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What is Remote Sensing? “the measurement or acquisition of information of some property of an object or phenomenon, by a recording device that is not in physical contact with the object or phenomenon under study” (Colwell, 1997). A remote sensing instrument collects information about an object or phenomenon within the instantaneous- field-of-view (IFOV) of the sensor system without being in direct physical contact with it. The sensor is located on a suborbital or satellite platform Radiometric Resolution 7-bit (0 - 127), 8-bit (0 - 255), 9-bit (0 - 511), 10-bit (0 - 1023) Angular Information	 There is always an angle of incidence associated with the incoming energy that illuminates the terrain and an angle of exitance from the terrain to the sensor system. This bidirectional nature of remote sensing data collection is known to influence the spectral and polarization characteristics of the at-sensor radiance, L, recorded by the remote sensing system. Remote sensing Applications- Oceans The image shows sea surface height relative to normal ocean conditions on Feb. 5, 1998 and sea surface height is an indicator of the heat content of the ocean. The white and red areas indicate unusual patterns of heat storage; in the white areas, the sea surface is between 14 and 32 centimeters above normal; in the red areas, it's about 10 centimeters above normal. The green areas indicate normal conditions, while purple means at least 18 centimeters below normal sea level. Red and white areas in the central and eastern equatorial Pacific were 100 to 180 millimeters (4 to 7 inches) above normal. In the western equatorial Pacific, blue and purple areas show where sea levels were between 80 and 150 millimeters (3 and 6 inches) below normal. Energy-matter interactions in the atmosphere, at the surface, and at the remote sensing system (sensor). atmospheric effects are more important at shorter wavelengths so for the visible part of the EM spectrum. Electromagnetic Energy Interactions Energy recorded by remote sensing systems undergoes fundamental interactions: that should be understood to properly interpret the remotely sensed data. For example, if the energy being remotely sensed comes from the Sun, the energy: • is radiated by atomic particles at the source (the Sun), • propagates through the vacuum of space at the speed of light,• interacts with the Earth's atmosphere, • interacts with the Earth's surface, • interacts with the Earth's atmosphere once again, and • finally reaches the remote sensor where it interacts with various How is Energy Transferred? Energy may be transferred three ways: conduction, convection, and radiation. a) Energy may be conducted directly from one object to another as when a pan is in direct physical contact with a hot burner. b) The Sun bathes the Earths surface with radiant energy causing the air near the ground to increase in temperature. The less dense air rises, creating convectional currents in the atmosphere. c) Electromagnetic energy in the Jensen form of electromagnetic waves may be transmitted through the vacuum of space from the 2007 Sun to the Earth. Wave Model of Electromagnetic Radiation In the 1860s, James Clerk Maxwell (1831–1879) conceptualized electromagnetic radiation (EMR) as an electromagnetic wave that travels through space at the speed of light, c, which is 3 x 108 meters per second (hereafter referred to as m s-1). The electromagnetic wave consists of two fluctuating fields—one electric and the other magnetic. The two vectors are at right angles (orthogonal) to one another, and both are perpendicular to the direction of travel. The Wave Model of Electromagnetic Energy Electromagnetic radiation is generated when an electrical charge is accelerated. • The wavelength of electromagnetic radiation (!) depends upon the length of time that the charged particle is accelerated and its frequency (!) depends on the number of accelerations per second. • Wavelength is formally defined as the mean distance between maximums (or minimums) of a roughly periodic pattern and is normally measured in micrometers (μm) or nanometers (nm). • Frequency is the number of wavelengths that pass a point per unit time. A wave that sends one crest by every second (completing one cycle) is said to have a frequency of one cycle per second or one hertz, abbreviated 1 Hz. Wave Model of Electromagnetic Energy This cross-section of an electromagnetic wave illustrates the inverse relationship between wavelength and frequency. The amplitude of an electromagnetic wave is the height of the wave crest above the undisturbed position. Successive wave crests are numbered 1, 2, 3, and 4. An observer at the position of the clock records the number of crests that pass by in a second. This frequency is measured in cycles per second, or hertz. - The electromagnetic energy from the Sun travels in eight minutes across the intervening 150 million km of space to the Earth. • The Sun produces a continuous spectrum of electromagnetic radiation ranging from very short, extremely high frequency gamma and cosmic waves to long, very low frequency radio waves. • The Earth approximates a 300 K (27 ̊C) blackbody and has a dominant wavelength at approximately 9.7 μm. Stefan-Boltzmann Law Using the wave model, it is possible to characterize the energy of the Sun which represents the initial source of most of the electromagnetic energy recorded by remote sensing systems (except radar). We may think of the Sun as a 6 000 K blackbody (a theoretical construct which radiates energy at the maximum possible rate per unit area at each wavelength for any given temperature). The total emitted (integrated) radiation (M in W m-2 ) from a blackbody is proportional to the fourth power of its absolute temperature. Sources of Electromagnetic Energy Thermonuclear fusion taking place on the surface of the Sun yields a continuous spectrum of electromagnetic energy. The 5770 – 6000 kelvin (K) temperature of this process produces a large amount of relatively short wavelength energy that travels through the vacuum of space at the speed of light. Some of this energy is intercepted by the Earth, where it interacts with the atmosphere and surface materials. The Earth reflects some of the energy directly back out to space or it may absorb the short wavelength energy and then re-emit it at a longer wavelength. Electromagnetic Spectrum The Sun produces a continuous spectrum of energy from gamma rays to radio waves that continually bathe the Earth in energy. The visible portion of the spectrum may be measured using wavelength (measured in micrometers or nanometers, i.e., μm or nm) or electron volts (eV). All units are interchangeable. we can determine the dominant wavelength or peak spectral exitance of a blackbody based on Wein's displacement law.we can determine the spectral radiant exitance of a blackbody using Planck␣s equation: Blackbody Radiation Curves Blackbody radiation curves for several objects including the Sun and the Earth which approximate 6000 K and 300 K blackbodies, respectively. The area under each curve may be summed to compute the total radiant energy (M) exiting each object. Thus, the Sun produces more radiant exitance than the Earth because its temperature is greater. As the temperature of an object increases, its dominant wavelength (!max ) shifts toward the shorter wavelengths of the spectrum. Radiant Intensity of the Sun The Sun approximates a 6000 K blackbody with a dominant wavelength of 0.48 μm (green light). Earth approximates a 300 K blackbody with a dominant wavelength of 9.66 μm. The 6000 K Sun produces 41% of its energy in the visible region from 0.4 - 0.7 μm (blue, green, and red light). The other 59% of the energy is in wavelengths shorter than blue light (<0.4 μm) and longer than red light (>0.7 μm). Eyes are only sensitive to light from the 0.4 to 0.7 μm. Atmospheric Energy - Matter Interactions-Refraction The index of refraction (n) is a measure of the optical density of a substance. The speed of light in a substance can never reach the speed of light in a vacuum. Therefore, its index of refraction must always be greater than 1. Snell␣s Law Refraction can be described by Snell␣s law, which states that for a given frequency of light (we must use frequency since, unlike wavelength, it does not change when the speed of light changes), the product of the index of refraction and the sine of the angle between the ray and a line normal to the interface is constant. From the next figure, we can see that a nonturbulent atmosphere can be thought of as a series of layers of gases, each with a slightly different density. Anytime energy is propagated through the atmosphere for any appreciable distance at any angle other than vertical, refraction occurs. Atmospheric Refraction Refraction in three nonturbulent atmospheric layers. The incident energy is bent from its normal trajectory as it travels from one atmospheric layer to another. Snell␣s law can be used to predict how much bending will take place, based on a knowledge of the angle of incidence (#) and the index of refraction of each atmospheric level, n1, n2, n3. Scattering Once electromagnetic radiation is generated, it is propagated through the earth's atmosphere almost at the speed of light in a vacuum. Unlike a vacuum in which nothing happens, however, the atmosphere may affect not only the speed of radiation but also its wavelength, intensity, spectral distribution, and/or direction. Scatter differs from reflection in that the direction associated with scattering is unpredictable, whereas the direction of reflection is predictable. There are essentially three types of scattering: • Rayleigh, • Mie, and • Non-selective. Atmospheric Layers and Constituents Major subdivisions of the atmosphere and the types of molecules and aerosols found in each layer. Atmospheric Scattering. Type of scattering is a function of:• the wavelength of the incident radiant energy, and• the size of the gas molecule, dust particle, and/or water vapor droplet encountered. Rayleigh Scattering Rayleigh scattering occurs when the diameter of the matter (usually air molecules) are many times smaller than the wavelength of the incident electromagnetic radiation. All scattering is accomplished through absorption and re-emission of radiation by atoms or molecules. It is impossible to predict the direction in which a specific atom or molecule will emit a photon, hence scattering. The amount of scattering is inversely related to the fourth power of the radiation's wavelength. For example, blue light (0.4 μm) is scattered 16 times more than near-infrared light (0.8 μm). Earth from Space Using Remote Sensing The intensity of Rayleigh scattering varies inversely with the fourth power of the wavelength. Mie Scattering • Mie scattering takes place when there are essentially spherical particles present in the atmosphere with diameters approximately equal to the wavelength of radiation being considered. For visible light, water vapor, dust, and other particles ranging from a few tenths of a micrometer to several micrometers in diameter are the main scattering agents. The amount of scatter is greater than Rayleigh scatter and the wavelengths scattered are longer. • Pollution also contributes to beautiful sunsets and sunrises. The greater the amount of smoke and dust particles in the atmospheric column, the more violet and blue light will be scattered away and only the longer orange and red wavelength light will reach our eyes. Non-selective Scattering • Non-selective scattering is produced when there are particles in the atmosphere several times the diameter of the radiation being transmitted. This type of scattering is non-selective, i.e. all wavelengths of light are scattered, not just blue, green, or red. Thus, water droplets, which make up clouds and fog banks, scatter all wavelengths of visible light equally well, causing the cloud to appear white (a mixture of all colors of light in approximately equal quantities produces white). • Scattering can severely reduce the information content of remotely sensed data to the point that the imagery looses contrast and it is difficult to differentiate one object from another. Absorption • Absorption is the process by which radiant energy is absorbed and converted into other forms of energy. An absorption band is a range of wavelengths (or frequencies) in the electromagnetic spectrum within which radiant energy is absorbed by substances such as water (H2O), carbon dioxide (CO2), oxygen (O2), ozone (O3), and nitrous oxide (N2O). • The cumulative effect of the absorption by the various constituents can cause the atmosphere to close down in certain regions of the spectrum. This is bad for remote sensing because no energy is available to be sensed. Absorption and Transmission • In certain parts of the spectrum such as the visible region (0.4 - 0.7 μm), the atmosphere does not absorb all of the incident energy but transmits it effectively. Parts of the spectrum that transmit energy effectively are called ␣atmospheric windows␣. • Absorption occurs when energy of the same frequency as the resonant frequency of an atom or molecule is absorbed, producing an excited state. If, instead of re-radiating a photon of the same wavelength, the energy is transformed into heat motion and is reradiated at a longer wavelength, absorption occurs. When dealing with a medium like air, absorption and scattering are frequently combined into an extinction coefficient. • Transmission is inversely related to the extinction coefficient times the thickness of the layer. Certain wavelengths of radiation are affected far more by absorption than by scattering. This is particularly true of infrared and wavelengths shorter than visible light. The combined effects of atmospheric absorption, scattering, and reflectance reduce the amount of solar irradiance reaching the Earth␣s surface at sea level. Reflectance: • Reflectance is the process whereby radiation ␣bounces off␣␣ an object like a cloud or the terrain. Actually, the process is more complicated, involving re-radiation of photons in unison by atoms or molecules in a layer one-half wavelength deep. There are various types of reflecting surfaces: • When specular reflection occurs, the surface from which the radiation is reflected is essentially smooth (i.e. the average surface profile is several times smaller than the wavelength of radiation striking the surface). • If the surface is rough, the reflected rays go in many directions, depending on the orientation of the smaller reflecting surfaces. This diffuse reflection does not yield a mirror image, but instead produces diffused radiation. White paper, white powders and other materials reflect visible light in this diffuse manner. • If the surface is so rough that there are no individual reflecting surfaces, then scattering may occur. Lambert defined a perfectly diffuse surface; hence the commonly designated Lambertian surface is one for which the radiant flux leaving the surface is constant for any angle of reflectance to the surface normal. Terrain Energy-Matter Interactions Radiometric quantities have been identified that allow analysts to keep a careful record of the incident and exiting radiant flux. The time rate of flow of energy onto, off of, or through a surface is called radiant flux, is of critical importance in remote sensing. In fact, this is the fundamental focus of much remote sensing research. By carefully monitoring the exact nature of the incoming (incident) radiant flux in selective wavelengths and how it interacts with the terrain, it is possible to learn important information about the terrain. The Hemispherical reflectance: is defined as the dimensionless ratio of the radiant flux reflected from a surface to the radiant flux incident to it Hemispherical transmittance is defined as the dimensionless ratio of the radiant flux transmitted through a surface to the radiant flux incident to it Hemispherical absorptance is defined by the dimensionless relationship.These radiometric quantities are useful for producing general statements about the spectral reflectance, absorptance, and transmittance characteristics of terrain features. Irradiance and Exitance: The amount of radiant flux incident upon a surface per unit area of that surface is called Irradiance, where The amount of radiant flux leaving per unit area of the plane surface is called Exitance. Radiance is the radiant flux per unit solid angle leaving an extended source in a given direction per unit projected source area in that direction and is measured in watts per meter squared per steradian. Spectral Signatures: For any given material, the amount of solar radiation that is reflected, absorbed or transmitted varies with wavelength. This important property of matter makes it possible to identify different substances or classes and separate them by their spectral signatures (spectral curves), as shown in the figure below. Spectral Reflectance Measurement using a Spectroradiometer. Reference Lambertian surface (Calibration target). GPS. Total radiance, (Lt) recorded by a remote sensing system over water is a function of the electromagnetic energy received from: Lp = atmospheric path radiance, Ls = free-surface layer reflectance, Lv = subsurface volumetric, reflectance Lb = bottom reflectance Absorption and Scattering Attenuation in Pure Water Molecular water absorption dominates in the ultraviolet (<400 nm) and in the yellow through the near-infrared portion of the spectrum (>580 nm). Almost all of the incident near-infrared and middle- infrared (740 - 2500 nm) radiant flux entering a pure water body is absorbed with negligible scattering taking place. Scattering in the water column is important in the violet, dark blue, and light blue portions of the spectrum (400 - 500 nm). This is the reason water appears blue to our eyes. The graph truncates the absorption data in the ultraviolet and in the yellow through near-infrared regions because the attenuation is so great. Monitoring the Surface Extent of Water Bodies • The best wavelength region for discriminating land from pure water is in the near-infrared and middle-infrared from 740 - 2500 nm (0.74 - 2.5 μm) • In the near- and middle-infrared regions, water bodies appear very dark, even black, because they absorb almost all of the incident radiant flux, especially when the water is deep and pure and contains little suspended sediment or organic matter. Spectral Response of Water as a Function of Organic and Inorganic Constituents Monitoring Suspended Minerals (Turbidity), Chlorophyll, and Dissolved Organic Matter: For water-quality studies we are usually most interested in measuring the subsurface volumetric radiance, Lv exiting the water column toward the sensor. The characteristics of this radiant energy are a function of: concentration of pure water (w), inorganic suspended minerals (SM), organic chlorophyll a (Chl), dissolved organic material (DOM), and the total amount of absorption and scattering attenuation that takes place in the water column due to each of these constituents Spectral Response of Water as a Function of Inorganic and Organic Constituents: • Minerals such as silicon, aluminum, and iron oxides are found in suspension in most natural water bodies. The particles range from fine clay particles ( 3 - 4 μm in diameter), to silt (5 - 40 μm), to fine-grain sand (41 - 130 μm), and coarse grain sand (131 - 1250 μm). • Sediment comes from a variety of sources including agriculture erosion, weathering of mountainous terrain, shore erosion caused by waves or boat traffic, and volcanic eruptions (ash). • Most suspended mineral sediment is concentrated in the inland and nearshore water bodies. Clear, deep ocean far from shore rarely contains suspended minerals greater than 1 μm in diameter. Spectral Response of Water as a Function of Organic Constituents - Plankton • Plankton is the generic term used to describe all the living organisms (plant and animal) present in a waterbody that cannot resist the current (unlike fish). Plankton may be subdivided further into algal plant organisms (phytoplankton), animal organisms (zooplankton), bacteria (bacterio-plankton), and lower plant forms such as algal fungi. • Phytoplankton are small single-celled plants smaller than the size of a pinhead. Phytoplankton, like plants on land, are composed of substances that contain carbon. Phytoplankton sink to the ocean or water-body floor when they die. All phytoplankton in water bodies contain the photosynthetically active pigment chlorophyll a. Chlorophyll in Ocean Water: A remote estimate of near-surface chlorophyll concentration generally constitutes an estimate of near-surface biomass (or primary productivity) for deep ocean water where there is little danger of suspended mineral sediment contamination.Numerous studies have documented a relationship between selected spectral bands and ocean chlorophyll (Chl) concentration Spectral Response of Water as a Function of Dissolved Organic Constituents: • Sunlight penetrates into the water column a certain photic depth (the vertical distance from the water surface to the 1 percent subsurface irradiance level). • Phytoplankton within the photic depth of the water column consume nutrients and convert them into organic matter via photosynthesis. This is called primary production. • Zooplankton eat the phytoplankton and create organic matter. Bacterioplankton decompose this organic matter. All this conversion introduces dissolved organic matter (DOM) into oceanic, nearshore, and inland water bodies. • In certain instances, there may be sufficient dissolved organic matter in the water to reduce the penetration of light in the water column. • The decomposition of phytoplankton cells yields carbon dioxide, inorganic nitrogen, sulfur, and phosphorous compounds. The more productive the phytoplankton, the greater the release of dissolved organic matter. In addition, humic substances may be produced. These often have a yellow appearance and represent an important colorant agent in the water column, which may need to be taken into consideration. These dissolved humic substances are called yellow substance and can 1) impact the absorption and scattering of light in the water column, and 2) change the color of the water. • There are sources of DOM other than phytoplankton. For example, the brownish-yellow color of the water in many rivers in the northern US/Canada is due to the high concentrations of tannin from various tree species and plants grown in bogs. These tannins can create problems when remote sensing inland water bodies. Spectral Signatures of Water: Solid State: The spectral signature of snow is influenced by the size and shape of crystals at/near the surface, the water content near the surface, the presence of impurities (e.g. soot), the depth (thin layer), and roughness of the snow cover. Spectral Signatures of Vegetation: Photosynthesis is the process by which green plants use the energy of light and a special substance called chlorophyll to synthesize carbohydrates (energy) from carbon dioxide and water. Cross-section Through A Hypothetical and Real Leaf Revealing the Major Structural Components that Determine the Spectral Reflectance of Vegetation Spectral Signatures of Soils: Spectral Reflectance Characteristics of Soils Are a Function of Several Important Characteristics • soil texture (percentage of sand, silt, and clay), • soil moisture content (e.g. dry, moist, saturated), • organic matter content, • iron-oxide content, and • surface roughness. Reflectance from Dry versus Wet Soils: Radiant energy may be reflected from the surface of the dry soil, or it penetrates into the soil particles, where it may be absorbed or scattered. Total reflectance from the dry soil is a function of specular reflectance and the internal volume reflectance. As soil moisture increases, each soil particle may be encapsulated with a thin membrane of capillary water. The interstitial spaces may also fill with water. The greater the amount of water in the soil, the greater the absorption of incident energy and the lower the soil reflectance. Organic Matter in a Sandy Soil: Generally, the greater the amount of organic content in a soil, the greater the absorption of incident energy and the lower the spectral reflectance. Spectral Signatures of Rocks/ Minerals: • Rocks are assemblages of minerals that have interlocking grains or are bound together by various types of cement (usually silica or calcium carbonate). • When there is minimal vegetation and soil present and the rock material is visible directly by the remote sensing system, it maybe possible to differentiate between several rock types and obtain information about their characteristics using remote sensing techniques. • Most rock surfaces consist of several types of minerals. Earth Observing System - Terra Instruments (5) ASTER - Advanced Spaceborne Thermal Emission and Reflection Radiometer, CERES - Clouds and the Earth’s Radiant Energy System, MISR - Multi-angle Imaging Spectroradiometer, MODIS - Moderate-resolution Imaging Spectroradiometer, MOPITT - Measurement of Pollution in the Troposphere
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