is operated by the 50th Space Wing of the United States Air Force.

Block I satellites
Beginning with Navstar 1 in 1978, ten "Block I" GPS satellites were successfully launched. One satellite, "Navstar 7", was lost due to an unsuccessful launch on December 18, 1981. The Block I satellites were launched from Vandenberg Air Force Base using Atlas rockets that were converted intercontinental ballistic missiles. The satellites were built by Rockwell International at the same plant in Seal Beach, CA where the S-II second stages of the Saturn V rockets had been built. The final Block I launch was conducted on October 9, 1985.

The last Block I satellite was taken out of service on November 18, 1995.

Block II satellites
The first of the nine satellites in the initial Block II series was launched February 14, 1989; the last was launched October 1, 1990. The final satellite of the series to be taken out of service was decommissioned March 15, 2007.

Block IIA series
Nineteen satellites in the Block IIA series were launched, the first on November 26, 1990 and the last on November 6, 1997. As of January 17, 2009, six satellites of this series have been removed from service.

Two of the satellites in this series, numbers 35 and 36, are equipped with laser retro-reflectors, allowing them to be tracked independently of their radio signals, providing unambiguous separation of clock and ephemeris errors.

Block IIR series
The Block IIR series are "replenishment" satellites developed by Lockheed Martin. Each satellite weighs 4480 lb at launch and 2370 lb once on orbit. The first attempted launch of a Block IIR satellite failed on January 17, 1997 when the Delta II rocket exploded 12 seconds into flight. The first successful launch was on July 23, 1997. Twelve satellites in the series were successfully launched.

Block IIR-M series
The Block IIR-M satellites include a new military signal and a more robust civil signal, known as L2C. There are eight satellites in the Block IIR-M series, which were built by Lockheed Martin. The first Block IIR-M satellite was launched on September 26, 2005. The final launch of a IIR-M was on August 17, 2009.

Block IIF series
GPS Block IIF, or GPS IIF is an interim class of GPS satellite, which will be used to keep the Navstar Global Positioning System operational until the Block IIIA satellites become operational. They are being built by Boeing, and will be operated by the United States Air Force following their launch by United Launch Alliance, using Evolved Expendable Launch Vehicles. They will be the final component of the Block II GPS constellation to be launched. The spacecraft have a mass of 1630 kg and a design life of 12 years. Like earlier GPS satellites, Block IIF spacecraft operate in semi-synchronous medium Earth orbits, with an altitude of approximately 20460 km, and an orbital period of twelve hours.

The satellites will replace the GPS Block IIA satellites which were launched between 1990 and 1997 and were designed to last 7.5 years. Eleven of those satellites are still in use, including four that were launched in 1992.

Because the Evolved Expendable Launch Vehicles are more powerful than the Delta II, which was used to orbit earlier Block II GPS satellites, they can place the satellites directly into their operational orbits. As a result, Block IIF satellites do not carry apogee motors. The original contract for Block IIF, signed in 1996, called for 33 spacecraft. This was later reduced to 12, and programme delays and technical problems pushed the first launch from 2006 to 2010.

New characteristics

 * Broadcasting L5 "safety of life" navigation signal demonstrated on USA-203
 * Broadcasting a new M-code signal
 * Doubling in the predicted accuracy
 * Better resistance to jamming
 * Reprogrammable processors that can receive software uploads
 * The first GPS satellites not to have hardware Selective Availability installed which could degrade civilian accuracy

Launches
The first GPS IIF satellite, Space Vehicle 1, was launched by a Delta IV-M+(4,2) rocket at 03:00 UTC on 28 May 2010. The launch took place from Space Launch Complex 37B at the Cape Canaveral Air Force Station. It was the 61st U.S. GPS satellite to launch and the 50th GPS launched on a Delta.

The second GPS IIF satellite, GPS IIF-2 was launched on July 16, 2011 at 2:41am ET from Space Launch Complex 37B at the Cape Canaveral Air Force Station.

Cross reference of PRNs to satellite blocks
The following table is for the purpose of making it possible to determine the block associated with a PRN by looking at one column in one table rather than having to search through all rows of three tables. Thus this table can be used to quickly and easily determine the number of satellites in orbit and healthy associated with each block.

Control segment


The control segment is composed of
 * 1) a master control station (MCS),
 * 2) an alternate master control station,
 * 3) four dedicated ground antennas and
 * 4) six dedicated monitor stations

The MCS can also access U.S. Air Force Satellite Control Network (AFSCN) ground antennas (for additional command and control capability) and NGA (National Geospatial-Intelligence Agency) monitor stations. The flight paths of the satellites are tracked by dedicated U.S. Air Force monitoring stations in Hawaii, Kwajalein, Ascension Island, Diego Garcia, Colorado Springs, Colorado and Cape Canaveral, along with shared NGA monitor stations operated in England, Argentina, Ecuador, Bahrain, Australia and Washington DC. The tracking information is sent to the Air Force Space Command's MCS at Schriever Air Force Base 25 km ESE of Colorado Springs, which is operated by the 2nd Space Operations Squadron (2 SOPS) of the U.S. Air Force. Then 2 SOPS contacts each GPS satellite regularly with a navigational update using dedicated or shared (AFSCN) ground antennas (GPS dedicated ground antennas are located at Kwajalein, Ascension Island, Diego Garcia, and Cape Canaveral). These updates synchronize the atomic clocks on board the satellites to within a few nanoseconds of each other, and adjust the ephemeris of each satellite's internal orbital model. The updates are created by a Kalman filter that uses inputs from the ground monitoring stations, space weather information, and various other inputs.

Satellite maneuvers are not precise by GPS standards. So to change the orbit of a satellite, the satellite must be marked unhealthy, so receivers will not use it in their calculation. Then the maneuver can be carried out, and the resulting orbit tracked from the ground. Then the new ephemeris is uploaded and the satellite marked healthy again.

The Operation Control Segment (OCS) currently serves as the control segment of record. It provides the operational capability that supports global GPS users and keeps the GPS system operational and performing within specification.

OCS successfully replaced the legacy 1970’s-era mainframe computer at Schriever Air Force Base in September 2007. After installation, the system helped enable upgrades and provide a foundation for a new security architecture that supported the U.S. armed forces. OCS will continue to be the ground control system of record until the new segment, Next Generation GPS Operation Control System (OCX), is fully developed and functional.

The new capabilities provided by OCX will be the cornerstone for revolutionizing GPS’s mission capabilities, and enabling Air Force Space Command to greatly enhance GPS operational services to U.S. combat forces, civil partners and myriad of domestic and international users.

The GPS OCX program also will reduce cost, schedule and technical risk. It is designed to provide 50% sustainment cost savings through efficient software architecture and Performance-Based Logistics. In addition, GPS OCX expected to cost millions less than the cost to upgrade OCS while providing four times the capability.

The GPS OCX program represents a critical part of GPS modernization and provides significant information assurance improvements over the current GPS OCS program.


 * OCX will have the ability to control and manage GPS legacy satellites as well as the next generation of GPS III satellites, while enabling the full array of military signals.
 * Built on a flexible architecture that can rapidly adapt to the changing needs of today’s and future GPS users allowing immediate access to GPS data and constellations status through secure, accurate and reliable information.
 * Empowers the warfighter with more secure, actionable and predictive information to enhance situational awareness.
 * Enables new modernized signals (L1C, L2C, and L5) and has M-code capability, which the legacy system is unable to do.
 * Provides significant information assurance improvements over the current program including detecting and preventing cyber attacks, while isolating, containing and operating during such attacks.
 * Supports higher volume near real-time command and control capability.

On September 14, 2011, the U.S. Air Force announced the completion of GPS OCX Preliminary Design Review and confirmed that the OCX program is ready for the next phase of development.

The GPS OCX program has achieved major milestones and is on track to support the GPS IIIA launch in May 2014.

User segment


The user segment is composed of hundreds of thousands of U.S. and allied military users of the secure GPS Precise Positioning Service, and tens of millions of civil, commercial and scientific users of the Standard Positioning Service. In general, GPS receivers are composed of an antenna, tuned to the frequencies transmitted by the satellites, receiver-processors, and a highly stable clock (often a crystal oscillator). They may also include a display for providing location and speed information to the user. A receiver is often described by its number of channels: this signifies how many satellites it can monitor simultaneously. Originally limited to four or five, this has progressively increased over the years so that,, receivers typically have between 12 and 20 channels.

GPS receivers may include an input for differential corrections, using the RTCM SC-104 format. This is typically in the form of an RS-232 port at 4,800 bit/s speed. Data is actually sent at a much lower rate, which limits the accuracy of the signal sent using RTCM. Receivers with internal DGPS receivers can outperform those using external RTCM data. , even low-cost units commonly include Wide Area Augmentation System (WAAS) receivers.

Many GPS receivers can relay position data to a PC or other device using the NMEA 0183 protocol. Although this protocol is officially defined by the National Marine Electronics Association (NMEA), references to this protocol have been compiled from public records, allowing open source tools like gpsd to read the protocol without violating intellectual property laws. Other proprietary protocols exist as well, such as the SiRF and MTK protocols. Receivers can interface with other devices using methods including a serial connection, USB, or Bluetooth.

GPS navigation devices
A GPS navigation device is any device that receives Global Positioning System (GPS) signals for the purpose of determining the device's current location on Earth. GPS devices provide latitude and longitude information, and some may also calculate altitude, although this is not considered sufficiently accurate or continuously available enough (due to the possibility of signal blockage and other factors) to rely on exclusively to pilot aircraft. GPS devices are used in military, aviation, marine and consumer product applications.

GPS devices may also have additional capabilities such as:


 * containing maps, which may be displayed in human readable format via text or in a graphical format
 * providing suggested directions to a human in charge of a vehicle or vessel via text or speech
 * providing directions directly to an autonomous vehicle such as a robotic probe
 * providing information on traffic conditions (either via historical or real time data) and suggesting alternative directions
 * providing information on nearby amenities such as restaurants, fueling stations, etc.

In other words, all GPS devices can answer the question "Where am I?", and may also be able to answer:


 * which roads or paths are available to me now?
 * which roads or paths should I take in order to get to my desired destination?
 * if some roads are usually busy at this time or are busy right now, what would be a better route to take?
 * where can I get something to eat nearby or where can I get fuel for my vehicle?

Consumer applications
Consumer GPS navigation devices include:
 * Dedicated GPS navigation devices
 * GPS modules that need to be connected to a computer to be used
 * GPS loggers that record trip information for download. Such GPS tracking is useful for trailblazing, mapping by hikers and cyclists, and the production of geocoded photographs.
 * Converged devices, including GPS Phones and GPS cameras, in which GPS is a feature rather than the main purpose of the device. Those devices are the majority, and may use assisted GPS or standalone (not network dependent) or both.  The vulnerability of consumer GPS to radio frequency interference from planned wireless data services is controversial.

Dedicated GPS navigation devices


Dedicated devices have various degrees of mobility. Hand-held, outdoor, or sport receivers have replaceable batteries that can run them for several hours, making them suitable for hiking, bicycle touring and other activities far from an electric power source. Their screens are small, and some do not show color, in part to save power. Cases are rugged and some are water resistant.

Other receivers, often called mobile are intended primarily for use in a car, but have a small rechargeable internal battery that can power them for an hour or two away from the car. Special purpose devices for use in a car may be permanently installed and depend entirely on the automotive electrical system.

The pre-installed embedded software of early receivers did not display maps; 21st century ones commonly show interactive street maps (of certain regions) that may also show points of interest, route information and step-by-step routing directions, often in spoken form with a feature called "text to speech".

Manufacturers include:
 * Navman products
 * TomTom products
 * Garmin products
 * Mio products
 * Navigon products
 * Magellan Navigation consumer products
 * TeleType products

Mobile phones with GPS capability
Due in part to regulations encouraging mobile phone tracking, including E911, the majority of GPS receivers are built into mobile telephones, with varying degrees of coverage and user accessibility. Commercial navigation software is available for most 21st century smartphones as well as some Java-enabled phones that allows them to use an internal or external GPS receiver (in the latter case, connecting via serial or Bluetooth). Some phones with GPS capability work by assisted GPS (A-GPS) only, and do not function when out of range of their carrier's cell towers. Others can navigate worldwide with satellite GPS signals as a dedicated portable GPS receiver does, upgrading their operation to A-GPS mode when in range. Still others have a hybrid positioning system that can use other signals when GPS signals are inadequate.

More bespoke solutions also exist for smartphones with inbuilt GPS capabilities. Some such phones can use tethering to double as a wireless modem for a laptop, while allowing GPS-navigation/localisation as well. One such example is marketed by Verizon Wireless in the United States, and is called VZ Navigator. The system uses gpsOne technology to determine the location, and then uses the mobile phone's data connection to download maps and calculate navigational routes. Other products including iPhone are used to provide similar services. Nokia gives Ovi Maps free on its smartphones and maps can be preloaded. According to market research from the independent analyst firm Berg Insight, the sales of GPS-enabled GSM/WCDMA handsets was 150 million units in 2009, while only 40 million separate GPS receivers were sold.

GPS navigation applications for mobile phones include Waze and Google Maps Navigation. Google Maps Navigation included with Android means most smartphone users only need their phone to have a personal navigation assistant.

Laptop PC GPS
Various software companies have made available GPS navigation software programs for in-vehicle use on laptop computers. Benefits of GPS on a laptop include larger map overview, ability to use the keyboard to control GPS functions, and some GPS software for laptops offers advanced trip-planning features not available on other platforms.

GPS modules
Other GPS devices need to be connected to a computer in order to work. This computer can be a home computer, laptop or even a PDAs, or smartphones. Depending on the type of computer and available connectors, connections can be made through a serial or USB cable, as well as Bluetooth, CompactFlash, SD, PCMCIA and the newer ExpressCard. Some PCMCIA/ExpressCard GPS units also include a wireless modem. Devices usually do not come with pre-installed GPS navigation software, thus, once purchased, the user must install or write their own software. As the user can choose which software to use, it can be better matched to their personal taste. It is very common for a PC-based GPS receiver to come bundled with a navigation software suite. Also, GPS modules are significantly cheaper than complete stand-alone systems (around €50 to €100). The software may include maps only for a particular region, or the entire world, if softwares such as Google Maps, Networks in Motion's AtlasBook mobile navigation platform, etc., are used.

Some hobbyists have also made some GPS devices and open-sourced the plans. Examples include the Elektor GPS units. These are based around a SiRFstarIII chip and are comparable to their commercial counterparts.

Commercial aviation
Commercial aviation applications include GPS devices that calculate location and feed that information to large multi-input navigational computers for autopilot, course information and correction displays to the pilots, and course tracking and recording devices.

Military
Military applications include devices similar to consumer sport products for foot soldiers (commanders and regular soldiers), small vehicles and ships, and devices similar to commercial aviation applications for aircraft and missiles. Examples are the United States military's Commander's Digital Assistant and the Soldier Digital Assistant.

Applications
While originally a military project, GPS is considered a dual-use technology, meaning it has significant military and civilian applications.

GPS has become a widely deployed and useful tool for commerce, scientific uses, tracking, and surveillance. GPS's accurate time facilitates everyday activities such as banking, mobile phone operations, and even the control of power grids by allowing well synchronized hand-off switching.

Civilian
Many civilian applications use one or more of GPS's three basic components: absolute location, relative movement, and time transfer.


 * Clock synchronization: The accuracy of GPS time signals (±10 ns) is second only to the atomic clocks upon which they are based.
 * Cellular telephony: Clock synchronization enables time transfer, which is critical for synchronizing its spreading codes with other base stations to facilitate inter-cell handoff and support hybrid GPS/cellular position detection for mobile emergency calls and other applications. The first handsets with integrated GPS launched in the late 1990s. The U.S. Federal Communications Commission (FCC) mandated the feature in either the handset or in the towers (for use in triangulation) in 2002 so emergency services could locate 911 callers. Third-party software developers later gained access to GPS APIs from Nextel upon launch, followed by Sprint in 2006, and Verizon soon thereafter.
 * Disaster relief/emergency services: Depend upon GPS for location and timing capabilities.
 * Geofencing: Vehicle tracking systems, person tracking systems, and pet tracking systems use GPS to locate a vehicle, person, or pet. These devices are attached to the vehicle, person, or the pet collar. The application provides continuous tracking and mobile or Internet updates should the target leave a designated area.
 * Geotagging: Applying location coordinates to digital objects such as photographs and other documents for purposes such as creating map overlays.
 * Navigation: Navigators value digitally precise velocity and orientation measurements.
 * Phasor measurements: GPS enables highly accurate timestamping of power system measurements, making it possible to compute phasors.
 * Robotics: Self-navigating, autonomous robots using a GPS sensors, which calculate latitude, longitude, time, speed, and heading.
 * Tectonics: GPS enables direct fault motion measurement in earthquakes.
 * Telematics: GPS technology integrated with computers and mobile communications technology in automotive navigation systems
 * Fleet Tracking: The use of GPS technology to identify, locate and maintain contact reports with one or more fleet vehicles in real-time.

Navigation
Navigators value digitally precise velocity and orientation measurements.


 * Automobiles can be equipped with GNSS receivers at the factory or as aftermarket equipment. Units often display moving maps and information about location, speed, direction, and nearby streets and points of interest.




 * Aircraft navigation systems usually display a "moving map" and are often connected to the autopilot for en-route navigation. Cockpit-mounted GNSS receivers and glass cockpits are appearing in general aviation aircraft of all sizes, using technologies such as WAAS or LAAS to increase accuracy.  Many of these systems may be certified for instrument flight rules navigation, and some can also be used for final approach and landing operations.  Glider pilots use GNSS Flight Recorders to log GNSS data verifying their arrival at turn points in gliding competitions. Flight computers installed in many gliders also use GNSS to compute wind speed aloft, and glide paths to waypoints such as alternate airports or mountain passes, to aid en route decision making for cross-country soaring.


 * Boats and ships can use GNSS to navigate all of the world's lakes, seas and oceans. Maritime GNSS units include functions useful on water, such as “man overboard” (MOB) functions that allow instantly marking the location where a person has fallen overboard, which simplifies rescue efforts.  GNSS may be connected to the ships self-steering gear and Chartplotters using the NMEA 0183 interface. GNSS can also improve the security of shipping traffic by enabling AIS.
 * Heavy Equipment can use GNSS in construction, mining and precision agriculture. The blades and buckets of construction equipment are controlled automatically in GNSS-based machine guidance systems.  Agricultural equipment may use GNSS to steer automatically, or as a visual aid displayed on a screen for the driver.  This is very useful for controlled traffic and row crop operations and when spraying.  Harvesters with yield monitors can also use GNSS to create a yield map of the paddock being harvested.


 * Bicycles often use GNSS in racing and touring. GNSS navigation allows cyclists to plot their course in advance and follow this course, which may include quieter, narrower streets, without having to stop frequently to refer to separate maps.  Some GNSS receivers are specifically adapted for cycling with special mounts and housings.


 * Hikers, climbers, and even ordinary pedestrians in urban or rural environments can use GNSS to determine their position, with or without reference to separate maps. In isolated areas, the ability of GNSS to provide a precise position can greatly enhance the chances of rescue when climbers or hikers are disabled or lost (if they have a means of communication with rescue workers).


 * Spacecraft are now beginning to use GNSS as a navigational tool. The addition of a GNSS receiver to a spacecraft allows precise orbit determination without ground tracking. This, in turn, enables autonomous spacecraft navigation, formation flying, and autonomous rendezvous. The use of GNSS in MEO, GEO, HEO, and highly elliptical orbits is feasible only if the receiver can acquire and track the much weaker (15 - 20 dB) GNSS side-lobe signals. This design constraint, and the radiation environment found in space, prevents the use of COTS receivers. Low earth orbit satellite constellations such as the one operated by Orbcomm uses GPS receivers on all satellites

GPS for the visually impaired
There have been many attempts to integrate Global Positioning System into a navigation-assistance system for the blind. GPS was introduced in the late 1980s and since then there have been several research projects to increase it's usefulness for the visually impaired.

Satellite navigation complements existing aids, such as the white cane or guide dog, but does not replace them.

Loadstone GPS
The Loadstone project is developing an open source software for satellite navigation for blind and visually impaired users. The software is free and runs currently on many different Nokia devices with the S60 platform under all versions of the Symbian operating system. A GPS receiver must be connected to the cell phone by Bluetooth. Many blind people around the world are using Nokia cell phones because there are two screen reader products for the S60 Symbian platform; Talks from Nuance Communications and Mobile Speak from the Spanish company Code Factory. This makes these devices accessible by output of synthetic speech and also allow the use of third party software, such as Loadstone GPS.

The Loadstone developers, who are blind, are from Vancouver, Glasgow, and Amsterdam. Many users from around the world have contributed improvement proposals as they know exactly what functionality helps to increase their pedestrian mobility. In 2004 the project was started by Monty Lilburn and Shawn Kirkpatrick. After the first development successes, they made it public in May 2006. Since then, other volunteers have found their way to this project of global selfhelp. The program is under the GNU General Public License (GPL), and was financed entirely by the private developers and by donations of users. This product provides blind people with more independence from the trading policy and prices of the few global vendors of accessible satellite navigation solutions.

In large rural regions and developing, or newly industrializing, countries nearly no exact map data is available in common map databases. As such, the Loadstone software provides users an option to create and store their own waypoints for navigation and share it with others. The Loadstone community is working on importing coordinates from free sources, such as the OpenStreetMap project. In addition they are searching for a sponsor of licenses for commercial map data, such as is offered by the company Tele Atlas. The other major supplier is Navteq, which belongs to Nokia.

Loadstone is the name of a natural magnetic iron that was used throughout history in the manufacturing of compasses. Sighted owners of S60 devices can use Loadstone for leisure-time activities geocaching.

Wayfinder Access
Wayfinder Access is an innovative GPS solution from the Swedish company Wayfinder Systems AB. This application for Symbian phones is designed especially to work with screen readers, such as Mobile Speak from Code Factory or TALKS from Nuance Communications and offers text-to-speech technology. It is able to take the special needs of the blind and visually impaired into consideration. Symbian screen reader software offers more than just the reading of the application’s screens, but also provides Braille support.

Highlights of Wayfinder Access include, but are not limited to:  Information provided for both pedestrian and vehicular navigation. A database of 20 million points of interest. Online maps that are regularly updated. The "Where am I?" feature that readily gives information about your current location. The "What is in my surrounding?" feature that initiates a scan of the immediate area to inform you of street names, intersections and nearby points of interest such as restaurants, banks, and much more. The new “Vicinity View” feature that allows you to hear audible references for an area with a scope that you can later adjust based on the radius of the scanned vicinity. Feedback on points of Interest (POI), crossings or favorites that can be restricted, prioritized, and presented according to their distance from your location. 

Trekker
The Victor Trekker, designed and manufactured by HumanWare (previously known as VisuAide), was launched on March 2003. It is a personal digital assistant (PDA) application operating on a Pocket PC, adapted for the blind and visually impaired with talking menus, talking maps, and GPS information. Fully portable (weight 600g), it offered features enabling a blind person to determine position, create routes and receive information on navigating to a destination. It also provided search functions for an exhaustive database of point of interests, such as restaurants, hotels, etc.

It is fully upgradeable, so it can expand to accommodate new hardware platforms and more detailed geographic information.

Trekker and Maestro, which is the first off-the-shelf accessible PDA based on Windows Mobile Pocket PC, are integrated and available since May 2005.

Trekker Breeze
The Trekker Breeze is a standalone hardware. Routes need to be recorded before they can be used. POIs are supported.

BrailleNote GPS
The BrailleNote GPS device is developed by Sendero Group, LLC, and Pulse Data International, now called HumanWare, in 2002. It is like a combination of a personal digital assistant, Map-quest software and a mechanical voice.

With a receiver about the size of a small cell phone, the BrailleNote GPS utilizes the GPS network to pinpoint a traveler’s position on earth and nearby points of interest. The personal computers receive radio signals from satellites to chart the location of users and direct them to their destination with recorded voice commands. The system uses satellites to triangulate the carrier’s position, much like a ship finding its location at sea.

Visually impaired people can encode points of interest such as local restaurants or any other location, into the computer’s database. Afterward, they can punch keys on the unit’s keyboard to direct themselves to a specific point of interest.

Mobile Geo
Mobile Geo is Code Factory’s GPS navigation software for Windows Mobile-based Smartphones, Pocket PC phones and personal digital assistants (PDAs). Powered by GPS and mapping technology from the Sendero Group, a leading provider of GPS products for the blind, Mobile Geo is the first solution specifically designed to serve as a navigation aid for people with a visual impairment which works with a wide range of mainstream mobile devices. Though it is a separately licensed product, Mobile Geo is seamlessly integrated with Code Factory’s popular screen readers – Mobile Speak for Pocket PCs and Mobile Speak for Windows Mobile Smartphones.

With Mobile Geo, you can pinpoint your location, learn about the points of interest (POIs) in your immediate vicinity, plan a route between specified points of origin and destination, and get instructions on maneuvers to make as well as information about waypoints along a route that you are following. However, Mobile Geo by no means replaces a cane or dog guide, and use of this product when traveling independently is recommended only when the blind user has received training in the skills of orientation and mobility.

The following is a list of what makes Mobile Geo a unique GPS product for the blind, eclipsing similar solutions currently available. Please note that most of these features will be present in the first version, while a few will be released in future updates of the product:

<ol> <li>Mobile Geo supports versions 5.0, 6.0, and 6.1 of the Windows Mobile operating system. This includes support for devices running Windows Mobile Professional (Pocket PC phone), Standard (Smartphone), and Classic (stand-alone PDA), editions of the operating system. This means that you can pick up a supported device from a local store or through your preferred mobile phone network operator, and upgrade to newer device models in the future whenever you wish.</li> <li>It is compatible with more than 300 personal digital assistants as well as mobile phones operating on GSM, CDMA and WCDMA networks.</li> <li>It functions as an add-on to Code Factory’s leading screen readers for Windows Mobile, which means that you get, besides the GPS solution, a fully accessible smartphone, PDA or phone+PDA hybrid device.</li> <li>It is available with map data and POI files for different countries including the USA, Canada, the UK and Ireland, Spain, France, Italy, Germany, Austria, the Netherlands, Denmark, Norway, Sweden, South Africa, Australia, and the Singapore/Malaysia/Hong Kong region.</li> <li>It functions with speech output from various text-to-speech engines developed by leading providers, such as Fonix, Loquendo, and Acapela. It is, therefore, able to speak more than 20 languages.</li> <li>It works with external Bluetooth GPS receivers as well as the built-in GPS receivers present in some mobile devices.</li> <li>It can be used with the speech output routed to a Bluetooth headset.</li> <li>Application commands can be performed using numeric or QWERTY keyboards integrated with the device, the Pocket PC touch screen, or an external keyboard.</li> <li>It can be used with more than 20 different Braille devices for input and output.</li> <li>It is activated using Code Factory’s User-Centered Licensing system, which allows you to easily transfer your product license from one mobile device to another of the same platform type and version.</li> </ol>

Kapsys Kapten
The French company Kapsys offers a navigation system without a display, that works with speech input and output, called Kapten.

Trinetra
The Trinetra project aims to develop cost-effective, independence-enhancing technologies to benefit blind people. One such system addresses accessibility concerns of blind people using public transportation systems. Using GPS receivers and staggered Infrared sensors, information is relayed to a centralized fleet management server via a cellular modem. Blind people, using common text-to-speech enabled cell phones can query estimated time of arrival, locality, and current bus capacity using a web browser.

Trinetra, spearheaded by Professor Priya Narasimhan, is an ongoing project at the Electrical and Computer Engineering department of Carnegie Mellon University. Additional research topics include item-level UPC and RFID identification while grocery shopping and indoor navigation in retail settings.

MoBIC
MoBIC means Mobility of Blind and Elderly people Interacting with Computers, which was carried out from 1994 to 1996 supported by the Commission of the European Union. It was developing a route planning system which is designed to allow a blind person access to information from many sources such as bus and train timetables as well as electronic maps of the locality. The planning system helps blind people to study and plan their routes in advance, indoors.

With the addition of devices to give the precise current position and orientation of the blind pedestrian, the system could then be used outdoors. The outdoor positioning system is based on signals and satellites which give the longitude and latitude to within a metre; the computer converts this data to a position on an electronic map of locality. The output from the system is in the form of spoken messages.

Drishti
Drishti is a wireless pedestrian navigation system. It integrates several technologies including wearable computers, voice recognition and synthesis, wireless networks, Geographic information system (GIS) and GPS. It augments contextual information to the visually impaired and computed optimized routes based on user preference, temporal constraints (e.g. traffic congestion), and dynamic obstacles (e.g. ongoing ground work, road blockade for special events).

The system constantly guides the blind user to navigate based on static and dynamic data. Environmental conditions and landmark information queries from a spatial database along their route are provided on the fly through detailed explanatory voice cues. The system also provides capability for the user to add intelligence, as perceived by the blind user, to the central server hosting the spatial database.

UCSB Personal Guidance System
In 1985, Jack Loomis, a Professor of Psychology at the University of California, Santa Barbara, came up with the idea of GPS-based navigation system for the visually impaired. A short unpublished paper (Loomis, 1985) outlined the concept and detailed some ideas for implementation, including the idea of a virtual sound interface. Loomis directed the project for over 20 years, in collaboration with Reginald Golledge (1937-2009), Professor of Geography at UCSB, and Roberta Klatzky, Professor of Psychology (now at Carnegie Mellon University). Their combination of development and applied research was supported by three multi-year grants from the National Eye Institute (NEI) and another multi-year consortium grant from the National Institute on Disability and Rehabilitation Research (NIDRR), headed by Michael May of Sendero Group. In 1993, the UCSB group first publicly demonstrated the Personal Guidance System (PGS) using a bulky prototype carried in a backpack. Since then, they created several versions of the PGS, one of which was carried in a small pack worn at the waist. Their project mostly focused on the user interface and the resulting research has defined the legacy of the project. As indicated earlier in this entry, several wearable systems are now commercially available. These systems provide verbal guidance and environmental information via speech and Braille displays. But just as drivers and pilots want pictorial information from their navigation systems, survey research by the UCSB group has shown that visually impaired people often want direct perceptual information about the environment. Most of their R&D has dealt with several types of “spatial display”, with researchers Jim Marston and Nicholas Giudice contributing to the recent efforts. The first is a virtual acoustic display, which provides auditory information to the user via earphones (as proposed in the 1985 concept paper). With this display, the user hears important environmental locations, such as turn points along the route and points of interest. The labels of these locations are converted to synthetic speech and then displayed using auditory direction and distance cues, such that the spoken labels appear in the auditory space of the user. A second type of display, which the group calls a “haptic pointer interface”, was inspired by the hand-held receiver used in the Talking Signs© system of remote signage. The user holds a small wand, to which are attached an electronic compass and a small loudspeaker or vibrator. When the hand is pointing toward some location represented in the computer database, the user hears a tone or feels a vibration. Supplementary verbal information can be provided by synthetic speech. The user moves toward the desired location by aligning the body with the hand while maintaining the "on-course" auditory or vibratory signal. Other variants of the pointer interface involve putting the compass on the body or head and turning the body or head until the on-course signal is perceived. Six published route-guidance studies indicate that spatial displays provide effective route guidance, entail less cognitive load than speech interfaces, and are generally preferred by visually impaired users.

Brunel navigation system for the blind
Prof. W. Balachandran is the pioneer and the head of GPS research group at Brunel University. He and his research team are pursuing research on navigation system for blind and visually impaired people. The system is based on the integration of state of the art current technologies, including high-accuracy GPS positioning, GIS, electronic compass and wireless digital video transmission (remote vision) facility with an accuracy of 3~4m. It provides an automated guidance using the information from daily updated digital map datasets e.g. roadworks. If required the remote guidance of visually impaired pedestrians by a sighted human guide using the information from the digital map and from the remote video image provides flexibility.

The difficulties encountered includes the availability of up to date information and what information to offer including the navigation protocol. Levels of functionality have been created to tailor the information to the user’s requirements.

NOPPA
NOPPA navigation and guidance system was designed to offer public transport passenger and route information using GPS technology for the visually impaired. This was a three-year (2002~2004) project in VTT Industrial Systems in Finland. The system provides an unbroken trip chain for a pedestrian using buses, commuter trains and trams in three neighbor cities’ area. It is based on an information server concept, which has user-centered and task oriented approach for solving information needs of special needs groups.

In the system, the Information Server is an interpreter between the user and Internet information systems. It collects, filters and integrates information from different sources and delivers results to the user. The server handles speech recognition and functions requiring either heavy calculations or data transfer. The data transfer between the server and the client is minimized. The user terminal holds speech synthesis and most of route guidance.

NOPPA is currently able to offer basic route planning and navigation services in Finland. In practice, the limits are map data can have outdated information or inaccuracies, positioning can be unavailable or inaccurate, or wireless data transmission is not always available.

Navig
NAVIG is an innovative multidisciplinary project, with fundamental and applied aspects. The main objective is to increase the autonomy of blind people in their navigation capabilities. Reaching a destination while avoiding obstacles is one of the most difficult issue that blind individuals have to face.

Achieving autonomous navigation will be pursued indoor and outdoor, in known and unknown environments. The project consortium is composed by two research centers in computer sciences specialized in human-machine interaction (IRIT) for handicapped people and in auditory perception, spatial cognition, sound design and augmented reality (LIMSI). Another research center is specialized in human and computer vision (CERCO), and two industrial partners are active in artificial vision (Spikenet Technology) and in pedestrian geolocalisation (Navocap). The last member of the consortium is an educational research center for the visually impaired (CESDV – IJA, Institute of Blind Youth).

Cartography and surveying
Both civilian and military cartographers use GPS extensively. Surveyors use absolute locations to make maps and determine property boundaries.


 * Surveying — Survey-Grade GNSS receivers can be used to position survey markers, buildings, and road construction. These units use the signal from both the L1 and L2 GPS frequencies. Even though the L2 code data are encrypted, the signal's carrier wave enables correction of some ionospheric errors.  These dual-frequency GPS receivers typically cost US$10,000 or more, but can have positioning errors on the order of one centimeter or less when used in carrier phase differential GPS mode.


 * Mapping and geographic information systems (GIS) — Most mapping grade GNSS receivers use the carrier wave data from only the L1 frequency, but have a precise crystal oscillator which reduces errors related to receiver clock jitter. This allows positioning errors on the order of one meter or less in real-time, with a differential GNSS signal received using a separate radio receiver. By storing the carrier phase measurements and differentially post-processing the data, positioning errors on the order of 10 centimeters are possible with these receivers.
 * Several projects, including OpenStreetMap and TierraWiki, allow users to create maps collaboratively, much like a wiki, using consumer-grade GPS receivers.


 * Geophysics and geology — High precision measurements of crustal strain can be made with differential GNSS by finding the relative displacement between GNSS sensors. Multiple stations situated around an actively deforming area (such as a volcano or fault zone) can be used to find strain and ground movement. These measurements can then be used to interpret the cause of the deformation, such as a dike or sill beneath the surface of an active volcano.


 * Archaeology — As archaeologists excavate a site, they generally make a three-dimensional map of the site, detailing where each artifact is found.


 * Survey-grade GNSS receiver industry include a relatively small number of major players who specialize in the design of complex dual-frequency GNSS receivers capable of precise tracking of carrier phases for all or most of available signals in order to bring the accuracy of relative positioning down to cm-level values required by these applications. The most known companies are Javad, Leica, NovAtel, Septentrio, Topcon, Trimble.

Geocaching
Geocaching is an outdoor sporting activity in which the participants use a Global Positioning System (GPS) receiver or mobile device and other navigational techniques to hide and seek containers, called "geocaches" or "caches", anywhere in the world.

A typical cache is a small waterproof container containing a logbook where the geocacher enters the date they found it and signs it with their established code name. Larger containers such as plastic storage containers (tupperware or similar) or ammunition boxes can also contain items for trading, usually toys or trinkets of little value. Geocaching is often described as a "game of high-tech hide and seek", sharing many aspects with benchmarking, trigpointing, orienteering, treasure-hunting, letterboxing, and waymarking.

Geocaches are currently placed in over 100 countries around the world and on all seven continents, including Antarctica. After 10 years of activity there are over 1,532,000 active geocaches published on various websites. There are over 5 million geocachers worldwide.

History
Geocaching is similar to the 150-year-old game letterboxing, which uses clues and references to landmarks embedded in stories. Geocaching was conceived shortly after the removal of Selective Availability from GPS on May 2, 2000, because the improved accuracy of the system allowed for a small container to be specifically placed and located. The first documented placement of a GPS-located cache took place on May 3, 2000, by Dave Ulmer of Beavercreek, Oregon. The location was posted on the Usenet newsgroup as 45.291°N, -122.41333°W. By May 6, 2000, it had been found twice and logged once (by Mike Teague of Vancouver, Washington). According to Dave Ulmer's message, the original stash was a black plastic bucket buried most of the way in the ground and contained software, videos, books, food, money, and a slingshot.

The Oregon Public Broadcasting program Oregon Field Guide covered the topic of geocaching in a February 2010 episode, paying a visit to the original site. A memorial plaque now sits at the actual site, the Original Stash Tribute Plaque (GCGV0P).

Origin of the name
The activity was originally referred to as GPS stash hunt or gpsstashing. This was changed after a discussion in the gpsstash discussion group at eGroups (now Yahoo!). On May 30, 2000, Matt Stum suggested that "stash" could have negative connotations, and suggested instead "geocaching."

Geocaches
For the traditional geocache, a geocacher will place a waterproof container containing a log book (with pen or pencil) and trade items then record the cache's coordinates. These coordinates, along with other details of the location, are posted on a listing site (see list of some sites below). Other geocachers obtain the coordinates from that listing site and seek out the cache using their GPS handheld receivers. The finding geocachers record their exploits in the logbook and online. Geocachers are free to take objects (except the logbook, pencil, or stamp) from the cache in exchange for leaving something of similar or higher value.

Typical cache "treasures" are not high in monetary value but may hold personal value to the finder. Aside from the logbook, common cache contents are unusual coins or currency, small toys, ornamental buttons, CDs, or books. Also common are objects that are moved from cache to cache called "hitchhikers", such as Travel Bugs or Geocoins, whose travels may be logged and followed online. Cachers who initially place a Travel Bug or Geocoins often assign specific goals for their trackable items. Examples of goals are to be placed in a certain cache a long distance from home, or to travel to a certain country, or to travel faster and farther than other hitchhikers in a race. Higher value items are occasionally included in geocaches as a reward for the First to Find (called "FTF"), or in locations which are harder to reach. Dangerous or illegal items, weapons, and pornography are generally not allowed and are specifically against the rules of most geocache listing sites.

Geocache container sizes range from "nanos", which can be smaller than the tip of finger and only have enough room to store the log sheet, to 20 liter (5 gallon) buckets or even larger containers. The most common cache containers in rural areas are lunch-box sized plastic storage containers or surplus military ammunition cans. Ammo cans are considered the gold standard of containers because they are very sturdy, waterproof, animal and fire resistant, relatively cheap, and have plenty of room for trade items. Smaller containers are more common in urban areas because they can be more easily hidden. If a geocache has been vandalized or stolen it is said to have been "muggled" or "plundered." The former term plays off the fact that those not familiar with geocaching are called muggles, a term borrowed from the Harry Potter series of books which was rising in popularity at the same time Geocaching got its start.

Variations
Geocaches vary in size, difficulty, and location. Simple caches are often called "drive-bys," "park 'n grabs" (PNGs), or "cache and dash." Geocaches may also be complex, involving lengthy searches or significant travel. Examples include staged multi-caches; underwater caches, caches located 50 feet (15 m) up a tree, caches found only after long offroad drives, caches on high mountain peaks,   caches located in challenging environments (such as Antarctica or north of the Arctic Circle ), and magnetic caches attached to metal structures and/or objects. Different geocaching websites list different variations per their own policies (e.g. Geocaching.com does not list new Webcam, Virtual, Locationless, or Moving geocaches). The traditional Geocaching gave birth to GeoCaching – one of active urban games of Encounter project. The game is quite similar to Geocaching but has time limitations and hints in it.



Variations of geocaches (as listed on geocaching.com) include:
 * Traditional: The basic cache type, a traditional cache must include a log book of some sort. It may or may not include trade or traceable items. A traditional cache is distinguished from other cache variations in that the geocache is found at the coordinates given and involves only one stage.


 * Multi-cache: This variation consists of multiple discoveries of one or more intermediate points containing the coordinates for the next stage; the final stage contains the log book and trade items.


 * Offset: This cache is similar to the multi-cache except that the initial coordinates are for a location containing information that encodes the final cache coordinates. An example would be to direct the finder to a plaque where the digits of a date on the plaque correspond to coordinates of the final cache.


 * Mystery/puzzle: This cache requires one to discover information or solve a puzzle to find the cache. Some mystery caches provide a false set of coordinates with a puzzle that must be solved to determine the final cache location. In other cases, the given location is accurate, but the name of the location or other features are themselves a puzzle leading to the final cache. Alternatively, additional information is necessary to complete the find, such as a padlock combination to access the cache.


 * Night Cache: These multi-stage caches are designed to be found at night and generally involve following a series of reflectors with a flashlight to the final cache location.


 * Letterbox Hybrid:  A letterbox hybrid cache is a combination of a geocache and a letterbox in the same container. A letterbox has a rubber stamp and a logbook instead of tradable items. Letterboxers carry their own stamp with them, to stamp the letterbox's log book and inversely stamp their personal log book with the letterbox stamp. The hybrid cache contains the important materials for this and may or may not include trade items. Whether the letterbox hybrid contains trade items is up to the owner.


 * Locationless/Reverse: This variation is similar to a scavenger hunt. A description is given for something to find, such as a one-room schoolhouse, and the finder locates an example of this object. The finder records the location using their GPS hand-held receiver and often takes a picture at the location showing the named object and his or her GPS receiver. Typically others are not allowed to log that same location as a find.


 * Moving/Travelling: Similar to a traditional geocache, this variation is found at a listed set of coordinates. The finder uses the log book, trades trinkets, and then hides the cache in a different location. By updating this new location on the listing, the finder essentially becomes the hider, and the next finder continues the cycle. The hitchhiker concept (see above) has superseded this cache type on geocaching.com.


 * Virtual: Caches of this nature are coordinates for a location that does not contain the traditional box, log book, or trade items. Instead, the location contains some other described object. Validation for finding a virtual cache generally requires you to email the cache hider with information such as a date or a name on a plaque, or to post a picture of yourself at the site with GPS receiver in hand.


 * Earthcache: A type of virtual-cache which is maintained by the Geological Society of America. The cacher usually has to perform a task which teaches him/her an educational lesson about the earth science of the cache area.


 * Webcam: Similar to a virtual cache; there is no container, log book, or trade items for this cache type. Instead, the coordinates are for a location with a public webcam. Instead of signing a log book, the finder is often required to capture their image from the webcam for verification of the find.


 * Event Cache: This is a gathering organized and attended by geocachers. Physical caches placed at events are often active only for the event date.


 * Cache-In Trash-Out (CITO) Events: This variation on event caching is a coordinated activity of trash pickup and other maintenance to improve the environment.


 * Mega Event: An event that is attended by over 500 people. Mega Events are typically annual events, usually attracting geocachers from all over the world.


 * GPS Adventures Maze Exhibit: An exhibit at various museums and science centers in which participants in the maze learn about geocaching. These "events" have their own cache type on Geocaching.com and include many non-geocachers.


 * Wherigo cache: A Wherigo cache is similar to a multi-stage cache hunt that uses a Wherigo cartridge to guide the player. The player plays the cartridge and finds a physical cache sometime during cartridge play, usually at the end. Not all Wherigo cartridges incorporate geocaches into game play. Wherigo caches are unique to the geocaching.com website.


 * BIT Cache(tm): Physical yet containerless caches, they are laminated cards with a URL and the password needed for logging. More information is available at www.BITcaching.com. They are listed exclusively on Opencaching.us -


 * Guest Book Cache: Physical guest books often found in museums, tourist information centers, etc. They are listed exclusively at Opencaching.us -


 * USB Cache: Paperless caches stored inside USB drives and embedded (with permission) into walls or other structures. The cache is retrieved by connecting a device that has a USB port and that is able to read standard text files. Also known as Dead Drop caches.

Terminology
There are various acronyms and words commonly used when discussing geocaching.

General:
 * Cache – A box or container that contains, at the very least, a logbook.
 * Geoswag – The items that can be found in some larger caches.
 * Muggle – A non-geocacher.
 * Muggled - Being caught by a non-geocacher while retrieving/replacing a cache; also, a muggled cache has been removed or vandalized by a non-geocacher, usually out of misunderstanding or lack of knowledge.
 * Smiley – A cache find. Refers to the "smiley-face" icon attached to "Found It" logs on some listing sites.
 * BYOP – (Bring Your Own Pen/Pencil) The cache in question lacks a writing device for the logbook.
 * CITO – (Cache In Trash Out) and refers to picking up trash on the hunt.
 * CO – (Cache Owner) The person who is responsible for maintaining a cache, usually the person who hid it.
 * DNF – (Did Not Find) Did not find the cache container being searched for.
 * FTF – (First To Find) The first person to find a cache container.
 * GPS – Short for Global Positioning System, also occasionally refers to the receiver itself.
 * GPSr – Short for GPS receiver.
 * PAF - Phone-A-Friend.

Logging a hunt: Note: the various acronyms in this section are often combined in various ways, such as "TNLNSL, TFTC!"
 * TFTC – (Thanks For The Cache) This is often used at the end of logs to thank the cache owner.
 * TFTH – (Thanks For The Hunt or Hide or Hike) It shares the same purpose as TFTC, but can also be used when the cache was not found.
 * TN – (Took Nothing) no trade or traveling item was removed from the cache.
 * LN – (Left Nothing) no trade or traveling item was added to the cache.
 * XN – (eXchanged Nothing) combines the previous two acronyms; nothing was removed or added.
 * SL – (Signed Log) used when the participant visited the cache and signed its logbook.

Location description or hint:
 * GRC – (GuardRail Cache) used in the description on where a cache may be hidden.
 * GZ – (Ground Zero or Geo-zone) refers to the general area in which a cache is hidden.
 * ICT – (Ivy Covered Tree) used in the description on where a cache may be hidden.
 * LPC – (Light/Lamp Post Cache) used in the description on where a cache may be hidden.
 * MKH – (Magnetic Key Holder) used in the description on the type of container used for the cache.
 * PLC – (Parking Lot Cache) used in the description on where a cache may be hidden.
 * POR – (Pile Of Rocks) used in the description on where a cache may be hidden.
 * POS – (Pile Of Sticks or Stones) used in the description on where a cache may be hidden.
 * UFO – (Unnatural Formation of Objects) a pile of material that obviously did not form naturally and is a likely cache hiding spot.
 * UPS – (Unnatural Pile of Sticks) a piles of sticks that did not form naturally and where a cache may be hidden.

10/10/10
On 10 October 2010 geocachers around the world held events and went caching to commemorate 10 years of geocaching. In the process they broke the record for the most geocachers to find a cache in a day, with 78,313 accounts logging a cache.

Ethics
Individual geocaching websites have developed their own guidelines for acceptable geocache publications. Though not universally required, the Geocacher's Creed provides ethical search guidelines. Government agencies and others responsible for public use of land often publish guidelines for geocaching. Generally accepted rules are to not endanger others, to minimize the impact on nature, to respect private property, and to avoid public alarm.

Controversy and issues
Cachers have been approached by police and questioned when they were seen as acting suspiciously. Other times, investigation of a cache location after suspicious activity was reported has resulted in police and bomb squad discovery of the geocache. Schools have been occasionally evacuated when a cache has been seen by teachers or police, as in the case of Fairview High School in 2009. A number of caches have been destroyed by bomb squads.

The placement of geocaches has critics among some government personnel and the public at large who consider it littering. Some geocachers try to mitigate this perception by picking up litter while they search for geocaches. Geocaching is not illegal in the United States and is usually positively received when explained to law enforcement officials. However, certain types of placements can be problematic. Although disallowed, hiders could place caches on private property without adequate permission (intentionally or otherwise), which encourages cache finders to trespass. Caches might also be hidden in places where the act of searching can make a finder look suspicious (e.g. near schools, children's playgrounds, banks, courthouses, or in residential neighborhoods), or where the container placement could be mistaken for a drug stash or a bomb (especially in urban settings, under bridges, near banks, courthouses, or embassies). Hides in these areas are discouraged, and cache listing websites enforce guidelines that disallow certain types of placements. However, as cache reviewers typically cannot see exactly where and how every particular cache is hidden, problematic hides can slip through. Ultimately it is also up to cache finders to use discretion when attempting to search for a cache, and report any problems.

The South Carolina House of Representatives passed Bill 3777 in 2005, stating, "It is unlawful for a person to engage in the activity of geocaching or letterboxing in a cemetery or in an historic or archeological site or property publicly identified by an historical marker without the express written consent of the owner or entity which oversees that cemetery site or property." The bill was referred to committee on first reading in the Senate and has been there ever since.

Geodashing
Geodashing is an outdoor sport in which teams of players use GPS receivers to find and visit randomly-selected "dashpoints" (also called "waypoints") around the world and report what they find. The objective is to visit as many dashpoints as possible.

Unlike geocaching, nothing is to be left at the dashpoints; the sole objective is to visit them within the time limit.

The first game organized by gpsgames.org ran for two months (June and July 2001); each subsequent game has run for one month. Players are often encouraged to take pictures at the dashpoints and upload them to the site.

GPS drawing
GPS Drawing combines art, travelling (walking, flying and driving) and technology and is a method of drawing that uses GPS to create large-scale artwork.

Methods
Tracks of a journey can automatically be recorded into the GPS receiver's memory and can subsequentially be downloaded onto an electronic computer as a basis for drawing, sculpture or animation. This journey may be on the surface (e.g. walking) or taken in 3D (e.g. while flying).

Practitioners
The idea was first implemented by artists Hugh Pryor and Jeremy Wood, who have drawn a 13-mile wide fish in Oxfordshire and spiders whose legs reach across cities. They have also provided an answer to the question "What is the world's biggest "IF"?" It happens to be a pair of letters, "I", which goes from Iffley in Oxford to Southampton and back, and "F" which traverses through the Ifield Road in London down to Iford in East Sussex, through Iford and back up through Ifold in West Sussex. The total length is 537 km, and the height of the drawing in typographic units is 319,334,400 points. The text you are reading is about 10 points.

"Wood and Pryor tend toward cartoonish shapes that look as if they're drawn by an Etch-a-Sketch. But they lately have also managed a few artfully nervous abstractions, made by strapping a G.P.S. device to a poodle, a border collie and a Jack Russell terrier, which betray a certain perhaps unwitting animal attraction to the work of Giacometti."

Waymarking
Waymarking is an activity where people locate and log interesting locations around the world, usually with a GPS receiver and a digital camera. Waymarking differs from geocaching in that there is no physical container to locate at the given coordinates. Waymarking identifies points of interest for GPS users. There are many categories of waymarks, from pay phones through various restaurant chains, covered bridges, churches, places where one can take a factory tour and places of geologic significance, to name only a few. , there were over 1000 different categories.

Participation in waymarking leads some to become more knowledgeable of their own areas and to become interested in local history. Others have developed games (such as "What's in a Name?") that require the assistance of other players in remote areas.

Local governments have also adopted waymarking as a method for increasing awareness of local points of interest.

History
Waymarking.com was introduced by Groundspeak in 2005 as a counterpart to its geocaching listing service, Geocaching.com. The history of waymarking is tied directly to geocaching: when it first began operation in 2000, Geocaching.com accepted listings for locations at which a physical geocache ("traditional" caches consisting of, at a minimum, an actual physical container and log book) had been placed, as well as for locations of containerless ("virtual") caches. Virtual caches were intended to bring visitors to an interesting or special location which was considered to be unsuitable for a container and log book. In September 2001, Groundspeak began publishing listings for another variation of containerless cache known as "locationless" caches (or "reverse" caches). Locationless caches were essentially challenge themes: the purpose was for seekers to find the location of an object that met the definition detailed in the locationless cache description, and post coordinates for that location under the related locationless cache listing.

Virtual and locationless cache types gained popularity on Geocaching.com, but for several reasons, both cache types became problematic for the web site and its operators. There were performance issues associated with locationless cache listings, and the architecture of the web site did not provide an adequate mechanism for preventing the logging of duplicate locations for a given locationless theme. In addition, the review process for new virtual cache submissions had become a considerable challenge for the volunteer geocache reviewers who were faced with the difficult task of determining which virtual cache submissions were "novel enough" to be listed on the site. A moratorium was eventually placed on new listings for locationless caches and virtual caches while Groundspeak attempted to find a solution for the issues that made these containerless types problematic for the web site.

In August 2005, Groundspeak launched Waymarking.com, and shortly afterward archived all locationless caches on Geocaching.com (existing virtual caches remained grandfathered on the site). Waymarking.com combines elements of both locationless caches and virtual caches. Individual waymark listings (such as a particular museum, a historic marker, or a monument) are similar in nature to the original virtual cache listings on Geocaching.com (in that they are intended to identify interesting or special locations, and visitors who venture to the location can log their visit on the web site), and the various categories into which each individual waymark is added (History Museums, Pennsylvania Historic Markers, World War II Memorials / Monuments) play a similar role to that of the original locationless caches (all waymarks in a particular waymarking category closely follow the theme represented by the category). A number of the original locationless caches became waymarking categories, and a number virtual cache locations now are listed as waymarks.

Waymarks
A waymark serves to document a location (or an object at a location) that fits within a specific well-defined category. Waymarks are created by contributors who visit a location, take pictures of the object at that location (and/or the surrounding area), obtain coordinates for the location using a handheld GPS device, and later submit this information (along with additional descriptive information) to the Waymarking.com web site, specifically targeting the waymark to a particular waymarking category. Each waymarking category has its own group of volunteer reviewers, and submitted waymarks are reviewed by a group member who checks the submission for fit and completeness.

Once a waymark submission has been accepted by a reviewer, it becomes visible to the general public. Published waymarks allow visitors to submit "visit" logs, describing their experience or uploading their own pictures taken at the site.

Categories
Each published waymark falls under a single specific waymarking category. Categories (and the "Departments" into which they are grouped) represent the taxonomy of waymarking, providing an organized structure for focused filtering and searching. Each category description includes particular and appropriate requirements for all waymarks submitted to that category. For example, waymarks submitted to the Murals category require a description of the type of media used in the mural; waymarks submitted to the Battlefields category require the date of the battle.

New categories can (and are) added to Waymarking.com on a regular basis. New categories are created when small groups of individuals with interest in a potential category concept develop a proposal for the new category which is then submitted for peer review. Members of the waymarking community vote to accept or decline the proposal, based on a loose set of category guidelines (new categories are expected to be global in nature rather than local, prevalent, interesting, and minimize overlap with existing categories). Categories that are accepted in peer review can then begin accepting waymark submissions.

Waymarking signage
Waymarking is also used as a term to describe creating a walking, cycling, or other route which is traveled by following a specific symbol ('waymark', sometimes 'way-mark' or 'way mark') along the route. These waymarks sometimes follow the route in one direction, or in other cases allow a route to be followed in both directions. One example is the standardized sign posted along the 49-Mile Scenic Drive in San Francisco, California, which can be started at any point along the route.

Retailers sometimes also use waymarkers to draw motorists to the location of their store or car park. A similar process is also used by local fairs, fates or even firework nights. The signs are typically posters strapped to railings or lampposts but sometimes the standard symbol is used.

GPS aircraft tracking
GPS (Global Position System) aircraft tracking are systems installed on aircraft to give position reports over a satellite and/or cellular network. This information is typically accessed from a web-based mapping interface where current and historical information can be viewed. These devices come in many different forms: some are portable devices that can be moved between aircraft and others are fixed installations. There are varying degrees of fixed installations: some with only the antenna permanently installed and others where the electronics must be installed in the dash of the cockpit.

GPS aircraft tracking systems report aircraft-specific information such as speed, bearing and altitude and sometimes have built in voice or data communications capabilities. These systems have varying configurations for reporting intervals, typically from one-minute to fifteen-minute time intervals but cellular based systems can also report at shorter intervals. Some devices also have the ability to report for AFF.

Some common satellite networks include Iridium, Globalstar and Inmarsat.

Safety
Unless an aircraft is in an air traffic control zone it is not being tracked, and this is especially true for the general aviation market. Most general aviation aircraft frequent remote locations where the traditional check system includes calling the base upon arrival using a satellite phone. If the aircraft were to go down, someone would have to alert search and rescue who typically spend multiple days searching for a missing aircraft. GPS aircraft tracking reduces the time spent searching for aircraft by giving position reports indicating the last known location of the aircraft, the altitude, the direction it was heading and at what speed.

Accountability
Aircraft operators are required by law to report hours flown per aircraft and per pilot to their respective government agency. By having GPS aircraft tracking on board an aircraft, aircraft operators are given access to real historical information for each aircraft to verify logs. Also, this is a way for aircraft operators to verify the information recorded by pilots, possibly saving hundreds of thousands of dollars annually by reducing the frequency of maintenance and repair.

Government contractors are required to have GPS aircraft tracking by law. This is called AFF (Automated Flight Following).

Situational Awareness
Situational awareness of one's entire fleet of aircraft gives an aircraft operator several advantages. The first is the classic peace of mind. Some GPS aircraft tracking systems have the ability to track aircraft remotely from a PDA or SmartPhone, or to receive alerts by e-mail upon certain events such as OOOI. The second is the ability to improve operational efficiencies from identifying late arrivals and otherwise unexpected events, and plan for them in advance of landing.

Manufacturers of GPS aircraft tracking devices for general aviation

 * Guardian Mobility
 * Flightcell
 * v2track

GPS tours
A GPS tour is an audio tour or a multimedia tour that provides pre-recorded spoken commentary, normally through a handheld device, for mobile applications such as walking tours, boats, buses, trolleys and trains. GPS tours can either be GPS guided or self-directed tours that provide visitors with location relevant content about points of interest along a route or within a destination or region. GPS tours are predominately for outdoor applications, but some audio guides offer the flexibility to manually continue tours indoors.

Using satellite technology (GPS), audio and/or multimedia content is triggered based on a user's location, providing location relevant information to visitors depending on who they are, where they are, and what they are viewing.

A GPS audio tour provides "background, context, and information on the works being viewed". The Economist magazine has stated that "aiming such services at tourists makes sense — since people are more likely to want information when in an unfamiliar place."

GPS Tours are often unilingual, but advances in technology have made GPS tours for mobile applications available in multiple languages simultaneously. GPS tours can be created by using a combination of software and hardware and can be downloaded from the Internet for mobile phones, often in MP3 format and are available from organizations specializing in GPS tour development. Some GPS tours are free, included in the ticket fee, others have to be purchased separately.

Restrictions on civilian use
The U.S. Government controls the export of some civilian receivers. All GPS receivers capable of functioning above 18 km altitude and 515 m/s are classified as munitions (weapons) for which State Department export licenses are required. These limits attempt to prevent use of a receiver in a ballistic missile. They would not prevent use in a cruise missile because their altitudes and speeds are similar to those of ordinary aircraft.

This rule applies even to otherwise purely civilian units that only receive the L1 frequency and the C/A (Coarse/Acquisition) code and cannot correct for Selective Availability (SA), etc.

Disabling operation above these limits exempts the receiver from classification as a munition. Vendor interpretations differ. The rule targets operation given the combination of altitude and speed, while some receivers stop operating even when stationary. This has caused problems with some amateur radio balloon launches that regularly reach 30 km.

These limits only apply to units exported from (or which have components exported from) the USA - there is a growing trade in various components, including GPS units, supplied by other countries, which are expressly sold as ITAR-free.

Military
As of 2009, military applications of GPS include:
 * Navigation: GPS allows soldiers to find objectives, even in the dark or in unfamiliar territory, and to coordinate troop and supply movement. In the United States armed forces, commanders use the Commanders Digital Assistant and lower ranks use the Soldier Digital Assistant.
 * Target tracking: Various military weapons systems use GPS to track potential ground and air targets before flagging them as hostile. These weapon systems pass target coordinates to precision-guided munitions to allow them to engage targets accurately. Military aircraft, particularly in air-to-ground roles, use GPS to find targets (for example, gun camera video from AH-1 Cobras in Iraq show GPS co-ordinates that can be viewed with specialized software).
 * Missile and projectile guidance: GPS allows accurate targeting of various military weapons including ICBMs, cruise missiles and precision-guided munitions. Artillery projectiles. Embedded GPS receivers able to withstand accelerations of 12,000 g or about 118 km/s2 have been developed for use in 155 mm howitzers.
 * Search and Rescue: Downed pilots can be located faster if their position is known.
 * Reconnaissance: Patrol movement can be managed more closely.
 * GPS satellites carry a set of nuclear detonation detectors consisting of an optical sensor (Y-sensor), an X-ray sensor, a dosimeter, and an electromagnetic pulse (EMP) sensor (W-sensor), that form a major portion of the United States Nuclear Detonation Detection System. ''

Communication
GPS satellites broadcast radio signals to enable GPS receivers on or near the Earth's surface to determine location and synchronized time. The GPS system itself is operated by the U.S. Department of Defense for both military use and use by the general public.

GPS signals include ranging signals, used to measure the distance to the satellite, and navigation messages. The navigation messages include ephemeris data, used to calculate the position of each satellite in orbit, and information about the time and status of the entire satellite constellation, called the almanac.

Basic GPS signals
The original GPS design contains two ranging codes: the Coarse/Acquisition (C/A) code, which is freely available to the public, and the restricted Precision (P) code, usually reserved for military applications.

Coarse/Acquisition code
The C/A code is a 1,023 bit deterministic sequence called pseudorandom noise (also pseudorandom binary sequence) (PN or PRN code) which, when transmitted at 1.023 megabits per second (Mbit/s), repeats every millisecond. These sequences only match up, or strongly correlate, when they are exactly aligned. Each satellite transmits a unique PRN code, which does not correlate well with any other satellite's PRN code. In other words, the PRN codes are highly orthogonal to one another. This is a form of code division multiple access (CDMA), which allows the receiver to recognize multiple satellites on the same frequency.

Precision code
The P-code is also a PRN; however, each satellite's P-code PRN code is 6.1871 × 1012 bits long (6,187,100,000,000 bits, ~720.213 gigabytes) and only repeats once a week (it is transmitted at 10.23 Mbit/s). The extreme length of the P-code increases its correlation gain and eliminates any range ambiguity within the Solar System. However, the code is so long and complex it was believed that a receiver could not directly acquire and synchronize with this signal alone. It was expected that the receiver would first lock onto the relatively simple C/A code and then, after obtaining the current time and approximate position, synchronize with the P-code.

Whereas the C/A PRNs are unique for each satellite, the P-code PRN is actually a small segment of a master P-code approximately 2.35 × 1014 bits in length (235,000,000,000,000 bits, ~26.716 terabytes) and each satellite repeatedly transmits its assigned segment of the master code.

To prevent unauthorized users from using or potentially interfering with the military signal through a process called spoofing, it was decided to encrypt the P-code. To that end the P-code was modulated with the W-code, a special encryption sequence, to generate the Y-code. The Y-code is what the satellites have been transmitting since the anti-spoofing module was set to the "on" state. The encrypted signal is referred to as the P(Y)-code.

The details of the W-code are kept secret, but it is known that it is applied to the P-code at approximately 500 kHz, which is a slower rate than that of the P-code itself by a factor of approximately 20. This has allowed companies to develop semi-codeless approaches for tracking the P(Y) signal, without knowledge of the W-code itself.

Navigation message

 * {| class="wikitable" style="float:right; margin:0 0 0.5em 1em;" border="1"

! Subframes !! Words !! Description
 * + GPS message format
 * rowspan=2|1 || 1–2 || Telemetry and handover words (TLM and HOW)
 * 3–10 || Satellite clock, GPS time relationship
 * rowspan=2|2–3 || 1–2 || Telemetry and handover words (TLM and HOW)
 * 3–10 || Ephemeris (precise satellite orbit)
 * rowspan=2|4–5 || 1–2 || Telemetry and handover words (TLM and HOW)
 * 3–10 || Almanac component (satellite network synopsys, error correction)
 * }
 * 3–10 || Ephemeris (precise satellite orbit)
 * rowspan=2|4–5 || 1–2 || Telemetry and handover words (TLM and HOW)
 * 3–10 || Almanac component (satellite network synopsys, error correction)
 * }
 * 3–10 || Almanac component (satellite network synopsys, error correction)
 * }

In addition to the PRN ranging codes, a receiver needs to know detailed information about each satellite's position and the network. The GPS design has this information modulated on top of both the C/A and P(Y) ranging codes at 50 bit/s and calls it the Navigation Message.

The navigation message is made up of three major components. The first part contains the GPS date and time, plus the satellite's status and an indication of its health. The second part contains orbital information called ephemeris data and allows the receiver to calculate the position of the satellite. The third part, called the almanac, contains information and status concerning all the satellites; their locations and PRN numbers.

Whereas ephemeris information is highly detailed and considered valid for no more than four hours, almanac information is more general and is considered valid for up to 180 days. The almanac assists the receiver in determining which satellites to search for, and once the receiver picks up each satellite's signal in turn, it then downloads the ephemeris data directly from that satellite. A position fix using any satellite can not be calculated until the receiver has an accurate and complete copy of that satellite's ephemeris data. If the signal from a satellite is lost while its ephemeris data is being acquired, the receiver must discard that data and start again.

The navigation message itself is constructed from a 1,500 bit frame, which is divided into five subframes of 300 bits each and transmitted at 50 bit/s. Each subframe, therefore, requires 6 seconds to transmit. Each subframe has the GPS time. Subframe 1 contains the GPS date (week number) and information to correct the satellite's time to GPS time, plus satellite status and health. Subframes 2 and 3 together contain the transmitting satellite's ephemeris data. Subframes 4 and 5 contain components of the almanac. Each frame contains only 1/25th of the total almanac; a receiver must process 25 whole frames worth of data retrieve the entire 15,000 bit almanac message. At this rate, 12.5 minutes are required to receive the entire almanac from a single satellite.

The orbital position data, or ephemeris, from the navigation message is used to calculate precisely where the satellite was at the start of the message. A more sensitive receiver will potentially acquire the ephemeris data more quickly than a less sensitive receiver, especially in a noisy environment.

Each subframe is divided into 10 words. It begins with a Telemetry Word (TLM), which enables the receiver to detect the beginning of a subframe and determine the receiver clock time at which the navigation subframe begins. The next word is the handover word (HOW), which gives the GPS time (actually the time when the first bit of the next subframe will be transmitted) and identifies the specific subframe within a complete frame. The remaining eight words of the subframe contain the actual data specific to that subframe.

After a subframe has been read and interpreted, the time the next subframe was sent can be calculated through the use of the clock correction data and the HOW. The receiver knows the receiver clock time of when the beginning of the next subframe was received from detection of the Telemetry Word thereby enabling computation of the transit time and thus the pseudorange. The receiver is potentially capable of getting a new pseudorange measurement at the beginning of each subframe or every 6 seconds.

Almanac
The almanac, provided in subframes 4 and 5 of the frames, consists of coarse orbit and status information for each satellite in the constellation, an ionospheric model, and information to relate GPS derived time to Coordinated Universal Time (UTC). Each frame contains a part of the almanac (in subframes 4 and 5) and the complete almanac is transmitted by each satellite in 25 frames total (requiring 12.5 minutes). The almanac serves several purposes. The first is to assist in the acquisition of satellites at power-up by allowing the receiver to generate a list of visible satellites based on stored position and time, while an ephemeris from each satellite is needed to compute position fixes using that satellite. In older hardware, lack of an almanac in a new receiver would cause long delays before providing a valid position, because the search for each satellite was a slow process. Advances in hardware have made the acquisition process much faster, so not having an almanac is no longer an issue. The second purpose is for relating time derived from the GPS (called GPS time) to the international time standard of UTC. Finally, the almanac allows a single-frequency receiver to correct for ionospheric error by using a global ionospheric model. The corrections are not as accurate as augmentation systems like WAAS or dual-frequency receivers. However, it is often better than no correction, since ionospheric error is the largest error source for a single-frequency GPS receiver. Each satellite transmits not only its own ephemeris, but transmits an almanac for all satellites.

Data updates
Satellite data is updated typically every 24 hours, with up to 60 days data loaded in case there is a disruption in the ability to make updates regularly. Typically the updates contain new ephemerides, with new almanacs uploaded less frequently. The Control Segment guarantees that during normal operations a new almanac will be uploaded at least every 6 days.

A new ephemeris is broadcast by the satellite every 2 hours and is generally valid for 4 hours, with provisions for updates every 4 hours or longer in non-nominal conditions. The time needed to acquire the ephemeris is becoming a significant element of the delay to first position fix, because, as the hardware becomes more capable, the time to lock onto the satellite signals shrinks, but the ephemeris data requires 18 to 36 seconds before it is received, due to the low data transmission rate.

Frequency information


For the ranging codes and navigation message to travel from the satellite to the receiver, they must be modulated onto a carrier frequency. In the case of the original GPS design, two frequencies are utilized; one at 1575.42 MHz (10.23 MHz × 154) called L1; and a second at 1227.60 MHz (10.23 MHz × 120), called L2.

The C/A code is transmitted on the L1 frequency as a 1.023 MHz signal using a bi-phase shift keying (BPSK) modulation technique. The P(Y)-code is transmitted on both the L1 and L2 frequencies as a 10.23 MHz signal using the same BPSK modulation, however the P(Y)-code carrier is in quadrature with the C/A carrier (meaning it is 90° out of phase).

Besides redundancy and increased resistance to jamming, a critical benefit of having two frequencies transmitted from one satellite is the ability to measure directly, and therefore remove, the ionospheric delay error for that satellite. Without such a measurement, a GPS receiver must use a generic model or receive ionospheric corrections from another source (such as the Wide Area Augmentation System or EGNOS). Advances in the technology used on both the GPS satellites and the GPS receivers has made ionospheric delay the largest remaining source of error in the signal. A receiver capable of performing this measurement can be significantly more accurate and is typically referred to as a dual frequency receiver.

Demodulation and decoding
Since all of the satellite signals are modulated onto the same L1 carrier frequency, there is a need to separate the signals after demodulation. This is done by assigning each satellite a unique binary sequence known as a Gold code, and the signals are decoded, after demodulation, using modulo 2 addition of the Gold codes corresponding to satellites n1 through nk, where k is the number of channels in the GPS receiver and n1 through nk are the PRN identifiers of the satellites. Each satellite's PRN identifier is unique and in the range from 1 through 32. The results of these modulo 2 additions are the 50 bit/s navigation messages from satellites n1 through nk. The Gold codes used in GPS are a sequence of 1,023 bits with a period of one millisecond. These Gold codes are highly mutually orthogonal, so that it is unlikely that one satellite signal will be misinterpreted as another. As well, the Gold codes have good auto-correlation properties.

There are 1,025 different Gold codes of length 1,023 bits, but only 32 are used. These Gold codes are quite often referred to as pseudo random noise since they contain no data and are said to look like random sequences. However, this may be misleading since they are actually deterministic sequences.

If the almanac information has previously been acquired, the receiver picks which satellites to listen for by their PRNs. If the almanac information is not in memory, the receiver enters a search mode and cycles through the PRN numbers until a lock is obtained on one of the satellites. To obtain a lock, it is necessary that there be an unobstructed line of sight from the receiver to the satellite. The receiver can then acquire the almanac and determine the satellites it should listen for. As it detects each satellite's signal, it identifies it by its distinct C/A code pattern.

The receiver uses the C/A Gold code with the same PRN number as the satellite to compute an offset, O, that generates the best correlation. The offset, O, is computed in a trial and error manner. The 1,023 bits of the satellite PRN signal are compared with the receiver PRN signal. If correlation is not achieved, the 1,023 bits of the receiver's internally generated PRN code are shifted by one bit relative to the satellite's PRN code and the signals are again compared. This process is repeated until correlation is achieved or all 1,023 possible cases have been tried. If all 1,023 cases have been tried without achieving correlation, the frequency oscillator is offset to the next value and the process is repeated.

Since the carrier frequency received can vary due to Doppler shift, the points where received PRN sequences begin may not differ from O by an exact integral number of milliseconds. Because of this, carrier frequency tracking along with PRN code tracking are used to determine when the received satellite's PRN code begins. Unlike the earlier computation of offset in which trials of all 1,023 offsets could potentially be required, the tracking to maintain lock usually requires shifting of half a pulse width or less. To perform this tracking, the receiver observes two quantities, phase error and received frequency offset. The correlation of the received PRN code with respect to the receiver generated PRN code is computed to determine if the bits of the two signals are misaligned. Comparisons of the received PRN code with receiver generated PRN code shifted half a pulse width early and half a pulse width late are used to estimate adjustment required. The amount of adjustment required for maximum correlation is used in estimating phase error. Received frequency offset from the frequency generated by the receiver provides an estimate of phase rate error. The command for the frequency generator and any further PRN code shifting required are computed as a function of the phase error and the phase rate error in accordance with the control law used. The Doppler velocity is computed as a function of the frequency offset from the carrier nominal frequency. The Doppler velocity is the velocity component along the line of sight of the receiver relative to the satellite.

As the receiver continues to read successive PRN sequences, it will encounter a sudden change in the phase of the 1,023 bit received PRN signal. This indicates the beginning of a data bit of the navigation message. This enables the receiver to begin reading the 20 millisecond bits of the navigation message. The TLM word at the beginning of each subframe of a navigation frame enables the receiver to detect the beginning of a subframe and determine the receiver clock time at which the navigation subframe begins. The HOW word then enables the receiver to determine which specific subframe is being transmitted. There can be a delay of up to 30 seconds before the first estimate of position because of the need to read the ephemeris data before computing the intersections of sphere surfaces.

After a subframe has been read and interpreted, the time the next subframe was sent can be calculated through the use of the clock correction data and the HOW. The receiver knows the receiver clock time of when the beginning of the next subframe was received from detection of the Telemetry Word thereby enabling computation of the transit time and thus the pseudorange. The receiver is potentially capable of getting a new pseudorange measurement at the beginning of each subframe or every 6 seconds.

Then the orbital position data, or ephemeris, from the navigation message is used to calculate precisely where the satellite was at the start of the message. A more sensitive receiver will potentially acquire the ephemeris data more quickly than a less sensitive receiver, especially in a noisy environment.

Modernization and additional GPS signals
Having reached full operational capability on July 17, 1995 the GPS system had completed its original design goals. However, additional advances in technology and new demands on the existing system led to the effort to "modernize" the GPS system. Announcements from the Vice President and the White House in 1998 heralded the beginning of these changes and in 2000, the U.S. Congress reaffirmed the effort, referred to as GPS III.

The project involves new ground stations and new satellites, with additional navigation signals for both civilian and military users, and aims to improve the accuracy and availability for all users. A goal of 2013 has been established with incentives offered to the contractors if they can complete it by 2011.

General features
Modernized GPS civilian signals have two general improvements over their legacy counterparts: a dataless acquisition aid and forward error correction (FEC) coding of the NAV message.

A dataless acquisition aid is an additional signal, called a pilot carrier in some cases, broadcast alongside the data signal. This dataless signal is designed to be easier to acquire than the data encoded and, upon successful acquisition, can be used to acquire the data signal. This technique improves acquisition of the GPS signal and boosts power levels at the correlator.

The second advancement is to use forward error correction (FEC) coding on the NAV message itself. Due to the relatively slow transmission rate of NAV data (usually 50 bits per second) small interruptions can have potentially large impacts. Therefore, FEC on the NAV message is a significant improvement in overall signal robustness.

L2C
One of the first announcements was the addition of a new civilian-use signal, to be transmitted on a frequency other than the L1 frequency used for the coarse/acquisition (C/A) signal. Ultimately, this became the L2C signal, so called because it is broadcast on the L2 frequency. Because it requires new hardware onboard the satellite, it is only transmitted by the so-called Block IIR-M and later design satellites. The L2C signal is tasked with improving accuracy of navigation, providing an easy to track signal, and acting as a redundant signal in case of localized interference.

Unlike the C/A code, L2C contains two distinct PRN code sequences to provide ranging information; the Civilian Moderate length code (called CM), and the Civilian Long length code (called CL). The CM code is 10,230 bits long, repeating every 20 ms. The CL code is 767,250 bits long, repeating every 1500 ms. Each signal is transmitted at 511,500 bits per second (bit/s); however, they are multiplexed together to form a 1,023,000 bits/s signal.

CM is modulated with the CNAV Navigation Message (see below), whereas CL does not contain any modulated data and is called a dataless sequence. The long, dataless sequence provides for approximately 24 dB greater correlation (~250 times stronger) than L1 C/A-code.

When compared to the C/A signal, L2C has 2.7 dB greater data recovery and 0.7 dB greater carrier-tracking, although its transmission power is 2.3 dB weaker.

CNAV Navigation message
The CNAV data is an upgraded version of the original NAV navigation message. It contains higher precision representation and nominally more accurate data than the NAV data. The same type of information (Time, Status, Ephemeris, and Almanac) is still transmitted using the new CNAV format; however, instead of using a frame / subframe architecture, it features a new pseudo-packetized format made up of 12-second 300-bit message packets.

In CNAV, two out of every four packets are ephemeris data and at least one of every four packets will include clock data, but the design allows for a wide variety of packets to be transmitted. With a 32-satellite constellation, and the current requirements of what needs to be sent, less than 75% of the bandwidth is used. Only a small fraction of the available packet types have been defined; this enables the system to grow and incorporate advances.

There are many important changes in the new CNAV message:
 * It uses forward error correction (FEC) in a rate 1/2 convolution code, so while the navigation message is 25 bps, a 50 bps signal is transmitted.
 * The GPS week number is now represented as 13 bits, or 8192 weeks, and only repeats every 157.0 years, meaning the next return to zero won't occur until the year 2137. This is longer compared to the L1 NAV message's use of a 10-bit week number, which returns to zero every 19.6 years.
 * There is a packet that contains a GPS-to-GNSS time offset. This allows for interoperability with other global time-transfer systems, such as Galileo and GLONASS, both of which are supported.
 * The extra bandwidth enables the inclusion of a packet for differential correction, to be used in a similar manner to satellite based augmentation systems and which can be used to correct the L1 NAV clock data.
 * Every packet contains an alert flag, to be set if the satellite data can not be trusted. This means users will know within 6 seconds if a satellite is no longer usable. Such rapid notification is important for safety-of-life applications, such as aviation.
 * Finally, the system is designed to support 63 satellites, compared with 32 in the L1 NAV message.

L2C Frequency information
An immediate effect of having two civilian frequencies being transmitted is the civilian receivers can now directly measure the ionospheric error in the same way as dual frequency P(Y)-code receivers. However, if a user is utilizing the L2C signal alone, they can expect 65% more position uncertainty than with the L1 signal.

Military (M-code)
A major component of the modernization process is a new military signal. Called the Military code, or M-code, it was designed to further improve the anti-jamming and secure access of the military GPS signals.

Very little has been published about this new, restricted code. It contains a PRN code of unknown length transmitted at 5.115 MHz. Unlike the P(Y)-code, the M-code is designed to be autonomous, meaning that a user can calculate their position using only the M-code signal. From the P(Y)-code's original design, users had to first lock onto the C/A code and then transfer the lock to the P(Y)-code. Later, direct-acquisition techniques were developed that allowed some users to operate autonomously with the P(Y)-code.

MNAV Navigation Message
A little more is known about the new navigation message, which is called MNAV. Similar to the new CNAV, this new MNAV is packeted instead of framed, allowing for very flexible data payloads. Also like CNAV it can utilize Forward Error Correction (FEC) and advanced error detection (such as a CRC).

M-code Frequency Information
The M-code is transmitted in the same L1 and L2 frequencies already in use by the previous military code, the P(Y)-code. The new signal is shaped to place most of its energy at the edges (away from the existing P(Y) and C/A carriers).

In a major departure from previous GPS designs, the M-code is intended to be broadcast from a high-gain directional antenna, in addition to a full-Earth antenna. This directional antenna's signal, called a spot beam, is intended to be aimed at a specific region (several hundred kilometers in diameter) and increase the local signal strength by 20 dB, or approximately 100 times stronger. A side effect of having two antennas is that the GPS satellite will appear to be two GPS satellites occupying the same position to those inside the spot beam. While the whole Earth M-code signal is available on the Block IIR-M satellites, the spot beam antennas will not be deployed until the Block III satellites are deployed, tentatively in 2013.

An interesting side effect of having each satellite transmit four separate signals is that the MNAV can potentially transmit four different data channels, offering increased data bandwidth.

The modulation method is binary offset carrier, using a 10.23 MHz subcarrier against the 5.115 MHz code. This signal will have an overall bandwidth of approximately 24 MHz, with significantly separated sideband lobes. The sidebands can be used to improve signal reception.

L5, Safety of Life
Civilian, safety of life signal planned to be available with first GPS IIF launch (2010).

Two PRN ranging codes are transmitted on L5: the in-phase code (denoted as the I5-code); and the quadrature-phase code (denoted as the Q5-code). Both codes are 10,230 bits long and transmitted at 10.23 MHz (1ms repetition). In addition, the I5 stream is modulated with a 10-bit Neuman-Hofman code that is clocked at 1 kHz and the Q5-code is modulated with a 20-bit Neuman-Hofman code that is also clocked at 1 kHz.


 * Improves signal structure for enhanced performance
 * Higher transmitted power than L1/L2 signal (~3 db, or twice as powerful)
 * Wider bandwidth provides a 10× processing gain
 * Longer spreading codes (10× longer than C/A)
 * Uses the Aeronautical Radionavigation Services band

The recently launched GPS IIR-M7 satellite transmits a demonstration of this signal.

L5 Navigation message
The L5 CNAV data includes SV ephemerides, system time, SV clock behavior data, status messages and time information, etc. The 50 bit/s data is coded in a rate 1/2 convolution coder. The resulting 100 symbols per second (sps) symbol stream is modulo-2 added to the I5-code only; the resultant bit-train is used to modulate the L5 in-phase (I5) carrier. This combined signal will be called the L5 Data signal. The L5 quadrature-phase (Q5) carrier has no data and will be called the L5 Pilot signal.

L5 Frequency information
Broadcast on the L5 frequency (1176.45 MHz, 10.23 MHz × 115), which is an aeronautical navigation band. The frequency was chosen so that the aviation community can manage interference to L5 more effectively than L2.

L1C
Civilian use signal, broadcast on the L1 frequency (1575.42 MHz), which currently contains the C/A signal used by all current GPS users. The L1C will be available with first Block III launch, currently scheduled for 2014.

The PRN codes are 10,230 bits long and transmitted at 1.023 Mbps. It uses both Pilot and Data carriers like L2C.

The modulation technique used is BOC(1,1) for the data signal and TMBOC for the pilot. The time multiplexed binary offset carrier (TMBOC) is BOC(1,1) for all except 4 of 33 cycles, when it switches to BOC(6,1). Of the total L1C signal power, 25% is allocated to the data and 75% to the pilot.


 * Implementation will provide C/A code to ensure backward compatibility
 * Assured of 1.5 dB increase in minimum C/A code power to mitigate any noise floor increase
 * Data-less signal component pilot carrier improves tracking
 * Enables greater civil interoperability with Galileo L1

CNAV-2 Navigation message
The L1C navigation message, called CNAV-2, is 1800 bits (including FEC) and is transmitted at 100 bps. It contains 9 bits of time information, 600 bits of ephemeris data, and 274 bits of packetized data payload.

Frequencies used by GPS

 * {| class="wikitable" style="float:right;font-size:80%; margin:0 0 0.5em 1em;" border="1"

! Band ! Frequency (MHz) ! Phase ! Original Usage ! Modernized Usage Military (M) code
 * + GPS Frequencies
 * rowspan=2|L1 || rowspan=2|1575.42 10.23×154 || In-Phase (I) ||colspan=2 align=center| Encrypted Precision P(Y) code
 * Quadrature- Phase (Q) || Coarse-acquisition (C/A) code || C/A, L1 Civilian (L1C), and Military (M) code
 * rowspan=2|L2 || rowspan=2|1227.60 10.23×120 || In-Phase (I) ||colspan=2 align=center| Encrypted Precision P(Y) code
 * Quadrature- Phase (Q) || Unmodulated carrier || L2 Civilian (L2C) code and
 * rowspan=2|L2 || rowspan=2|1227.60 10.23×120 || In-Phase (I) ||colspan=2 align=center| Encrypted Precision P(Y) code
 * Quadrature- Phase (Q) || Unmodulated carrier || L2 Civilian (L2C) code and
 * Quadrature- Phase (Q) || Unmodulated carrier || L2 Civilian (L2C) code and
 * Quadrature- Phase (Q) || Unmodulated carrier || L2 Civilian (L2C) code and
 * L3 || 1381.05 10.23×135 || || Used by Nuclear Detonation (NUDET) Detection System Payload (NDS); signals nuclear detonations/ high-energy infrared events. Used to enforce nuclear test ban treaties.
 * L4 || 1379.913 10.23×1214/9 || || (No transmission) || Being studied for additional ionospheric correction
 * rowspan=2|L5 || rowspan=2|1176.45 10.23×115 || In-Phase (I) ||rowspan=2| (No transmission) || Safety-of-Life (SoL) Data signal
 * Quadrature- Phase (Q) || Safety-of-Life (SoL) Pilot signal
 * }
 * rowspan=2|L5 || rowspan=2|1176.45 10.23×115 || In-Phase (I) ||rowspan=2| (No transmission) || Safety-of-Life (SoL) Data signal
 * Quadrature- Phase (Q) || Safety-of-Life (SoL) Pilot signal
 * }
 * }

All satellites broadcast at the same two frequencies, 1.57542 GHz (L1 signal) and 1.2276 GHz (L2 signal). The satellite network uses a CDMA spread-spectrum technique where the low-bitrate message data is encoded with a high-rate pseudo-random (PRN) sequence that is different for each satellite. The receiver must be aware of the PRN codes for each satellite to reconstruct the actual message data. The C/A code, for civilian use, transmits data at 1.023 million chips per second, whereas the P code, for U.S. military use, transmits at 10.23 million chips per second. The L1 carrier is modulated by both the C/A and P codes, while the L2 carrier is only modulated by the P code. The P code can be encrypted as a so-called P(Y) code which is only available to military equipment with a proper decryption key. Both the C/A and P(Y) codes impart the precise time-of-day to the user.

Each composite signal (in-phase and quadrature phase) becomes:

S(t) = \sqrt{P_\text{I}} X_\text{I} (t) \cos (\omega t + \phi_0) \underbrace{{} - \sqrt{P_\text{Q}} X_\text{Q} (t) \sin (\omega t + \phi_0)}_{+ \sqrt{P_\text{Q}} X_\text{Q} (t) \cos\left(\omega t + \phi_0 + \frac{\pi}{2}\right)} , $$ where $$\scriptstyle\ P_\text{I}\,$$ and $$\scriptstyle\ P_\text{Q}\,$$ represent signal powers; $$\scriptstyle\ X_\text{I}(t)\,$$ and  $$\scriptstyle\ X_\text{Q}(t)\,$$ represent codes with/without data $$(\scriptstyle\ = \pm 1)\,$$.

Navigation equations
The receiver uses messages received from satellites to determine the satellite positions and time sent. The x, y, and z components of satellite position and the time sent are designated as [xi, yi, zi, ti] where the subscript i denotes the satellite and has the value 1, 2, ..., n, where $$n \ge 4.$$ Knowing when the message was received $$\, t_\text{r}$$, the receiver computes the message's transit time as $$\, t_\text{r}- t_i$$. Note that the receiver indeed knows the reception time indicated by its on-board clock, $$\, \tilde{t}_\text{r}$$ rather than $$\, t_\text{r}$$. Assuming the message traveled at the speed of light (c) the distance traveled is (tr − ti)c. Knowing the distance from receiver to satellite and the satellite's position implies that the receiver is on the surface of a sphere centered at the satellite's position. Thus the receiver is at or near the intersection of the surfaces of the spheres. In the ideal case of no errors, the receiver is at the intersection of the surfaces of the spheres.

Let b denote the clock error or bias, the amount that the receiver's clock is off. The receiver has four unknowns, the three components of GPS receiver position and the clock bias [x, y, z, b]. The equations of the sphere surfaces are given by:
 * $$(x-x_i)^2 + (y-y_i)^2 + (z-z_i)^2 = \bigl([ t_\text{r} + b - t_i]c\bigr)^2, \; i=1,2,\dots,n$$

or in terms of pseudoranges, $$ p_i = \left ( t_\text{r} - t_i \right )c$$, as
 * $$p_i = \sqrt{(x-x_i)^2 + (y-y_i)^2 + (z-z_i)^2}- bc, \;i=1,2,...,n$$.

These equations can be solved by algebraic or numerical methods.

Bancroft's method
Bancroft's method involves an algebraic as opposed to numerical method and can be used for the case of four satelites or for the case of more than four satellites. If there are four satellites then Bancroft's method provides the unique solution for the four unknowns. If there are more than four satellites then Bancroft's method provides the solution which minimizes the sum of the squares of the errors for the over determined system.

Trilateration
The receiver can use trilateration  and one dimensional numerical root finding. Trilateration is used to determine the position based on three satellite's pseudoranges. In the usual case of two intersections, the point nearest the surface of the sphere corresponding to the fourth satellite is chosen. Let d denote the signed distance from the receiver position to the sphere around the fourth satellite. The notation, d(correction) shows this as a function of the correction, because it changes the pseudoranges. The problem is to determine the correction such that d(correction) = 0. This is the familiar problem of finding the zeroes of a one dimensional non-linear function of a scalar variable. Iterative numerical methods, such as those found in the chapter on root finding in Numerical Recipes can solve this type of problem.

Multidimensional Newton-Raphson calculations
<span id="pos_multi_nr"> Alternatively, multidimensional root finding method such as Newton-Raphson method can be used. The approach is to linearize around an approximate solution, say $$\ \left [x^{(k)}, y  ^{(k)}, z^{(k)}, b^{(k)}\right ]$$ from iteration k, then solve the linear equations derived from the quadratic equations above to obtain $$\left [x^{(k+1)}, y^{(k+1)}, z^{(k+1)}, b^{(k+1)}\right ]$$. Although there is no guarantee that the method always converges due to the fact that multidimensional roots cannot be bounded, when a neighborhood containing a solution is known as is usually the case for GPS, it is quite likely that a solution will be found. It has been shown that results are comparable in accuracy to those of the Bancroft's method.

Additional methods for more than four satellites
When more than four satellites are available, the calculation can use the four best or more than four, considering number of channels, processing capability, and geometric dilution of precision (GDOP). Using more than four is an over-determined system of equations with no unique solution, which must be solved by least-squares or a similar technique. If all visible satellites are used, the results are as good as or better than using the four best. Errors can be estimated through the residuals. With each combination of four or more satellites, a GDOP factor can be calculated, based on the relative sky directions of the satellites used. As more satellites are picked up, pseudoranges from various 4-way combinations can be processed to add more estimates to the location and clock offset. The receiver then takes the weighted average of these positions and clock offsets. After the final location and time are calculated, the location is expressed in a specific coordinate system such as latitude and longitude, using the WGS 84 geodetic datum or a country-specific system.

Error sources and analysis


The analysis of errors computed using the Global Positioning System is important for understanding how GPS works, and for knowing what magnitude errors should be expected. The Global Positioning System makes corrections for receiver clock errors and other effects but there are still residual errors which are not corrected. The Global Positioning System (GPS) was created by the United States Department of Defense (DOD) in the 1970s. It has come to be widely used for navigation both by the U.S. military and the general public.

User vehicle position is computed by the receiver based on data received from the satellites. Errors depend on geometric dilution of precision and the sources listed in the table below.

Overview
User equivalent range errors (UERE) are shown in the table. There is also a numerical error with an estimated value, $$\ \sigma_{num} $$, of about 1 meter. The standard deviations, $$\ \sigma_R$$, for the coarse/acquisition and precise codes are also shown in the table. These standard deviations are computed by taking the square root of the sum of the squares of the individual components (i.e., RSS for root sum squares). To get the standard deviation of receiver position estimate, these range errors must be multiplied by the appropriate dilution of precision terms and then RSS'ed with the numerical error. Electronics errors are one of several accuracy-degrading effects outlined in the table above. When taken together, autonomous civilian GPS horizontal position fixes are typically accurate to about 15 meters (50 ft). These effects also reduce the more precise P(Y) code's accuracy. However, the advancement of technology means that today, civilian GPS fixes under a clear view of the sky are on average accurate to about 5 meters (16 ft) horizontally.

The term user equivalent range error (UERE) refers to the error of a component in the distance from receiver to a satellite. These UERE errors are given as ± errors thereby implying that they are unbiased or zero mean errors. These UERE errors are therefore used in computing standard deviations. The standard deviation of the error in receiver position, $$\ \sigma_{rc}$$, is computed by multiplying PDOP (Position Dilution Of Precision) by $$\ \sigma_R$$, the standard deviation of the user equivalent range errors. $$\ \sigma_R$$ is computed by taking the square root of the sum of the squares of the individual component standard deviations.

PDOP is computed as a function of receiver and satellite positions. A detailed description of how to calculate PDOP is given in the section, geometric dilution of precision computation (GDOP).

$$\ \sigma_R$$ for the C/A code is given by:
 * $$\sigma_R= \sqrt{3^2+5^2+2.5^2+2^2+1^2+0.5^2} \, \mathrm{m} \,=\,6.7 \, \mathrm{m}$$

The standard deviation of the error in estimated receiver position $$\ \sigma_{rc}$$, again for the C/A code is given by:
 * $$\ \sigma_{rc} = \sqrt{PDOP^2 \times \sigma_R^2 + \sigma_{num}^2} = \sqrt{PDOP^2 \times 6.7^2 + 1^2} \, \mathrm{m}$$

The error diagram on the left shows the inter relationship of indicated receiver position, true receiver position, and the intersection of the four sphere surfaces.

Signal arrival time measurement
The position calculated by a GPS receiver requires the current time, the position of the satellite and the measured delay of the received signal. The position accuracy is primarily dependent on the satellite position and signal delay.

To measure the delay, the receiver compares the bit sequence received from the satellite with an internally generated version. By comparing the rising and trailing edges of the bit transitions, modern electronics can measure signal offset to within about one percent of a bit pulse width, $$\frac{0.01}{(1.023 \times 10^6 /\mathrm{s})}$$, or approximately 10 nanoseconds for the C/A code. Since GPS signals propagate at the speed of light, this represents an error of about 3 meters.

This component of position accuracy can be improved by a factor of 10 using the higher-chiprate P(Y) signal. Assuming the same one percent of bit pulse width accuracy, the high-frequency P(Y) signal results in an accuracy of $$\frac {(0.01 \times 300,000,000\ \mathrm{m/s})} {(10.23 \times 10^6 / \mathrm{s})}$$ or about 30 centimeters.

Atmospheric effects
Inconsistencies of atmospheric conditions affect the speed of the GPS signals as they pass through the Earth's atmosphere, especially the ionosphere. Correcting these errors is a significant challenge to improving GPS position accuracy. These effects are smallest when the satellite is directly overhead and become greater for satellites nearer the horizon since the path through the atmosphere is longer (see airmass). Once the receiver's approximate location is known, a mathematical model can be used to estimate and compensate for these errors.

Ionospheric delay of a microwave signal depends on its frequency. It arises from ionized atmosphere (see Total electron content). This phenomenon is known as dispersion and can be calculated from measurements of delays for two or more frequency bands, allowing delays at other frequencies to be estimated. Some military and expensive survey-grade civilian receivers calculate atmospheric dispersion from the different delays in the L1 and L2 frequencies, and apply a more precise correction. This can be done in civilian receivers without decrypting the P(Y) signal carried on L2, by tracking the carrier wave instead of the modulated code. To facilitate this on lower cost receivers, a new civilian code signal on L2, called L2C, was added to the Block IIR-M satellites, which was first launched in 2005. It allows a direct comparison of the L1 and L2 signals using the coded signal instead of the carrier wave.

The effects of the ionosphere generally change slowly, and can be averaged over time. Those for any particular geographical area can be easily calculated by comparing the GPS-measured position to a known surveyed location. This correction is also valid for other receivers in the same general location. Several systems send this information over radio or other links to allow L1-only receivers to make ionospheric corrections. The ionospheric data are transmitted via satellite in Satellite Based Augmentation Systems (SBAS) such as Wide Area Augmentation System (WAAS) (available in North America and Hawaii), EGNOS (Europe and Asia) or Multi-functional Satellite Augmentation System (MSAS) (Japan), which transmits it on the GPS frequency using a special pseudo-random noise sequence (PRN), so only one receiver and antenna are required.

Humidity also causes a variable delay, resulting in errors similar to ionospheric delay, but occurring in the troposphere. This effect both is more localized and changes more quickly than ionospheric effects, and is not frequency dependent. These traits make precise measurement and compensation of humidity errors more difficult than ionospheric effects.

Changes in receiver altitude also change the delay, due to the signal passing through less of the atmosphere at higher elevations. Since the GPS receiver computes its approximate altitude this error is relatively simple to correct, either by applying a function regression or correlating margin of atmospheric error to ambient pressure using a barometric altimeter.

Multipath effects
GPS signals can also be affected by multipath issues, where the radio signals reflect off surrounding terrain; buildings, canyon walls, hard ground, etc. These delayed signals can cause inaccuracy. A variety of techniques, most notably narrow correlator spacing, have been developed to mitigate multipath errors. For long delay multipath, the receiver itself can recognize the wayward signal and discard it. To address shorter delay multipath from the signal reflecting off the ground, specialized antennas (e.g., a choke ring antenna) may be used to reduce the signal power as received by the antenna. Short delay reflections are harder to filter out because they interfere with the true signal, causing effects almost indistinguishable from routine fluctuations in atmospheric delay.

Multipath effects are much less severe in moving vehicles. When the GPS antenna is moving, the false solutions using reflected signals quickly fail to converge and only the direct signals result in stable solutions.

Ephemeris and clock errors
While the ephemeris data is transmitted every 30 seconds, the information itself may be up to two hours old. If a fast time to first fix (TTFF) is needed, it is possible to upload a valid ephemeris to a receiver, and in addition to setting the time, a position fix can be obtained in under ten seconds. It is feasible to put such ephemeris data on the web so it can be loaded into mobile GPS devices. See also Assisted GPS.

The satellite's atomic clocks experience noise and clock drift errors. The navigation message contains corrections for these errors and estimates of the accuracy of the atomic clock. However, they are based on observations and may not indicate the clock's current state.

These problems tend to be very small, but may add up to a few meters (tens of feet) of inaccuracy.

For very precise positioning (e.g., in geodesy), these effects can be eliminated by differential GPS: the simultaneous use of two or more receivers at several survey points. In the 1990s when receivers were quite expensive, some methods of quasi-differential GPS were developed, using only one receiver but reoccupation of measuring points. At the TU Vienna the method was named qGPS and adequate software of post processing was developed.

Computation of geometric dilution of precision
The concept of geometric dilution of precision was introduced in the section, error sources and analysis. Computations were provided to show how PDOP was used and how it affected the receiver position error standard deviation.

When visible GPS satellites are close together in the sky (i.e., small angular separation), the DOP values are high; when far apart, the DOP values are low. Conceptually, satellites that are close together cannot provide as much information as satellites that are widely separated. Low DOP values represent a better GPS positional accuracy due to the wider angular separation between the satellites used to calculate GPS receiver position. HDOP, VDOP, PDOP and TDOP are respectively Horizontal, Vertical, Position (3-D) and Time Dilution of Precision.

Figure 3.1 Dilution of Precision of Navstar GPS data from the U.S. Coast Guard provide a graphical indication of how geometry affect accuracy.

We now take on the task of how to compute the dilution of precision terms. As a first step in computing DOP, consider the unit vector from the receiver to satellite i with components $$\frac{(x_i- x)}{R_i}$$, $$\frac {(y_i-y)}{R_i}$$, and $$\frac {(z_i-z)}{R_i}$$ where the distance from receiver to the satellite, $$\ R_i $$, is given by:
 * $$R_i\,=\,\sqrt{(x_i- x)^2 + (y_i-y)^2 + (z_i-z)^2}$$

where $$\ x, y, and\ z$$ denote the position of the receiver and $$\ x_i, y_i, and\ z_i$$ denote the position of satellite i. These x, y, and z components may be components in a North, East, Down coordinate system a South, East, Up coordinate system or other convenient system. Formulate the matrix A as:
 * $$A =

\begin{bmatrix} \frac {(x_1- x)} {R_1} & \frac {(y_1-y)} {R_1} & \frac {(z_1-z)} {R_1} & c \\ \frac {(x_2- x)} {R_2} & \frac {(y_2-y)} {R_2} & \frac {(z_2-z)} {R_2} & c \\ \frac {(x_3- x)} {R_3} & \frac {(y_3-y)} {R_3} & \frac {(z_3-z)} {R_3} & c \\ \frac {(x_4- x)} {R_4} & \frac {(y_4-y)} {R_4} & \frac {(z_4-z)} {R_4} & c \end{bmatrix}$$

The first three elements of each row of A are the components of a unit vector from the receiver to the indicated satellite. The elements in the fourth column are c where c denotes the speed of light. Formulate the matrix, Q, as
 * $$ Q = \left (A^T A \right )^{-1}

$$

This computation is in accordance with Chapter 11 of The global positioning system by Parkinson and Spilker where the weighting matrix, P, has been set to the identity matrix. The elements of the Q matrix are designated as:
 * $$Q =

\begin{bmatrix} d_x^2   & d_{xy}^2 & d_{xz}^2 & d_{xt}^2 \\ d_{xy}^2 & d_{y}^2 & d_{yz}^2 & d_{yt}^2 \\ d_{xz}^2 & d_{yz}^2 & d_{z}^2 & d_{zt}^2 \\ d_{xt}^2 & d_{yt}^2 & d_{zt}^2 & d_{t}^2 \end{bmatrix} $$

The Greek letter $$\ \sigma$$ is used quite often where we have used d. However the elements of the Q matrix do not represent variances and covariances as they are defined in probability and statistics. Instead they are strictly geometric terms. Therefore d as in dilution of precision is used. PDOP, TDOP and GDOP are given by
 * $$PDOP = \sqrt{d_x^2 + d_y^2 + d_z^2}$$,
 * $$\ TDOP = \sqrt{d_{t}^2} = |d_{t}|\ $$, and
 * $$ GDOP = \sqrt{PDOP^2 + TDOP^2}$$

in agreement with "Section 1.4.9 of PRINCIPLES OF SATELLITE POSITIONING".

The horizontal dilution of precision, $$ HDOP = \sqrt{d_x^2 + d_y^2}$$, and the vertical dilution of precision, $$\ VDOP = \sqrt{d_{z}^2}$$, are both dependent on the coordinate system used. To correspond to the local horizon plane and the local vertical, x, y, and z should denote positions in either a North, East, Down coordinate system or a South, East, Up coordinate system.

Derivation of equations for computing geometric dilution of precision
The equations for computing the geometric dilution of precision terms have been described in the previous section. This section describes the derivation of these equations. The method used here is similar to that used in "Global Positioning System (preview) by Parkinson and Spiker"

Consider the position error vector, $$\mathbf{e}$$, defined as the vector from the intersection of the four sphere surfaces corresponding to the pseudoranges to the true position of the receiver.$$\mathbf{e} = e_x\hat{x} + e_y\hat{y} + e_z\hat{z} $$ where bold denotes a vector and $$\hat{x}$$, $$\hat{y}$$, and $$\hat{z}$$ denote unit vectors along the x, y, and z axes respectively. Let $$\ e_t$$ denote the time error, the true time minus the receiver indicated time. Assume that the mean value of the three components of $$\mathbf {e}$$ and $$\ e_t$$ are zero.


 * $$A\

\begin{bmatrix} e_x \\ e_y \\ e_z \\ e_t \end{bmatrix} = \begin{bmatrix} \frac {(x_1- x)} {R_1} & \frac {(y_1-y)} {R_1} & \frac {(z_1-z)} {R_1} & c \\ \frac {(x_2- x)} {R_2} & \frac {(y_2-y)} {R_2} & \frac {(z_2-z)} {R_2} & c \\ \frac {(x_3- x)} {R_3} & \frac {(y_3-y)} {R_3} & \frac {(z_3-z)} {R_3} & c \\ \frac {(x_4- x)} {R_4} & \frac {(y_4-y)} {R_4} & \frac {(z_4-z)} {R_4} & c \end{bmatrix}\ \begin{bmatrix} e_x \\ e_y \\ e_z \\ e_t \end{bmatrix} = \begin{bmatrix} e_1 \\ e_2 \\ e_3 \\ e_4 \end{bmatrix} \ (1)$$ where $$\ e_1,\ e_2,\ e_3,\ and\ e_4 $$ are the errors in pseudoranges 1 through 4 respectively. This equation comes from linearizing the equation relating pseudoranges to receiver position, satellite positions, and receiver clock errors as shown in. Multiplying both sides by $$\ A^{-1}\ $$ there results

\begin{bmatrix} e_x \\ e_y \\ e_z \\ e_t \end{bmatrix} = A^{-1} \begin{bmatrix} e_1 \\ e_2 \\ e_3 \\ e_4 \end{bmatrix} \ (2)$$.

Transposing both sides:

\begin{bmatrix} e_x & e_y & e_z & e_t \end{bmatrix} = \begin{bmatrix} e_1 & e_2 & e_3 & e_4 \end{bmatrix}\left (A^{-1} \right )^T \ (3)$$. Post multiplying the matrices on both sides of equation (2) by the corresponding matrices in equation (3), there results

\begin{bmatrix} e_x \\ e_y \\ e_z \\ e_t \end{bmatrix} \begin{bmatrix} e_x & e_y & e_z & e_t \end{bmatrix} = A^{-1} \begin{bmatrix} e_1 \\ e_2 \\ e_3 \\ e_4 \end{bmatrix} \begin{bmatrix} e_1 & e_2 & e_3 & e_4 \end{bmatrix}\left (A^{-1} \right )^T \ (4) $$.

Taking the expected value of both sides and taking the non-random matrices outside the expectation operator, E, there results:
 * $$\ E

\left (\begin{bmatrix} e_x \\ e_y \\ e_z \\ e_t \end{bmatrix} \begin{bmatrix} e_x & e_y & e_z & e_t \end{bmatrix} \right ) = A^{-1} \ E \left (\begin{bmatrix} e_1 \\ e_2 \\ e_3 \\ e_4 \end{bmatrix} \begin{bmatrix} e_1 & e_2 & e_3 & e_4 \end{bmatrix} \right ) \left (A^{-1} \right )^T \ (5) $$ Assuming the pseudorange errors are uncorrelated and have the same variance, the covariance matrix on the right side can be expressed as a scalar times the identity matrix. Thus



\begin{bmatrix} \sigma_x^2   & \sigma_{xy}^2 & \sigma_{xz}^2 & \sigma_{xt}^2 \\ \sigma_{xy}^2 & \sigma_{y}^2 & \sigma_{yz}^2 & \sigma_{yt}^2 \\ \sigma_{xz}^2 & \sigma_{yz}^2 & \sigma_{z}^2 & \sigma_{zt}^2 \\ \sigma_{xt}^2 & \sigma_{yt}^2 & \sigma_{zt}^2 & \sigma_{t}^2 \end{bmatrix} = \sigma_R^2 \ A^{-1} \left (A^{-1} \right )^T = \sigma_R^2 \ \left (A^T A \right )^{-1} \ (6)$$ since $$\ A^{-1} \left (A^{-1} \right )^T \left (A^T A \right ) = I $$

Note: $$\left (A^{-1} \right )^T = \left (A^{T} \right )^{-1},\ $$ since $$\ I = \left(A A^{-1}\right)^T = \left(A^{-1}\right)^T A^T$$

Substituting for $$\left (A^T A \right )^{-1} = Q $$ there follows

\begin{bmatrix} \sigma_x^2   & \sigma_{xy}^2 & \sigma_{xz}^2 & \sigma_{xt}^2 \\ \sigma_{xy}^2 & \sigma_{y}^2 & \sigma_{yz}^2 & \sigma_{yt}^2 \\ \sigma_{xz}^2 & \sigma_{yz}^2 & \sigma_{z}^2 & \sigma_{zt}^2 \\ \sigma_{xt}^2 & \sigma_{yt}^2 & \sigma_{zt}^2 & \sigma_{t}^2 \end{bmatrix} = \sigma_R^2 \ \begin{bmatrix} d_x^2   & d_{xy}^2 & d_{xz}^2 & d_{xt}^2 \\ d_{xy}^2 & d_{y}^2 & d_{yz}^2 & d_{yt}^2 \\ d_{xz}^2 & d_{yz}^2 & d_{z}^2 & d_{zt}^2 \\ d_{xt}^2 & d_{yt}^2 & d_{zt}^2 & d_{t}^2 \end{bmatrix} \ (7) $$

From equation (7), it follows that the variances of indicated receiver position and time are
 * $$\sigma_{rc}^2 = \sigma_x^2 + \sigma_y^2 + \sigma_z^2 = \sigma_R^2\left(d_x^2 + d_y^2 + d_z^2\right) = PDOP^2 \sigma_R^2$$ and
 * $$\sigma_t^2 = \sigma_R^2 d_t^2 = TDOP^2 \sigma_R^2$$

The remaining position and time error variance terms follow in a straightforward manner.

Selective availability
GPS includes a (currently disabled) feature called Selective Availability (SA) that adds intentional, time varying errors of up to 100 meters (328 ft) to the publicly available navigation signals. This was intended to deny an enemy the use of civilian GPS receivers for precision weapon guidance.

SA errors are actually pseudorandom, generated by a cryptographic algorithm from a classified seed key available only to authorized users (the U.S. military, its allies and a few other users, mostly government) with a special military GPS receiver. Mere possession of the receiver is insufficient; it still needs the tightly controlled daily key.

Before it was turned off on May 1, 2000, typical SA errors were about 50 m (164 ft) horizontally and about 100 m (328 ft) vertically. Because SA affects every GPS receiver in a given area almost equally, a fixed station with an accurately known position can measure the SA error values and transmit them to the local GPS receivers so they may correct their position fixes. This is called Differential GPS or DGPS. DGPS also corrects for several other important sources of GPS errors, particularly ionospheric delay, so it continues to be widely used even though SA has been turned off. The ineffectiveness of SA in the face of widely available DGPS was a common argument for turning off SA, and this was finally done by order of President Clinton in 2000.

DGPS services are widely available from both commercial and government sources. The latter include WAAS and the U.S. Coast Guard's network of LF marine navigation beacons. The accuracy of the corrections depends on the distance between the user and the DGPS receiver. As the distance increases, the errors at the two sites will not correlate as well, resulting in less precise differential corrections.

During the 1990-91 Gulf War, the shortage of military GPS units caused many troops and their families to buy readily available civilian units. This significantly impeded the U.S. military's own battlefield use of GPS, so the military made the decision to turn off SA for the duration of the war.

In the 1990s, the FAA started pressuring the military to turn off SA permanently. This would save the FAA millions of dollars every year in maintenance of their own radio navigation systems. The amount of error added was "set to zero" at midnight on May 1, 2000 following an announcement by U.S. President Bill Clinton, allowing users access to the error-free L1 signal. Per the directive, the induced error of SA was changed to add no error to the public signals (C/A code). Clinton's executive order required SA to be set to zero by 2006; it happened in 2000 once the U.S. military developed a new system that provides the ability to deny GPS (and other navigation services) to hostile forces in a specific area of crisis without affecting the rest of the world or its own military systems.

Selective Availability is still a system capability of GPS, and could, in theory, be reintroduced at any time. In practice, in view of the hazards and costs this would induce for U.S. and foreign shipping, it is unlikely to be reintroduced, and various government agencies, including the FAA, have stated that it is not intended to be reintroduced.

One interesting side effect of the Selective Availability hardware is the capability to add corrections to the outgoing signal of the GPS cesium and rubidium atomic clocks to an accuracy of approximately 2 × 10−13. This represented a significant improvement over the raw accuracy of the clocks.

On 19 September 2007, the United States Department of Defense announced that future GPS III satellites will not be capable of implementing SA, eventually making the policy permanent.

Antispoofing
Another restriction on GPS, antispoofing, remains on. This encrypts the P-code so that it cannot be mimicked by a transmitter sending false information. Few civilian receivers have ever used the P-code, and the accuracy attainable with the public C/A code is so much better than originally expected (especially with DGPS) that the antispoof policy has relatively little effect on most civilian users. Turning off antispoof would primarily benefit surveyors and some scientists who need extremely precise positions for experiments such as tracking tectonic plate motion.

Relativity
A number of sources of error exist due to relativistic effects that would render the system useless if uncorrected. Three relativistic effects are the time dilation, gravitational frequency shift, and eccentricity effects. For example, the relativistic time slowing due to the speed of the satellite of about 1 part in 1010, the gravitational time dilation that makes a satellite run about 5 parts in 1010 faster than an Earth based clock, and the Sagnac effect due to rotation relative to receivers on Earth. These topics are examined below, one at a time.

Special and general relativity
According to the theory of relativity, due to their constant movement and height relative to the Earth-centered, non-rotating approximately inertial reference frame, the clocks on the satellites are affected by their speed. Special relativity predicts that the frequency of the atomic clocks moving at GPS orbital speeds will tick more slowly than stationary ground clocks by a factor of $$\frac{v^{2}}{2c^{2}}\approx 10 ^{-10}$$, or result in a delay of about 7 μs/day, where the orbital velocity is v = 4 km/s, and c = the speed of light. The time dilation effect has been measured and verified using the GPS.

The effect of gravitational frequency shift on the GPS due to general relativity is that a clock closer to a massive object will be slower than a clock farther away. Applied to the GPS, the receivers are much closer to Earth than the satellites, causing the GPS clocks to be faster by a factor of 5×10^(-10), or about 45.9 μs/day. This gravitational frequency shift is noticeable.

When combining the time dilation and gravitational frequency shift, the discrepancy is about 38 microseconds per day, a difference of 4.465 parts in 1010. Without correction, errors in the initial pseudorange of roughly 10 km/day would accumulate. This initial pseudorange error is corrected in the process of solving the navigation equations. In addition the elliptical, rather than perfectly circular, satellite orbits cause the time dilation and gravitational frequency shift effects to vary with time. This eccentricity effect causes the clock rate difference between a GPS satellite and a receiver to increase or decrease depending on the altitude of the satellite.

To compensate for the discrepancy, the frequency standard on board each satellite is given a rate offset prior to launch, making it run slightly slower than the desired frequency on Earth; specifically, at 10.22999999543 MHz instead of 10.23 MHz. Since the atomic clocks on board the GPS satellites are precisely tuned, it makes the system a practical engineering application of the scientific theory of relativity in a real-world environment. Placing atomic clocks on artificial satellites to test Einstein's general theory was proposed by Friedwardt Winterberg in 1955.

Calculation of time dilation
To calculate the amount of daily time dilation experienced by GPS satellites relative to Earth we need to separately determine the amounts due to special relativity (velocity) and general relativity (gravity) and add them together.

The amount due to velocity will be determined using the Lorentz transformation. This will be:
 * $$ \frac{1}{\gamma } = \sqrt{1-\frac{v^2}{c^2}} $$

For small values of v/c, by using binomial expansion this approximates to:
 * $$ \frac{1}{\gamma } \approx 1-\frac{v^2}{2 c^2} $$

The GPS satellites move at $3,874 m/s$ relative to Earth's center. We thus determine:
 * $$ \frac{1}{\gamma } \approx 1-\frac{3874^2}{2 \left(2.998\times 10^8\right)^2} \approx 1-8.349\times 10^{-11} $$

This difference below 1 of $8.349$ represents the fraction by which the satellites' clocks move slower than Earth's. It is then multiplied by the number of nanoseconds in a day:
 * $$ -8.349\times 10^{-11}\times 60\times 60\times 24\times 10^9\approx -7214 \text{ ns} $$

That is, the satellites' clocks lose 7,214 nanoseconds a day due to special relativity effects.


 * Note that this speed of $3,874 m/s$ is measured relative to Earth's center rather than its surface where the GPS receivers (and users) are. This is because Earth's equipotential makes net time dilation equal across its geodesic surface. That is, the combination of Special and General effects make the net time dilation at the equator equal to that of the poles, which in turn are at rest relative to the center. Hence we use the center as a reference point to represent the entire surface.

The amount of dilation due to gravity will be determined using the gravitational time dilation equation:
 * $$ \frac{1}{\gamma } =\sqrt{1-\frac{2G M}{r c^2}} $$

For small values of M/r, by using binomial expansion this approximates to:
 * $$ \frac{1}{\gamma } \approx 1-\frac{G M}{r c^2} $$

We are again only interested in the fraction below 1, and in the difference between Earth and the satellites. To determine this difference we take:
 * $$ \Delta \left(\frac{1}{\gamma }\right) \approx \frac{G M_{\text{earth}}}{R_{\text{earth}} c^2}-\frac{G M_{\text{earth}}}{R_{\text{gps}} c^2} $$

Earth has a radius of 6,357 km (at the poles) making Rearth = 6,357,000 m and the satellites have an altitude of 20,184 km making their orbit radius Rgps = 26,541,000 m. Substituting these in the above equation, with Mearth = $5.974$, G = $6.674$, and c = $2.998$ (all in SI units), gives:
 * $$ \Delta \left(\frac{1}{\gamma }\right) \approx 5.307\times 10^{-10} $$

This represents the fraction by which the satellites' clocks move faster than Earth's. It is then multiplied by the number of nanoseconds in a day:
 * $$ 5.307\times 10^{-10}\times 60\times 60\times 24\times 10^9\approx 45850 \text{ ns} $$

That is, the satellites' clocks gain 45,850 nanoseconds a day due to general relativity effects. These effects are added together to give (rounded to 10 ns):


 * 45850 - 7210 = 38640 ns

Hence the satellites' clocks gain approximately 38,640 nanoseconds a day or 38.6 &mu;s per day due to relativity effects in total.

In order to compensate for this gain, a GPS clock's frequency needs to be slowed by the fraction:



This fraction is subtracted from 1 and multiplied by the pre-adjusted clock frequency of 10.23 MHz:


 * (1 - $5.307$) × 10.23 = 10.22999999543

That is, we need to slow the clocks down from 10.23 MHz to 10.22999999543 MHz in order to negate the effects of relativity.

Sagnac distortion
GPS observation processing must also compensate for the Sagnac effect. The GPS time scale is defined in an inertial system but observations are processed in an Earth-centered, Earth-fixed (co-rotating) system, a system in which simultaneity is not uniquely defined. A Lorentz transformation is thus applied to convert from the inertial system to the ECEF system. The resulting signal run time correction has opposite algebraic signs for satellites in the Eastern and Western celestial hemispheres. Ignoring this effect will produce an east-west error on the order of hundreds of nanoseconds, or tens of meters in position.

Natural sources of interference
Since GPS signals at terrestrial receivers tend to be relatively weak, natural radio signals or scattering of the GPS signals can desensitize the receiver, making acquiring and tracking the satellite signals difficult or impossible.

Space weather degrades GPS operation in two ways, direct interference by solar radio burst noise in the same frequency band or by scattering of the GPS radio signal in ionospheric irregularities referred to as scintillation. Both forms of degradation follow the 11 year solar cycle and are a maximum at sunspot maximum although they can occur at anytime. Solar radio bursts are associated with solar flares and Coronal Mass Ejections (CMEs) and their impact can affect reception over the half of the Earth facing the sun. Scintillation occurs most frequently at tropical latitudes where it is a night time phenomenon. It occurs less frequently at high latitudes or mid-latitudes where magnetic storms can lead to scintillation. In addition to producing scintillation, magnetic storms can produce strong ionospheric gradients that degrade the accuracy of SBAS systems.

Artificial sources of interference
In automotive GPS receivers, metallic features in windshields, such as defrosters, or car window tinting films can act as a Faraday cage, degrading reception just inside the car.

Man-made EMI (electromagnetic interference) can also disrupt or jam GPS signals. In one well-documented case it was impossible to receive GPS signals in the entire harbor of Moss Landing, California due to unintentional jamming caused by malfunctioning TV antenna preamplifiers. Intentional jamming is also possible. Generally, stronger signals can interfere with GPS receivers when they are within radio range or line of sight. In 2002 a detailed description of how to build a short-range GPS L1 C/A jammer was published in the online magazine Phrack.

The U.S. government believes that such jammers were used occasionally during the 2001 war in Afghanistan, and the U.S. military claims to have destroyed six GPS jammers during the Iraq War, including one that was destroyed with a GPS-guided bomb. A GPS jammer is relatively easy to detect and locate, making it an attractive target for anti-radiation missiles. The UK Ministry of Defence tested a jamming system in the UK's West Country on 7 and 8 June 2007.

Some countries allow the use of GPS repeaters to allow the reception of GPS signals indoors and in obscured locations; however, under EU and UK laws, the use of these is prohibited as the signals can cause interference to other GPS receivers that receive data from both GPS satellites and the repeater.

Due to the potential for both natural and man-made noise, numerous techniques continue to be developed to deal with the interference. The first is to not rely on GPS as a sole source. According to John Ruley, "IFR pilots should have a fallback plan in case of a GPS malfunction". Receiver Autonomous Integrity Monitoring (RAIM) is a feature included in some receivers, designed to provide a warning to the user if jamming or another problem is detected. The U.S. military has also deployed since 2004 their Selective Availability / Anti-Spoofing Module (SAASM) in the Defense Advanced GPS Receiver (DAGR). In demonstration videos the DAGR was shown to detect jamming and maintain its lock on the encrypted GPS signals during interference which caused civilian receivers to lose lock.

Augmentation
Integrating external information into the calculation process can materially improve accuracy. Such augmentation systems are generally named or described based on how the information arrives. Some systems transmit additional error information (such as clock drift, ephemera, or ionospheric delay), others characterize prior errors, while a third group provides additional navigational or vehicle information.

Examples of augmentation systems include the Wide Area Augmentation System (WAAS), European Geostationary Navigation Overlay Service (EGNOS), Differential GPS, Inertial Navigation Systems (INS) and Assisted GPS.

Background
The Global Positioning System (GPS) is a satellite-based system for navigation. Receivers on or near the earth's surface can determine their locations based on signals received from any four or more of the satellites in the network.

All satellites in the work broadcast on the same two frequencies, known as L1 (1575.42 MHz) and L2 (1227.60 MHz). The network uses code division multiple access (CDMA) to allow separate messages from the individual satellites to be distinguished. Two distinct CDMA encodings are used: the coarse/acquisition (C/A) code, which is accessible by the general public, and the precise (P) code, that is encrypted so that only the U.S. military can access it. The messages sent from each satellite contain information ranging from the satellite health, the satellite's orbital path, the clock state of the satellite, and the configuration of the entire satellite network.

Precise monitoring
The accuracy of a calculation can also be improved through precise monitoring and measuring of the existing GPS signals in additional or alternate ways.

After Selective Availability was turned off by the U.S. government, the largest error in GPS was usually the unpredictable delay through the ionosphere. The spacecraft broadcast ionospheric model parameters, but errors remain. This is one reason the GPS spacecraft transmit on at least two frequencies, L1 and L2. Ionospheric delay is a well-defined function of frequency and the total electron content (TEC) along the path, so measuring the arrival time difference between the frequencies determines TEC and thus the precise ionospheric delay at each frequency.

Receivers with decryption keys can decode the P(Y)-code transmitted on both L1 and L2. However, these keys are reserved for the military and authorized agencies and are not available to the public. Without keys, it is still possible to use a codeless technique to compare the P(Y) codes on L1 and L2 to gain much of the same error information. However, this technique is slow, so it is currently limited to specialized surveying equipment. In the future, additional civilian codes are expected to be transmitted on the L2 and L5 frequencies (see GPS modernization). Then all users will be able to perform dual-frequency measurements and directly compute ionospheric delay errors.

A second form of precise monitoring is called Carrier-Phase Enhancement (CPGPS). The error, which this corrects, arises because the pulse transition of the PRN is not instantaneous, and thus the correlation (satellite-receiver sequence matching) operation is imperfect. The CPGPS approach utilizes the L1 carrier wave, which has a period of


 * $$ \frac{1 \ \mathrm{s}}{1575.42 \times 10^6} = 0.63475 \ \mathrm{ns} \approx 1 \ \mathrm{ns} \ $$

which is about one-thousandth of the C/A Gold code bit period of


 * $$ \frac{1 \ \mathrm{s}}{1023 \times 10^3} = 977.5 \ \mathrm{ns} \  \approx 1000 \ \mathrm{ns} \ $$

to act as an additional clock signal and resolve the uncertainty. The phase difference error in the normal GPS amounts to between 2 and 3 meters (6 to 10 ft) of ambiguity. CPGPS working to within 1% of perfect transition reduces this error to 3 centimeters (1 inch) of ambiguity. By eliminating this source of error, CPGPS coupled with DGPS normally realizes between 20 and 30 centimeters (8 to 12 inches) of absolute accuracy.

Relative Kinematic Positioning (RKP) is another approach for a precise GPS-based positioning system. In this approach, determination of range signal can be resolved to a precision of less than 10 centimeters (4 in). This is done by resolving the number of cycles in which the signal is transmitted and received by the receiver. This can be accomplished by using a combination of differential GPS (DGPS) correction data, transmitting GPS signal phase information and ambiguity resolution techniques via statistical tests—possibly with processing in real-time (real-time kinematic positioning, RTK).

Timekeeping and leap seconds
While most clocks are synchronized to Coordinated Universal Time (UTC), the atomic clocks on the satellites are set to GPS time (GPST; see the page of United States Naval Observatory). The difference is that GPS time is not corrected to match the rotation of the Earth, so it does not contain leap seconds or other corrections that are periodically added to UTC. GPS time was set to match Coordinated Universal Time (UTC) in 1980, but has since diverged. The lack of corrections means that GPS time remains at a constant offset with International Atomic Time (TAI) (TAI – GPS = 19 seconds). Periodic corrections are performed on the on-board clocks to keep them synchronized with ground clocks.

The GPS navigation message includes the difference between GPS time and UTC, which as of 2011 is 15 seconds because of the leap second added to UTC December 31, 2008. Receivers add this offset to GPS time to calculate UTC and specific timezone values. New GPS units may not show the correct UTC time until after receiving the UTC offset message. The GPS-UTC offset field can accommodate 255 leap seconds (eight bits) that, given the current period of the Earth's rotation (with one leap second introduced approximately every 18 months), should be sufficient to last until approximately the year 2300.

Timekeeping accuracy
GPS time is accurate to about 14 nanoseconds.

Timekeeping format
As opposed to the year, month, and day format of the Gregorian calendar, the GPS date is expressed as a week number and a seconds-into-week number. The week number is transmitted as a ten-bit field in the C/A and P(Y) navigation messages, and so it becomes zero again every 1,024 weeks (19.6 years). GPS week zero started at 00:00:00 UTC (00:00:19 TAI) on January 6, 1980, and the week number became zero again for the first time at 23:59:47 UTC on August 21, 1999 (00:00:19 TAI on August 22, 1999). To determine the current Gregorian date, a GPS receiver must be provided with the approximate date (to within 3,584 days) to correctly translate the GPS date signal. To address this concern the modernized GPS navigation message uses a 13-bit field that only repeats every 8,192 weeks (157 years), thus lasting until the year 2137 (157 years after GPS week zero).

Carrier phase tracking (surveying)
Utilizing the navigation message to measure pseudorange has been discussed. Another method that is used in GPS surveying applications is carrier phase tracking. The period of the carrier frequency times the speed of light gives the wave length, which is about 0.19 meters for the L1 carrier. With a 1% of wave length accuracy in detecting the leading edge, this component of pseudorange error might be as low as 2 millimeters. This compares to 3 meters for the C/A code and 0.3 meters for the P code.

However, this 2 millimeter accuracy requires measuring the total phase, that is the total number of wave lengths plus the fractional wavelength. This requires specially equipped receivers. This method has many applications in the field of surveying.

We now describe a method which could potentially be used to estimate the position of receiver 2 given the position of receiver 1 using triple differencing followed by numerical root finding, and a mathematical technique called least squares. A detailed discussion of the errors is omitted in order to avoid detracting from the description of the methodology. In this description differences are taken in the order of differencing between satellites, differencing between receivers, and differencing between epochs. This should not be construed to mean that this is the only order which can be used. Indeed other orders of taking differences are equally valid.

The satellite carrier total phase can be measured with ambiguity as to the number of cycles. Let $$\ \phi(r_i, s_j, t_k) $$ denote the phase of the carrier of satellite j measured by receiver i at time $$\ \ t_k $$. This notation has been chosen so as to make it clear what the subscripts i, j, and k mean. In view of the fact that the receiver, satellite, and time come in alphabetical order as arguments of $$\ \phi $$ and to strike a balance between readability and conciseness, let $$\ \phi_{i,j,k} = \phi(r_i, s_j, t_k) $$ so as to have a concise abbreviation. Also we define three functions, :$$\ \Delta^r, \Delta^s, \Delta^t $$ which perform differences between receivers, satellites, and time points respectively. Each of these functions has a linear combination of variables with three subscripts as its argument. These three functions are defined below. If $$\ \alpha_{i,j,k} $$ is a function of the three integer arguments, i, j, and k then it is a valid argument for the functions, :$$\ \Delta^r, \Delta^s, \Delta^t $$, with the values defined as


 * $$\ \Delta^r(\alpha_{i,j,k}) = \alpha_{i+1,j,k} - \alpha_{i,j,k} $$ ,
 * $$\ \Delta^s(\alpha_{i,j,k}) = \alpha_{i,j+1,k} - \alpha_{i,j,k} $$, and
 * $$\ \Delta^t(\alpha_{i,j,k}) = \alpha_{i,j,k+1} - \alpha_{i,j,k} $$.

Also if $$\ \alpha_{i,j,k}\ and\ \beta_{l,m,n} $$ are valid arguments for the three functions and a and b are constants then $$\ ( a\ \alpha_{i,j,k} + b\ \beta_{l,m,n} ) $$ is a valid argument with values defined as


 * $$\ \Delta^r(a\ \alpha_{i,j,k} + b\ \beta_{l,m,n}) = a \ \Delta^r(\alpha_{i,j,k}) + b \ \Delta^r(\beta_{l,m,n})$$ ,
 * $$\ \Delta^s(a\ \alpha_{i,j,k} + b\ \beta_{l,m,n} )= a \ \Delta^s(\alpha_{i,j,k}) + b \ \Delta^s(\beta_{l,m,n})$$, and
 * $$\ \Delta^t(a\ \alpha_{i,j,k} + b\ \beta_{l,m,n} )= a \ \Delta^t(\alpha_{i,j,k}) + b \ \Delta^t(\beta_{l,m,n})$$.

Receiver clock errors can be approximately eliminated by differencing the phases measured from satellite 1 with that from satellite 2 at the same epoch. This difference is designated as $$\ \Delta^s(\phi_{1,1,1}) = \phi_{1,2,1} - \phi_{1,1,1}$$

Double differencing can be performed by taking the differences of the between satellite difference observed by receiver 1 with that observed by receiver 2. The satellite clock errors will be approximately eliminated by this between receiver differencing. This double difference is:
 * $$\begin{align}

\Delta^r(\Delta^s(\phi_{1,1,1}))\,&=\,\Delta^r(\phi_{1,2,1} - \phi_{1,1,1}) &=\,\Delta^r(\phi_{1,2,1}) - \Delta^r(\phi_{1,1,1}) &=\,(\phi_{2,2,1} - \phi_{1,2,1}) - (\phi_{2,1,1} - \phi_{1,1,1}) \end{align}$$

Triple differencing can be performed by taking the difference of double differencing performed at time $$\ \ t_2 $$ with that performed at time $$\ \ t_1 $$. This will eliminate the ambiguity associated with the integral number of wave lengths in carrier phase provided this ambiguity does not change with time. Thus the triple difference result has eliminated all or practically all clock bias errors and the integer ambiguity. Also errors associated with atmospheric delay and satellite ephemeris have been significantly reduced. This triple difference is:
 * $$\ \Delta^t(\Delta^r(\Delta^s(\phi_{1,1,1}))) $$

Triple difference results can be used to estimate unknown variables. For example if the position of receiver 1 is known but the position of receiver 2 unknown, it may be possible to estimate the position of receiver 2 using numerical root finding and least squares. Triple difference results for three independent time pairs quite possibly will be sufficient to solve for the three components of position of receiver 2. This may require the use of a numerical procedure such as one of those found in the chapter on root finding and nonlinear sets of equations in Numerical Recipes. To use such a numerical method, an initial approximation of the position of receiver 2 is required. This initial value could probably be provided by a position approximation based on the navigation message and the intersection of sphere surfaces. Although multidimensional numerical root finding can have problems, this disadvantage may be overcome with this good initial estimate. This procedure using three time pairs and a fairly good initial value followed by iteration will result in one observed triple difference result for receiver 2 position. Greater accuracy may be obtained by processing triple difference results for additional sets of three independent time pairs. This will result in an over determined system with multiple solutions. To get estimates for an over determined system, least squares can be used. The least squares procedure determines the position of receiver 2 which best fits the observed triple difference results for receiver 2 positions under the criterion of minimizing the sum of the squares.

Possible threat
In January 2011, the FCC approved a wireless broadband network by the Virginia company LightSquared, to be operational in 92 percent of the United States by 2015. This approval came despite concerns by GPS equipment manufacturers that the network signals could interfere with GPS. The FCC believed LightSquared would not cause problems but vowed to keep the network from operating until testing showed GPS systems would not be affected. One problem was equipment designed to receive weak signals from satellites; LightSquared had up to 40,000 ground-based transmitters whose signals would be much stronger. Also, according to Chris Dancy of the Aircraft Owners and Pilots Association, airline pilots with the type systems that would be affected "may go off course and not even realize it." The problems could also affect the Federal Aviation Administration upgrade to the air traffic control system, United States Defense Department guidance, and local emergency services including 911. Aviation Week magazine reports that the latest testing (June 2011) confirms "significant jamming" of GPS by LightSquared's system. In response to these threats, a coalition of GPS users and stake holders has formed "The Coalition To Save Our GPS". This group serves as a coordinating body to facilitate communication between GPS users and policy makers. In the face of demonstrated disruption to GPS operations, LightSquared has turned to a strategy of blaming GPS manufacturers for building receiving equipment which "..looks into their (LightSquared) spectrum". This, despite the fact that the spectrum in question was never envisioned as being used for terrestrial broadcast. Further, it has come to light that the rapid approval of the LightSquared system by the FCC may be the result of pressure from the Obama Administration.

Other systems
Other satellite navigation systems in use or various states of development include:
 * GLONASS – Russia's global navigation system
 * Galileo – a global system being developed by the European Union and other partner countries, planned to be operational by 2014
 * Beidou – People's Republic of China's regional system, currently limited to Asia and the West Pacific
 * COMPASS – People's Republic of China's global system, planned to be operational by 2020
 * IRNSS – India's regional navigation system, planned to be operational by 2012, covering India and Northern Indian Ocean
 * QZSS – Japanese regional system covering Asia and Oceania