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Remote Sensing

REMOTE SENSING

CONCEPT

Scientists of many disciplines are accustomed to studying data that cannot be observed through direct contact. Physicists and chemists, for instance, know a great deal about the structure of the atom, even though even the most high-powered microscope cannot make an atom visible to the human eye. The objects of study for earth scientists are often similarly remote, though not necessarily because they are small. In some cases, the problem is quite the opposite: an area selected for study is too large to provide understanding to geologists working only on the ground. Other areas are simply inaccessible to human beings or even their equipment. This has necessitated the development of remote sensing equipment and techniques, primarily involving views from the air or from space and utilizing electromagnetic radiation across a wide spectrum.

HOW IT WORKS

An Introduction to Remote Sensing

The work of geologists would be much easier if Earth were transparent and they could simply look down into the ground as they would into the sky. But the ground is not transparent; nor, for that matter, is the sky, to which meteorologists look for information regarding atmospheric and weather patterns. Some places are hard to see, and many are difficult or even impossible to visit physically. Some places, such as the Sun or the Earth's core, could not be approached physically even by unmanned technology.

Hence the need for remote sensing, or the gathering of data without actual contact with the materials or objects being studied. Some earth scientists define the term more narrowly, restricting "remote sensing" to the use of techniques involving radiation on the electromagnetic spectrum. The latter category includes visible, infrared, and ultraviolet light as well as lower-frequency signals in the microwave range of the spectrum. This definition excludes the study of force fields involving gravitational or electromagnetic force. In general, in this essay we abide by that more narrow definition, primarily because most forms of remote sensing in use today involve electromagnetic radiation.

Remote sensing is used for a variety of measuring and mapping applications. The reader therefore is encouraged to consult the essay Measuring and Mapping Earth for more on this subject. Applications of remote sensing go far beyond cartography (mapmaking) and measurement, however. As suggested already, remote sensing makes it possible for earth scientists to collect data from places they could not possibly go. In addition, it allows for data collection in places where a human being would be "unable to see the forest for the trees"which in places such as the Amazon valley is quite literally the case.

The Military Influence

Scientists' understanding of the electromagnetic spectrum was still in its infancy in 1849, when the French army engineer Aimé Laussedat (1819-1907) introduced what was then called iconometry, from the Greek words icon ("image") and -metry ("measurement"). Laussedat, who experimented with aerial photography by means of cameras mounted on balloons or kites, is regarded as a pioneer of photogrammetry, the use of aerial or satellite photography to provide measurements of or between objects on the ground.

A few years later, the United States armies of the Civil War adopted the use of aerial photography for surveillance purposes, mounting cameras on balloons to provide intelligence regarding federal or Confederate positions and troop strength. This fact, combined with Laussedat's status as an army engineer, hints at one of the underlying themes in the history of remote sensing, and indeed of many another technological advance: the influence of the military. It is a fact of human existence that nations from at least the time of the Assyrians, if not the Egyptians of the New Kingdom, have devoted far more attention and resources to military applications than they have to peacetime activities.

On the other hand, societies have benefited enormously from technological and organizational innovations with military origins, innovations whose application later spread to a variety of peacetime uses. Some examples include the adoption of the chariot by the Egyptian army after the Hyksos invasion (ca. 1670 b.c.); the Assyrian introduction of logistics in an effort to supply imperial troops (ca. 800 b.c.); the Persian development of the postal service (ca. 600 b.c.); numerous Roman innovations, particularly in road building (ca. 200 b.c.-ca. a.d. 200); and the Chinese invention of the wheelbarrow (ca. 100 b.c.). And so the list goes, right up to such latter-day American developments as the Internet and GPS, or global positioning system.

MILITARY CONTRIBUTIONS TO REMOTE SENSING.

Forms of technology pioneered by military forces and now used in remote sensing include infrared photography, thermal imagery, radar scanning, and satellites. The first of these types of technology makes use of light in the infrared portion of the electromagnetic spectruma region that, as its name suggests, is adjacent to the red portion of visible light. Red has the longest wavelength and the lowest frequency of all colors, and infrared has an even longer wavelength and lower frequency. Military forces use infrared photography to distinguish between vegetation and camouflage designed to look like vegetation: live plants reflect infrared radiation, whereas dead ones and camouflaged material absorb it.

Whereas infrared photography measures reflection of infrared radiation, thermal imaging indicates the amount of such radiation that is emitted by the source. Its military origins lie in its use for reconnaissance during night bombing missions. Similarly, radar scanning makes it possible to view targets on the ground, regardless of lighting or cloud cover. Finally, there are satellites, which have extensive surveillance applications. Among the most important examples of military activity above Earth's atmosphere are the 24 satellites of GPS, which make allow U.S. forces to plot positions with amazing accuracy. Less accurate GPS intelligence is also available to civilians. (See Measuring and Mapping Earth for more on GPS.)

REAL-LIFE APPLICATIONS

Photogeology

All of these innovations introduced by the military, of course, have found application for civilian purposes. Thanks in part to improvements in aircraft during World War II, for instance, photogeologic data gathering has increased dramatically in the years since then. Efforts at gaining information by means of airborne sensing devices underwent enormous improvements throughout the middle and latter part of the twentieth century, with the development of technology that made it possible for earth scientists to gather information using techniques beyond ordinary photography, visible light, and airplanes.

Still, much of the remote-sensing activity that takes place today is performed aboard airplanes rather than satellites, using ordinary analogue photography within the visible spectrum. Stereoscopic techniques aid in the visualization of relief, or elevation and other in equalities on a land surface. Humans are used to seeing stereoscopically: the distance between the two eyes on our faces results in a difference between the two images each eye sees. The brain corrects for this difference, rendering a stereoscopic image that is more full and dimensional than anything a single eye could produce. The use of multiple cameras and stereoscopic technology replicates this activity of the human brain and thus provides earth scientists with much more information than they could gain simply by looking at "flat" photographs taken from an airplane.

The materials studied by a geologist, of course, are primarily underground, but Earth's surface furnishes many clues that a trained observer can interpret. Uplands and lowlands tend to suggest different types of rocks, while the direction of a dip in the land can supply volumes of information regarding the stratigraphic characteristics of the region. The presence of vegetation can make it harder to discern such clues, but a careful study of plant life can reveal much regarding minerals in the soil, local water resources, and so on.

Digital Photography

Within both photogeology and the larger realm of remote sensing, several innovations from the 1960s onward have underpinned more effective methods of observation. One of these is digital photography, which is as much of an improvement over old-fashioned photography as compact discs are over phonograph records. In both cases, the contrast is between analog technology and digital technology. In analog photography, for instance, the image is recorded by a camera and stored on photosensitive materials in a film emulsion. In digital photography the image is recorded on a solid-state device called an image sensor and stored in the camera's memory for transfer to a computer.

An analogue (the preferred spelling for the word as a noun) is just that, a "close copy," whereas digital methods make possible a more exact reproduction of images by assigning to each shade of color a number between 0 and 255. Instead of storing the image in a medium that can be destroyed or lost easily, as is the case with ordinary film, digital images can be saved on a computer, backed up, and sent anywhere in the world via the Internet. Furthermore, these images can be adjusted with the use of a computer, so as to make it easier to see certain features.

Computers and digital photography aid in the creation of false-color imaging, a means of representing invisible electromagnetic data by assigning specific colors to certain wavelengths. An example would be the use of red to depict areas of high energy. This is certainly a false use of color, since red actually has the lowest energy in the visible spectrum, with purple possessing the highest energy. (The reason we associate red, orange, and yellow with heat and green, blue, and purple with coldness is that in either case, these are the colors objects reflect, not the ones they absorb.)

RADAR.

Most remote-sensing technology uses light, whether infrared or visible, that falls at the middle to high end of the electromagnetic spectrum. By contrast, at least one important means of remote detection uses microwaves, which are much lower in energy levels. Microwaves carry FM radio and television signals, as well as radar, or RA dio D etection A nd R anging.

Radar makes it possible for pilots to "see" through clouds, rain, fog, and all manner of natural phenomenanot least of which is darkness. It also can identify objects, both natural and man-made, on the ground. In addition to its application in remote sensing, radar using the Doppler effect (the change in the observed frequency of a wave when the source of the wave is moving with respect to the observer) helps meteorologists track storms.

In the simplest model of radar operation, a sensing unit sends out microwaves toward the target, and the waves bounce back off the target to the unit. In a monostatic unitone in which the transmitter and receiver are in the same locationthe radar unit has to be switched continually between sending and receiving modes. Clearly, a bistatic unitone in which the transmitter and receiver antennas are at locations remote from one anotheris generally preferable, but on an airplane, for instance, there is no choice but to use a monostatic unit.

Satellite Data

The term satellite refers to any object orbiting a larger one; thus, Earth's Moon and all the other moons of the solar system are satellites, as are the many artificial satellites that orbit Earth. In practice, however, most people use the term to refer only to artificial satellites, of which there are many hundreds, launched by entities ranging from national governments to international associations to independent firms. Artificial satellites typically are intended for the purposes of gathering information (i.e., scientific research or military surveillance) or disseminating it (i.e., through satellite television broadcasting).

In launching a satellite, it is necessary to overcome the enormous pull of Earth's gravitational field. This is done by providing the satellite with power through rocket boosters that launch it far above Earth's atmosphere. At a height of 200 mi. (320 km) or more, the satellite is far above the dense gases of the atmosphere yet well within the gravitational field of the planet. The craft is then in a position to orbit Earth indefinitely without the need for additional power from man-made sources; instead, Earth's own gravitational energy keeps the satellite in orbit for as long as the satellite's structure remains intact. (See Gravity and Geodesy for more about the mechanics of orbit.)

The greater the altitude, the longer it takes a satellite to complete a single revolution. One of the most commonly used altitudes is at 22,500 mi. (36,000 km), at which height a satellite takes 24 hours to orbit Earth. Thus, it is said to be in geosynchronous orbit, meaning that it revolves at the same speed as the planet itself and therefore remains effectively stationary over a given area. Some satellites revolve at even higher altitudes25,000 mi. (40,225 km), which, while it is far beyond the atmosphere, is well within Earth's gravitational field.

LANDSAT.

One of the most impressive undertakings in the field of satellite research is Landsat, an Earth-monitoring satellite designed specifically for the use of earth scientists and resource managers. Conceived by the United States Department of the Interior in the mid-1960s, the Landsat project soon came to involve the National Aeronautics and Space Administration (NASA) and the U.S. Geological Survey (USGS; see Measuring and Mapping Earth for more about geologic surveys.) Landsat 1 went into orbit on July 23, 1972.

Over the years, Landsat has gone into six subsequent generations. Landsat 6, launched in 1993, was unable to achieve orbit, but Landsat 1 lasted more than five times as long as its projected life expectancy of one year. Since 1972 at least one Landsat satellite has been in orbit over Earth, and as of early 2001 both Landsat 5 (launched in March 1984) and Landsat 7 (launched in April 1999) were on line. (Landsat 5 was decommissioned in June 2001.) Over the course of the years, the Landsat governing body has changed. In the 1980s, NOAA (National Oceanic and Atmospheric Administration) took over from NASA, and in October 1985 the Landsat system came under the direction of a commercial organization, the Earth Observation Satellite Company (EOSat).

In contrast to communication satellites, which tend to maintain geosynchronous orbits, Landsat moves at a much lower altitude and therefore orbits Earth much more quickly. Landsat 7 takes approximately 99 minutes to orbit the planet, thus making 14 circuits in a 24-hour period. Though it never quite passes over the poles, it covers the rest of Earth in swaths 115 mi. (185 km) wide, meaning that eventually it passes over virtually all other spots on the planet.

SATELLITES AT WORK.

Landsat and other satellites, such as France's SPOT (Satellite Positioning and Tracking), provide data for governments, businesses, scientific institutions, and even the general public. Following the September 11, 2001, terrorist bombing of the World Trade Center in New York City, for instance, the SPOT U.S. Web site (<http://www.spot.com>) provided viewers with "Images of Infamy": views of downtown Manhattan before and just a few hours after the bombing.

Data from Landsat has been used to study disasters and potential disasters with particular application to the earth sciences. An example is the area of the tropical rainforest in Brazil's Amazon River valley, a region of about 1.9 million sq. mi. (five million sq km), in which deforestation is claiming between 4,250 sq. mi. and 10,000 sq. mi. (11,000-26,000 sq km) a year. This is an extremely serious issue, because the Amazon basin represents approximately one-third of the total rain-forest area on Earth. Earlier estimates, however, had suggested that deforestation was claiming up to three times as much as it actually is, and Landsat provided a more accurate figure.

Because of its acute spatial resolution (98 ft., or 30 m, compared with more than 0.6 mi., or 1 km), Landsat is much more effective for this purpose than other satellite systems operated by NOAA or other organizations. It is also cheaper to obtain images from it than from SPOT. Over the years, Landsat has provided data on urban sprawl in areas as widely separated as Las Vegas, Nevada, and Santiago, Chile. It has offered glimpses of disasters ranging from the eruption of Mount Saint Helens, Washington, in 1980 to some of the most potent recent examples of destruction caused by humans, including the nuclear disaster at Chernobyl, Ukraine, in 1986 and the fires and other effects of the Persian Gulf War of 1990-1991. (For more on this subject, see the Earthshots Web site, operated by USGS.)

Not all the news from Landsat is bad, as a visit to the Landsat 7 Web site (<http://landsat.gsfc.nasa.gov/>) in late 2001 revealed. Certainly there were areas of concern, among them, flooding in Mozambique and runaway development in Denver, Colorado. But images taken over the Aldabra atoll in the Seychelles showed the world's largest refuge for giant tortoises. And shots taken from Landsat over Lake Nasser in southern Egypt during the latter part of 2000 showed four lakes created by excess water from Nasser. As a result, that region of the Sahara had new lakes for the first time in 6,000 years.

WHERE TO LEARN MORE

Burtch, Robert. A Short History of Photogrammetry (Web site). <http://users.netonecom.net/~rburtch/sure340/history.html>.

"Earthshots: Satellite Images of Environmental Change," U.S. Geological Survey (Web site). <http://edcwww.cr.usgs.gov/earthshots/slow/tableofcontent>.

Hancock, Paul L., and Brian J. Skinner. The Oxford Companion to the Earth. New York: Oxford University Press, 2000.

NASA EOS IDS Volcanology (Web site). <http://eos.pgd.hawaii.edu/>.

Remote Sensing Data and Information (Web site). <http://rsd.gsfc.nasa.gov/rsd/RemoteSensing.html>.

Skinner, Brian J., Stephen C. Porter, and Daniel B. Botkin. The Blue Planet: An Introduction to Earth System Science, 2d ed. New York: John Wiley and Sons, 1999.

Smith, David G. The Cambridge Encyclopedia of Earth Sciences. New York: Cambridge University Press, 1981.

Strain, Priscilla, and Frederick Engle. Looking at Earth. Atlanta: Turner Publishing, 1992.

Visualization of Remote Sensing Data (Web site). <http://rsd.gsfc.nasa.gov/rsd/>.

The WWW Virtual Library: Remote Sensing (Web site). <http://www.vtt.fi/aut/rs/virtual/>.

KEY TERMS

CARTOGRAPHY:

The creation, production, and study of maps. Cartography is a subdiscipline of geography and involves not only science but also mathematics, technology, and even art.

DOPPLER EFFECT:

The change in the observed frequency of a wave when the source of the wave is moving with respect to the observer.

ELECTROMAGNETIC RADIATION:

See Electromagnetic spectrum and Radiation.

ELECTROMAGNETIC SPECTRUM:

The complete range of electromagnetic waves on a continuous distribution from a very low range of frequencies and energy levels, with a correspondingly long wavelength, to a very high range of frequencies and energy levels, with a correspondingly short wavelength. Included on the electromagnetic spectrum are long-wave and short-wave radio; microwaves; infrared, visible, and ultraviolet light; x rays; and gamma rays.

FALSE-COLOR IMAGING:

A means of representing invisible electromagnetic data by assigning specific colors to certainwavelengths.

FREQUENCY:

The number of waves, measured in Hertz, passing through a given point during the interval of one second. The higher the frequency, the shorter the wavelength.

GEODESY:

An area of geophysics devoted to the measurement of Earth's shape and gravitational field.

HERTZ:

A unit for measuring frequency equal to one cycle per second. High frequencies are expressed in terms of kilohertz (kHz; 103, or 1,000 cycles per second), megahertz (MHz; 106, or one million cycles per second), and gigahertz (GHz;109, or one billion cycles per second).

PHOTOGEOLOGY:

The use of aerial photographic data to make determinations regarding the geologic characteristics of an area.

PHOTOGRAMMETRY:

The use of aerial or satellite photography to provide measurements of or between objects on the ground.

RADIATION:

The transfer of energy by means of electromagnetic waves, which require no physical medium (for example, water or air) for the transfer. Earth receives the Sun's energy, via the electromagnetic spectrum by means of radiation.

RELIEF:

Elevation and other in equalities on a land surface.

REMOTE SENSING:

The gathering of data without actual contact with the materials or objects being studied.

STRATIGRAPHY:

The study of rock layers, or strata, beneath Earth's surface.

WAVELENGTH:

The distance between a crest and the adjacent crest or a trough and the adjacent trough of a wave. Wavelength is inversely related to frequency, meaning that the shorter the wavelength, the higher the frequency.

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Remote Sensing

Remote sensing

Remote sensing is the science and art of obtaining and interpreting information about an object, area , or phenomenon through the analysis of data acquired by a sensor that is not in contact with the object, area, or phenomenon being observed. There are four major characteristics of a remote sensing system, namely, an electromagnetic energy source, transmission path, target, and sensor.

The Sun is a common source of electromagnetic energy. It radiates solar energy in all directions. Earth reflects the energy from the Sun and emits some energy in the form of heat.

Based on the energy source, remote sensing systems can be grouped into two types, passive and active systems. Passive remote sensing systems detect radiation that is reflected and/or emitted from the surface features of Earth. Examples are the Landsat and European SPOT satellite systems. Active remote sensing systems provide their own energy source. For example, the Radarsat-1 synthetic aperture radar (SAR) system has an antenna that beams pulses of electromagnetic energy towards the target.

The transmission path is the space between the electro-magnetic energy source and the target, and back to the sensor. In the case of Earth observation, the transmission path is usually the atmosphere of Earth. While passing through Earth's atmosphere, the electromagnetic energy can be scattered by minute particles or absorbed by gases such that its strength and spectral characteristics are modified before being detected by the sensor.

The target could be a particular object, an area, or phenomenon. For example, it could be a ship, city, forest cover, mineralized zone, and water body contaminated by oil slick, a forest fire, or a combination thereof.

Electromagnetic energy that hits a target, called incident radiation, interacts with matter or the target in several ways. The energy could be reflected, absorbed, or transmitted. When incident radiation hits a smooth surface, it is reflected or bounced in the opposite direction like light bouncing off a mirror. If it hits a relatively rough surface, it could be scattered in all directions in a diffuse manner. When incident radiation is absorbed, it loses its energy largely to heating the matter. A portion of the energy may be emitted by the heated substance, usually at longer wavelengths. When incident radiation is transmitted, it passes through the substance such as from air into water.

The sensor is a device that detects reflected and/or emitted energy. Passive remote sensing systems carry optical sensors that detect energy in the visible, infrared, and thermal infrared regions of the electromagnetic spectrum . Common sensors used are cameras and charge-coupled detectors (CCD ) mounted on either airborne or space-borne platforms. In active remote sensing systems, the same antenna that sends out energy pulses detects the return pulse.

Present applications of remote sensing are numerous and varied. They include land cover mapping and analysis, land use mapping, agricultural plant health monitoring and harvest forecast, water resources, wildlife ecology, archeological investigations, snow and ice monitoring, disaster management, geologic and soil mapping, mineral exploration, coastal resource management, military surveillance, and many more.

One main advantage of a remote sensing system is its ability to provide a synoptic view of a wide area in a single frame. The width of a single frame, or swath width, could be 37 mi × 37 mi (60 km × 60 km) in the case of the European SPOT satellite, or as wide as 115 mi × 115 mi (185 km × 185 km) in the case of Landsat. Remote sensing systems can provide data and information in areas where access is difficult as rendered by terrain, weather , or military security. The towering Himalayas and the bitterly cold Antarctic regions provide good examples of these harsh environments. Active remote sensing systems provide cloud-free images that are available in all weather conditions, day or night. Such systems are particularly useful in tropical countries where constant cloud cover may obscure the target area. In 2002, the United States military initiatives in Afghanistan used remote sensing systems to monitor troops and vehicle convoy movements at spatial resolutions of less than one meter to a few meters. Spatial resolution or ground resolution is a measure of how small an object on Earth's surface can be "seen" by a sensor as separate from its surroundings.

The greater advantage of remote sensing systems is the capability of integrating multiple, interrelated data sources and analysis procedures. This could be a multistage sensing wherein data on a particular site is collected from the multiple sources at different altitudes like from a low altitude aircraft, a high altitude craft, a space shuttle and a satellite. It could also be a multispectral sensing wherein data on the same site are acquired in different spectral bands. Landsat-5, for example, acquires data simultaneously in seven wavelength ranges of the electromagnetic spectrum. Or, it could be a multitemporal sensing whereby data are collected on the same site at different dates. For example, data may be collected on rice-growing land at various stages of the crop's growth, or on a volcano before and after a volcanic eruption.

Two satellite systems in use today are the Landsat and Radarsat remote sensing systems. Landsat is the series of Earth observation satellites launched by the U.S. National Aeronautics and Space Administration (NASA) under the Landsat Program in 1972 to the present. The first satellite, originally named Earth Resources Technology Satellite-1 (ERTS-1), was launched on July 22, 1972. In 1975, NASA renamed the "ERTS" Program the "Landsat" Program and the name ERTS-1 was changed to Landsat-1. All following satellites carried the appellation of Landsat. As of 2002, there are seven Landsat satellites launched. The latest, Landsat-7 was launched on July 15, 1999.

Landsat-7 carries the Enhanced Thematic Mapper Plus (ETM+) sensor. The primary features of Landsat-7 include a panchromatic band with 49 ft (15 m) spatial resolution and a thermal infrared channel (Band 6) with 197 ft (60 m) spatial resolution. Like its predecessors the Landsat-4 and -5, Landsat-7 ETM+ includes the spectral bands 1,2,3,4,5,6 and 7. The spatial resolution remains at 98 ft (30 m), except for band 6 in which the resolution is increased from 394 ft (120 m) to 197 ft (60 m). Landsat-7 orbits Earth at an altitude of 438 mi (705 km). It has a repeat cycle of 16 days, meaning it returns to the same location every 16 days.

Radarsat is the series of space-borne SAR systems developed by Canada. Radarsat-1, launched on November 4, 1995 by NASA, carries a C-band 2.2 in (5.6 cm wavelength) antenna that looks to the right side of the platform. The antenna transmits at 5.3 GHz with an HH polarization (Horizontally transmitted, Horizontally received). It can be steered from 10 to 59 degrees. The swath width can be varied to cover an area from 31 mi (50 km) in fine mode to 311 mi (500 km) in Scan SAR Wide mode. Radarsat-1 orbits Earth at an altitude of 496 mi (798 km) and has a repeat cycle of 24 days.

Several space-borne remote sensing systems planned for launch in the near future include the Radarsat-2 and the Advanced Land Observing Satellite (ALOS) in 2003, and the Landsat-8 in 2005.

See also Archeological mapping; Earth, interior structure; Mapping techniques; Petroleum, history of exploration; RADAR and SONAR; Seismograph

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Remote Sensing

Remote Sensing

WILLIAM C. HANEBERG

Remote sensing is the acquisition of information about an object or phenomenon by a device located a considerable distance from the object or phenomenon. The term was coined in the mid-1950s by an Office of Naval Research scientist to distinguish the information obtained from the first generation of meteorological satellites from that which had been traditionally obtained by airplane-based aerial photography. In practice, however, information obtained from high-flying reconnaissance aircraft such as the U-2 and SR-71 can also be considered to be a product of remote sensing.

In addition to providing panchromatic (black and white) and multispectral color images that resemble photographs, some modern remote sensing satellites contain hyperspectral sensors that record information using dozens or hundreds of reflected electromagnetic energy wavelength bands that extend beyond the range of human vision. The simplest kind of multispectral image consists of red, blue, and green bands added together to form a color composite image. Image processing software can be used, particularly with hyperspectral data, to identify the chemical composition of rocks, vegetation type, soil or

water pollution, and other attributes that can be characterized in terms of spectral reflectance. Paired images can also be used to stereoscopically construct digital elevation models (DEMs), which can subsequently be transformed into topographic maps or three dimensional terrain models from space.

Other satellites contain active sensors that generate their own electromagnetic signals and record the reflections rather than passively recording reflected natural radiation. Synthetic aperture radar (SAR), in particular, is a useful tool because it can penetrate clouds and be used at night. The length of a radar antenna is known as its aperture and, in general, the resolution of a radar image is proportional to antenna length. The term synthetic aperture refers to a technique in which the constant movement of a satellite is combined with periodic radar pulses and computer processing to achieve the same effect as would be obtained by using a very large antenna. Pairs of SAR images can be combined to produce interferometric (InSAR) images that portray millimeter to centimeter scale changes in the elevation of Earth's surface. InSAR is becoming an increasingly important tool for monitoring tectonic movements of Earth's crust, subsidence associated with heavy groundwater pumping, and other geologic processes. It can also be used to construct digital elevation models. Another active source remote sensing technique is light detection and ranging (LIDAR), which is similar to radar but uses a laser instead of radio waves to produce extremely detailed topographic maps and images.

It is generally understood that remote sensing satellites must have a resolution of 5 meters (m) or less to be useful for intelligence work. The Landsat 1 satellite, launched by the United States in 1972 and from which imagery was freely available, had a resolution of 80 m. Landsat 7, launched in 1999 and still in service, has resolutions of 15 m for panchromatic images, 30 m for its six multispectral bands, and 60 m for its thermal band. The French SPOT 5 satellite offers commercially available images ranging in resolution from 5 m for panchromatic to 20 m for infrared. Publicly available images with these coarse resolutions are useful for such tasks as delineating large-scale geologic features, evaluating inaccessible or denied terrain, examining land use patterns, and inferring levels of crop stress, but not for detailed intelligence work. In recent years, however, commercial remote sensing satellites have been able to obtain high-resolution images that are of intelligence quality. The commercial Quick Bird satellite launched from Vandenberg Air Force Base in late 2001, for example, provides commercially available imagery with 61 cm panchromatic and 2.44 m multispectral resolution. The commercial IKONOS satellite, launched in 1999, can produce 1 m resolution color images.

Even the best publicly available imagery does not approach the resolution provided by classified intelligence satellites. The earliest KeyHole intelligence satellites (KH1 series), the first of which was launched by the United States in 1960, had a resolution of 2 m. Photographic film from KeyHole satellites was recovered using film drops until 1972, when digital imaging and transmission were instituted. The KH12 series is estimated to have a resolution of approximately 2 cm, although no images with this resolution have been released. Intelligence-quality images with sub-meter resolution can be used to assess details of troop or materiel movement, the progress of construction projects, and war damage in denied or otherwise inaccessible areas. Perhaps the most widely known application of remotely sensed images for intelligence work was the use of satellite and U-2 airplane photographs to detect the presence of Russian missiles in Cuba, which led to the 1962 Cuban missile crisis.

FURTHER READING:

BOOKS:

Campbell, James B. Introduction to Remote Sensing, 3rd ed. New York: Guilford Press, 2002.

ELECTRONIC:

Hardin, R. Winn. "Remote Sensing Satellite Market Pits Industry Against U.S. Policy." OE Reports. May 1999. <http://www.spie.org/app/publications/magazines/oerarchive/may/may99/cover1.html> (November 14, 2002).

Short, Nicholas M., Sr. "The Remote Sensing Tutorial." NASA. October 22, 2002. <http://rst.gsfc.nasa.gov/> (November 14, 2002).

Skorve, Johnny E. "Using Satellite Imagery to Map Military Bases of the Former Soviet Union." Earth Observation Magazine. April 2002. <http://www.eomonline.com/Common/currentissues/Apr02/skorve.htm> (November 14, 2002).

International Society for Photogrammetry and Remote Sensing, Department of Geomatic Engineering, University College London, Gower Street, London WC1E 6BT, United Kingdom. 44 207679 7226. <http://www.isprs.org/.> (November 14, 2002).

SEE ALSO

Bomb Damage, Forensic Assessment
Cameras
Cuban Missile Crisis
Electromagnetic Spectrum
Electro-optical Intelligence
Geospatial Imagery
LIDAR (Light Detection and Ranging)
Photographic Resolution
Photography, High-Altitude
RADAR, Synthetic Aperture
U-2 Spy Plane
Unmanned Aerial Vehicles (UAVs)

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Remote Sensing

Remote Sensing

Remote sensing is broadly defined as the act of obtaining images or data from a distance, typically using a manned spacecraft, a satellite, or a high-altitude spy aircraft. The term was invented in the 1950s to distinguish early satellite images from aerial photographs traditionally obtained from fixed wing aircraft. As such, remotely sensed images can be considered to be one kind of geospatial imagery . Although the application of unclassified remote sensing images to civil and criminal investigations has been limited, they have proven to be useful for documenting international atrocities in areas that are otherwise inaccessible to outside observers.

Sufficiently detailed satellite imagery has been used to document international crimes such as possible genocide in the Darfur region of Sudan and the existence of concealed mass graves in Iraq. In Iraq, potential gravesites were identified with the help of satellite image and aerial photograph interpretation and then investigated in more detail using ground-penetrating radar and other methods. A total of 270 mass graves were reported, of which 53 had been confirmed by early 2004, with some 400,000 bodies discovered. Features such as mass graves are generally not directly visible. Instead, analysis reveals features such as otherwise inexplicable areas of freshly moved earth or signs of heavy construction equipment used to excavate the graves. Comparison of publicly available Landsat satellite images obtained in 2003 and 2004 was also used to document the burning of 44 % of the villages in the Darfur region of Sudan during a period of civil strife, which some observers believe amounted to genocide. Burning was inferred in areas where the albedo, or amount of radiation reflected by the ground surface, had changed significantly during the times at which the two images were obtained. This was accomplished by using a computer algorithm to calculate albedo from the satellite data, then subtracting one albedo map from the other to calculate the change. This kind of mathematical operation on entire maps or digital images, as opposed to single numbers, is known as map algebra.

Modern remote sensing satellites provide panchromatic grayscale images (popularly known as black and white) and multispectral images in which channels representing discrete bands of the electromagnetic spectrum are combined. The most common multispectral images consist of some combination of red, green, blue, and near infrared bands. Hyperspectral sensors can produce images composed of dozens or hundreds of bands. Using information about the spectral reflectance characteristics of different kinds of soils , rocks, and plants, image analysts can fine tune the ratios of bands in multispectral and hyperspectral images to identify specific targets.

Image resolution has historically limited the use of satellite images, particularly those that are unclassified and easily available, in criminal and civil forensic work. The Landsat 1 satellite launched by the United States in the early 1970s, which provided the first publicly available satellite images, had a maximum resolution of 80 m. Therefore, objects smaller in size than several hundreds of meters could not be analyzed because objects must be many times larger than the maximum resolution in order to be clearly shown. Landsat 7, launched in 1999, had maximum resolution of 15 m for its panchromatic band, 30 m for its multispectral bands, and 60 m for its thermal infrared band. Although imagery with maximum resolution of 10 m or more can be useful for regional investigations, it is generally not useful for detailed forensic investigations of activities that have occurred through time on individual parcels of land. A new generation of commercial satellites such as the Quickbird satellite launched in 2001, however, has 0.61 m panchromatic resolution and 2.44 m multispectral resolution. The commercial IKONOS satellite, which was launched in 1999, has a maximum resolution of 1 m for color imagery. Although no images have been released as of early 2005, many intelligence experts believe that the most recent KeyHole surveillance satellites operated by the United States have a resolution of about 2 cm (0.02 m).

The resolution of panchromatic images is higher than that of multispectral or hyperspectral images because panchromatic information requirements are lower. In a panchromatic digital sensor, each light-sensitive photosite responds to all colors of light. In a multispectral sensor, however, the same number of photosites must be divided among each of the spectral bands. A multispectral sensor with infrared, red, green, and blue bands but the same number of photosites as a panchromatic sensor would have a resolution only 1/4 as high as the panchromatic sensor. This explains, for example, the ratio of 4 between the panchromatic 0.61 m resolution and multispectral 2.44 m resolution of the Quickbird satellite. In some cases, multispectral images can be combined with brightness information from more detailed panchromatic images. The apparent effect is a sharper image, although the resolution of the multispectral layer is not actually changed.

see also Digital imaging; Geospatial imagery; Satellites, non-governmental high resolution.

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Remote Sensing

Remote Sensing

At its simplest definition, remote sensing is obtaining information about an object by a device that is not in contact with the object. In ecology remote sensing usually involves sensors on satellite platforms or airplanes. Most devices have a series of sensors that record the intensity of electromagnetic radiation in particular segments of the spectrum for each point, or pixel, in an image. These sensors are designed to collect data in the visible wavelength as well as in other portions of the electromagnetic spectrum (such as the infrared region) that are needed to examine specific aspects of the physical world.

In addition to collecting data from a large part of the electromagnetic spectrum, remote sensing systems collect data over large areas. For instance, the U.S. Landsat satellites record continuous data over an area 71.4 square miles (185 square kilometers) wide. Since some satellites have been in orbit since the mid-1970s scientists can effectively "collect data" from this time period. Therefore, remote sensing offers scientists a wide spectral, spatial, and temporal data range.

For remote sensing to be of use to ecologists the spectral data must be related to some ground-based measurement such as land cover type or vegetation characteristics (biomass or net primary production, evapotranspiration rates, water stress, vegetation structure). Most work in ecology is done at the scale of a small plot, or piece of a field or forest. It can be difficult to extrapolate these small-scale measurements to larger, heterogeneous areas. Because sensors record continuous data over large areas, remote sensing can be used to "scale-up" plot-based measurements to examine landscape or even regional patterns. For example, ecologists have used remote sensing data to determine the rate at which rainforest in Brazil is being converted to agricultural land. In North America, scientists using satellite data have determined that one of the most endangered ecosystems , the tallgrass prairie, is being replaced by woody vegetation at an alarming rate.

Another set of questions that can be addressed with remote sensing data involves landscape heterogeneity. In these analyses, any of a number of spatial statistics can be applied to the original spectral data. Also, the original bands can be recombined to create indices. The most common of these is the Normalized Difference Vegetation Index, a ratio of red to near infrared bands, which has been useful in quantifying vegetation in numerous locations around the world.

Spectral data can be analyzed directly (total infrared reflected) or a classification can be performed on the data. With this method, the spectral data are analyzed and each pixel is assigned to a land cover type: forest, grassland, or urban. For instance, forests reflect less infrared than grasslands. These land cover data can then be incorporated into a Geographical Information System (GIS) for further analysis. A GIS is a computer-based system that can deal with virtually any type of information that can be referenced by geographical location.

Once the land cover types are identified and GIS coverage is generated, additional data such as soil type, elevation, and land use history can be entered into the GIS. Ecologists can then ask questions about landscape-level patterns such as the average patch size of a certain land cover type or its dispersion across the landscape. This information can then be related to some ecological process such as the movement or dispersal of animals.

see also Ecology; Ecosystem; Grassland; Landscape Ecology

Greg A. Hoch and John M. Briggs

Bibliography

Lillesand, Thomas M., and Ralph W. Kiefer. Remote Sensing and Image Interpretation, 4th ed. New York: John Wiley & Sons, 1999.

Schott, John R. Remote Sensing: The Image Chain Approach. Oxford: Oxford University Press, 1996.

Schowengerdt, Robert A. Remote Sensing: Models and Methods for Image Processing. San Diego, CA: Academic Press, 1997.

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remote sensing

remote sensing Any method of obtaining and recording information from a distance. The most common sensor is the camera; cameras are used in aircraft, satellites, and space probes to collect information and transmit it back to Earth (often by radio). The resulting photographs provide a variety of information, including archeological evidence and weather data. The images are also used in map-making. Microwave sensors use radar signals that penetrate cloud. Infrared sensors measure temperature differences over an area. Computers process data from sensors.

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remote sensing

remote sensing The technique whereby sensors located remotely from a computer are used to produce inputs for a digital system. These inputs are then transmitted either by wire or radio techniques to the computer. An example is the use of digital thermometers and humidity detectors in large buildings: the sensors transmit their readings to a central computer that optimizes energy use by regulating heat and air conditioning.

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remote sensing

remote sensing The gathering of information without actual physical contact with what is being observed. This involves the use of radars, sonars, spectrosocopy, and the use of airborne and satellite photography. See BISTATIC RADAR; IMAGING; LASER RANGING; POLARIMETRY; RADAR ALTIMETRY; RADIOMETRY; and RADIO OCCULTATION.

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