User:Ziggle/Draft of Sound Ranging

Sound Ranging is a technique that uses measurements of the arrival times of a sound to generate a position estimate for the source of the sound. Historically, sound ranging was mainly used for detecting the location of far distant artillery. However, other military uses have included locating submarines and aircraft. The civilian uses include locating wildlife and locating the shooting position of a firearm. The basic methodology of sound location is applied using radio signals in the LORAN navigation system. This article will focus on locating artillery using sound methods. The methods are applicable to pinpointing the source of any sound.

Basic Equipment Setup
A sound location system requires the following equipment.
 * An array of microphones
 * A system capable of measuring the sound wave arrival time differences between the microphones.
 * A means of analyzing the time differences to compute the position of the sound source.

Figure 1 illustrates the basic system.



Figure 1: Block Diagram of a Typical Sound Location System.

Some systems may not allow arbitrary placement of the microphones. For example, they may require the microphones to be placed on a straight line. These constraints would be imposed to simplify the calculation of the artillery position and are not a characteristic of the general approach.

The microphones also may be designed to pick up only the sound of the gun firing. There are three types of sounds that can be picked up by the microphone.
 * the gun firing (the desired signal)
 * the sound of the shell moving through the air
 * the impact of the shell

During World War I it was discovered that the gun firing makes a low rumbling sound that is best picked up with a microphone that is sensitive to low frequencies and rejects high frequencies.

Example
Figure 2 shows an example of an artillery location problem. Assume that we position three microphones with the following relative positions (all measurements made relative to Microphone 3).


 * Distance from Microphone 1 to Microphone 3: $$r_5 = 1267.9$$ meters
 * Distance from Microphone 2 to Microphone 3: $$r_4 = 499.1$$ meters
 * Angle between Microphone 1 and Microphone 2 measured from Microphone 3: 16.177o

These values would be established during an initial survey of the microphone layout.



Figure 2: Example of An Artillery Location Problem.

Assume that two time delays are measured (assume speed of sound = 330 meters per second).


 * Microphone 1 to Microphone 2 time delay: 0.455 s $$\Rightarrow$$ 150 meters
 * Microphone 1 to Microphone 3 time delay: 0.606 s $$\Rightarrow$$ 200 meters

There are a number of ways to determine the range to the artillery piece. One way is to apply the law of cosines twice.


 * $$\left(r_1+r_2\right)^2=\left(r_1+r_3\right)^2 + r_4^2-2\cdot\left(r_1+r_3\right)\cdot r_4 \cos\theta$$ (Equation 1)


 * $$r_1^2=\left(r_1+r_3\right)^2 + r_5^2-2\cdot\left(r_1+r_3\right)\cdot r_5 \cos\phi$$ (Equation 2)

This is a system of two equations with two unknowns ($$\theta$$, $$r_1$$). This system of equations, while nonlinear, can be solved using numerical methods to give a solution for r1of 1621 meters. While this approach would be usable today with computers, it would have been a problem in World War I and II. During these conflicts, the solutions were developed using one of the following methods.
 * graphically using hyperbolas drawn on paper
 * assuming the artillery is far away and using the asymptotes of the hyperbolas, which are lines, to find an approximate location of the artillery..
 * Approximate solutions can be generated using sets of metal disks whose radii differ by small increments. By selecting three discs that approximate the situation in question, an approximate solution can be generated.

Shortcomings
Sound location has a number of shortcomings.
 * the speed of sound varies with temperature and air pressure. Wind also introduces errors. There are means by which to compensate for these factors.
 * at a distance, the sound of a gun is not a sharp crack but more of a rumble (this makes it difficult to accurately measure the exact arrival time of the wavefront at different sensors)
 * artillery is often fired in large numbers, which makes it difficult to determine which wavefront is associated with which artillery piece)

The military has found various ways to mitigate these problems, but nonetheless they do create additional work and reduce the accuracy of the method.

World War I
World War I was the first conflict that brought together the necessary sensors, measurement technology, and analysis capabiliites required to do effective sound ranging. Like many technology concepts, the idea of using sound to locate enemy artillery pieces came to a number of people at about the same time.


 * A Russian's claimed to have invented artillery sound location in 1910
 * A German officer patented the method in 1913
 * The French developed the first operational equipment
 * The Americans proposed a scheme early in World War I

World War I provided the ideal environment for the development of sound location because:


 * electronic processing of sound was becoming mature because of the development of telephone and recording technology
 * sound recording on film technology became available (this facilitated making time difference measurements accurate to hundreths of a second)
 * the need for counter battery artillery fire provided a strong technology driver

While not the first to attempt the sound location of artillery, it was the British during WWI who actually fielded the first effective operational system. British sound location during WWI began with crews that used both sound and flash detection. The sound location operators used equipment that augmented human hearing. The instruments usually consisted of large horns or microphones connected to the operators ears using tubing, much like a very large stethoscope. Using the gun flash, the flash crew would determine a bearing to the gun using a theodolite or transit. The sound detection crew would determine the difference in time between the gun flash and the sound of the gun, which was used to determine the range of the gun. This provided range and bearing data needed for counter battery fire.

The Germans soon switched to flashless powders that limited the ability of the British to see the gun flash. The British responded by assigning the great Australian scientist Sir William Lawrence Bragg to the problem. When Bragg came on the scene, sound location was slow, unreliable, and inaccurate. At that time, sound location was being performed using equipment developed two Frenchman, Bull and Nordmann, which used photograhic film to record the artillery sound. This method was slow because the film took 5 minutes to develop. Also, Bragg found out that the nature of gun sounds was not well understood and that care needed to be taken to separate the sonic boom of the shell from the actual sound of the firing. A new type of low-frequency microphone was invented by a man named Taylor that separated the low frequency sound made by the firing of the gun from the sonic boom of the shell. With the proposed impovements, enemy artillery could be located accurately to within 25 to 50 meters under normal circumstances. This program was very well developed by the end of World War I. In fact, the method was expanded to determine the gun location, caliber, and the intended target.

Most of the work on anti-aircraft sound location was done by the British. They developed an extensive network of sound mirrors that were used from World War I through World War II.

Between the World Wars
Prior to the development of radar, sound location was the only technology available for detecting aircraft at a distance. Britain developed a sound location system. As World War II neared, radar began to become a credible alternative to the sound location of aircraft. However, the sound location stations were left in operation as a backup to radar.

World War II
During World War II, sound location was a mature technology. The emphasis was on developing more usable implementations. However, radar as a counter battery weapon proved superior to sound location and supplanted sound location in most situations. Radar was more accurate and less susceptible to weather conditions. However, radar did have some disadvantages. It was susceptible to jamming. Since radar emits radio signals, turning on a radar set gave away the position of the artillery spotting unit.

Even with the coming of radar, there were many instances of effective use of sound location for counter battery operations during World War II. The US Marines includes sound location units as standard parts of their defense battalions. These sound location units were active in the Marines both before and during World War II. The US Army also used sound locators. During the Okinawa campaign, the US Army used its sound location sets to provide effective counter battery fire.

During World War II, the British made extensive use of sound location for artillery spotting. There a number of excellent memoirs that address their use of sound location for artillery spotting available on the web. These memoirs include excellent details on the electronic equipment involved with these operations.

While sound location for artillery spotting had a secure place during the war, aircraft spotting proved less useful. During the Battle of Britain, sound location was used as a backup for radar in locating inbound aircraft. Since aircraft in World War II were much faster than in World War I, aircraft sound location only gave a few minutes of warning. This made it a much less attractive approach than radar. However, the sites are still in existence and are readily accessible.

After World War II, sound location played no further role in anti-aircraft operations.

Korean War
Sound location of artillery was done in Korea, but mainly was supplanted by aircraft-based artillery spotters. Since the UN had air superiority throughout the war, this approach was simpler and more accurate.

Vietnam
Most counter battery work in Vietnam was with artillery spotting done using radar or aircraft. Australia did deploy some sound location units in Vietnam with some successs.

Present-Day
Sound location is undergoing a renaissance. The US government has been showing interest in using sound location to determine the location of gunfire in large cities.

A number of military units are moving back to using sound location because it is a passive technology and cannot be detected by enemy forces, which is a failing of radar. The British army and US Marines are deploying new systems.

Because the cost of the associated sensors and electronics is dropping, the use of sound location technology is becoming accessible for other uses, such as for locating wildlife.