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(electricity) Sensing, by a power supply, of voltage directly at the load, so that variations in the load lead drop do not affect load regulation.
(engineering) The gathering and recording of information without actual contact with the object or area being investigated.
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 spectrum—a 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 phenomena—not 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 unit—one in which the transmitter and receiver are in the same location—the radar unit has to be switched continually between sending and receiving modes. Clearly, a bistatic unit—one in which the transmitter and receiver antennas are at locations remote from one another—is 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 altitudes—25,000 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/>.
The gathering and recording of information about terrain and ocean surfaces without actual contact with the object or area being investigated. Remote sensing uses the visual, infrared, and microwave portions of the electromagnetic spectrum. Remote sensing is generally conducted by means of remote sensors installed in aircraft and satellites.
Photography
Photography is probably the most useful remote sensing system. Much of the experience gained over the years from photographs of the terrain taken from aircraft is being drawn upon for use in space.
Multispectral photography isolates the reflected energy from a surface in a number of given wavelength bands and records each spectral band separately on film. This technique allows selection of the significant bandwidths in which a given area of terrain displays maximum tonal contrast and, hence, increases the effective spectral resolution of the system over conventional black-and-white or color systems. Because of its spectral selectivity capabilities, the multispectral approach provides a means of collecting a great amount of specific information.
Multispectral imagery
Multispectral scanning systems record the spectral reflectance by photoelectric means (rather than by photochemical means as in multispectral photography) simultaneously in several individual wavelengths within the visual and near-infrared portions of the electromagnetic spectrum.
In satellite applications, optical energy is sensed by an array of detectors simultaneously in four spectral bands from 0.47 to 1.1 micrometers. As the optical sensors for the various frequency bands sweep across the underlying terrain in a plane perpendicular to the flight direction of the satellite, they record energy from individual areas on the ground. The smallest individual area distinguished by the scanner is called a picture element or pixel, and a separate spectral reflectance is recorded in analog or digital form for each pixel. The spectral reflectance values for each pixel can be transmitted electronically to ground receiving stations in near-real time, or stored on magnetic tape in the satellite until it is over a receiving station. When the signal intensities are received on the ground, they can be reconstructed almost instantaneously into the virtual equivalent of conventional aerial photographs.
Infrared
Thermal infrared radiation is mapped by means of infrared scanners similar to multispectral scanners, but in this case radiated energy is recorded generally in the 8–14-μm portion of the electromagnetic spectrum. The imagery provided by an infrared scanning system gives information that is not available from ordinary photography or from multispectral scanners operating in the visual portion of the electromagnetic spectrum. See also Emissivity; Infrared radiation.
In the past, thermal infrared images were generally recorded on photographic film. Videotape records are replacing film as the primary recording medium and permit better imagery to be produced and greater versatility in interpretation of data.
Thermal infrared mapping (thermography) from satellite altitudes is proving to be useful for a number of purposes, one of which is the mapping of thermal currents in the ocean. Thermal infrared mapping from aircraft and satellite altitudes has many other uses also, including the mapping of volcanic activity and geothermal sites, location of groundwater discharge into surface and marine waters, and regional pollution monitoring.
Microwave radar
This type of remote sensing utilizes both active and passive sensors. The active sensors such as radar supply their own illumination and record the reflected energy. The passive microwave sensors record the natural radiation. A variety of sensor types are involved. These include imaging radars, radar scatterometers and altimeters, and over-the-hori-zon radars using large ground-based antenna arrays, as well as passive microwave radiometers and imagers. One of the mostsignificant advantages of these instruments is their all-weather capability, both day and night. See also Microwave; Radar.
High-frequency (hf) radar
Such radars utilize frequencies in the 3–30-MHz portion of the electromagnetic spectrum (median wavelength of about 20 m) and are thus not within the microwave part of the spectrum. The energy is transmitted by ground-based antennas in either a sky-wave or surface-wave mode. In the sky-wave mode, the energy is refracted by the various ionospheric layers back down to the Earth's surface some 480–1800 mi or 800–3000 km (on a single-hop basis) away from the hf radar antenna site. The incident waves are reflected from such surface features as sea waves.
Deriving digital models of an area on the earth. Using special cameras from airplanes or satellites, either the sun's reflections or the earth's temperature is turned into digital maps of the area. In order to view the results, the data must be rendered by specialized image processing software. See digital elevation model.
The scanning of the earth by satellite or high-flying aircraft in order to obtain information about it.
See the Introduction, Abbreviations and Pronunciation for further details.
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).
In the broadest sense, remote sensing is the short or large-scale acquisition of information of an object or phenomenon, by the use of either recording or real-time sensing device(s) that is not in physical or intimate contact with the object (such as by way of aircraft, spacecraft, satellite, buoy, or ship). In practice, remote sensing is the stand-off collection through the use of a variety of devices for gathering information on a given object or area. Thus, Earth observation or weather satellite collection platforms, ocean and atmospheric observing weather buoy platforms, monitoring of a pregnancy via ultrasound, Magnetic Resonance Imaging (MRI), Positron Emission Tomography (PET), and space probes are all examples of remote sensing. In modern usage, the term generally refers to the use of imaging sensor technologies including but not limited to the use of instruments aboard aircraft and spacecraft, and is distinct from other imaging-related fields such as medical imaging.
There are two kinds of remote sensing. Passive sensors detect natural energy (radiation) that is emitted or reflected by the object or surrounding area being observed. Reflected sunlight is the most common source of radiation measured by passive sensors. Examples of passive remote sensors include film photography, infra-red, charge-coupled devices and radiometers. Active collection, on the other hand, emits energy in order scan objects and areas whereupon a passive sensor then detects and measures the radiation that is reflected or backscattered from the target. RADAR is an example of active remote sensing where the time delay between emission and return is measured, establishing the location, height, speed and direction of an object.
Remote sensing makes it possible to collect data on dangerous or inaccessible areas. Remote sensing applications include monitoring deforestation in areas such as the Amazon Basin, the effects of global warming on glaciers and Arctic and Antarctic regions, and depth sounding of coastal and ocean depths. Military collection during the cold war made use of stand-off collection of dangerous border areas. Remote sensing also replaces costly and slow collection on the ground, ensuring in the process that areas or objects are not disturbed.
Orbital platforms collect and transmit data from different parts of the electromagnetic spectrum, in conjunction with smaller scale aerial or ground-based sensing and analysis provides researchers with enough information to monitor trends such as el niño and other natural long and short term phenomena. Other uses include different areas of earth sciences such as natural resource management, agricultural fields such as land usage and conservation, and national security, both overhead, ground-based and stand-off collection on border areas.[1].
The basis for multi-spectral collection and analysis is that of examined areas or objects that reflect or emit radiation that stand out from surrounding areas.
In order to coordinate a series of large-scale observations, most sensing systems depend on the following; platform location, what time it is, and the rotation and orientation of the sensor. High-end instruments now often use positional information from satellite navigation systems. The rotation and orientation is often provided within a degree or two with electronic compasses. Compasses can measure not just azimuth (i.e. degrees to magnetic north), but also altitude (degrees above the horizon), since the magnetic field curves into the Earth at different angles at different latitudes. More exact orientations require gyroscopic-aided orientation, periodically realigned by different methods including navigation from stars or known benchmarks.
Resolution impacts collection and is best explained with the following relationship; less resolution=less detail & larger coverage, More resolution=more detail, less coverage. The skilled management of collection results in cost-effective collection and avoid situations such as the use of multiple high resolution data which tends to clog transmission and storage infrastructure.
Generally speaking, remote sensing works on the principle of the inverse problem. While the object or phenomenon of interest (the state) may not be directly measured, there exists some other variable that can be detected and measured (the observation), which may be related to the object of interest through the use of a data-derived computer model. The common analogy given to describe this is trying to determine the type of animal from its footprints. For example, while it is impossible to directly measure temperatures in the upper atmosphere, it is possible to measure the spectral emissions from a known chemical species (such as carbon dioxide) in that region. The frequency of the emission may then be related to the temperature in that region via various thermodynamic relations.
The quality of remote sensing data consists of its spatial, spectral, radiometric and temporal resolutions. Spatial resolution refers to the size of a pixel that is recorded in a raster image - typically pixels may correspond to square areas ranging in side length from 1 to 1000 metres. Spectral resolution refers to the number of different frequency bands recorded - usually, this is equivalent to the number of sensors carried by the platform(s). Current Landsat collection is that of seven bands, including several in the infra-red spectrum. The MODIS satellites are the highest resolving at 31 bands. Radiometric resolution refers to the number of different intensities of radiation the sensor is able to distinguish. Typically, this ranges from 8 to 14 bits, corresponding to 256 levels of the gray scale and up to 16,384 intensities or "shades" of colour, in each band. The temporal resolution is simply the frequency of flyovers by the satellite or plane, and is only relevant in time-series studies or those requiring an averaged or mosaic image as in deforesting monitoring. This was first used by the intelligence community where repeated coverage revealed changes in infrastructure, the deployment of units or the modification/introduction of equipment . Cloud cover over a given area or object makes it necessary to repeat the collection of said location. Finally, some people also refer to the "economic resolution", that is, the cost-effective way to manage the collection of data.
In order to create sensor-based maps, most remote sensing systems expect to extrapolate sensor data in relation to a reference point including distances between known points on the ground. This depends on the type of sensor used. For example, in conventional photographs, distances are accurate in the center of the image, with the distortion of measurements increasing the farther you get from the center. Another factor is that of the platen against which the film is pressed can cause severe errors when photographs are used to measure ground distances. The step in which this problem is resolved is called georeferencing, and involves computer-aided matching up of points in the image (typically 30 or more points per image) which is extrapolated with the use of an established benchmark, "warping" the image to produce accurate spatial data. As of the early 1990s, most satellite images are sold fully georeferenced.
In addition, images may need to be radiometrically and atmospherically corrected. Radiometric correction gives a scale to the pixel values, e.g. the monochromatic scale of 0 to 255 will be converted to actual radiance values. Atmospheric correction eliminates atmospheric haze by rescaling each frequency band so that its minimum value (usually realised in water bodies) corresponds to a pixel value of 0. The digitizing of data also make possible to manipulate the data by changing gray-scale values.
Interpretation is the critical process of making sense of the data. The first application was that of aerial photographic collection which used the following process; spatial measurement through the use of a light table in both conventional single or stereographic coverage, added skills such as the use of photogrammetry, the use of photomosaics, repeat coverage, Making use of objects' known dimensions in order to detect modifications. Image Analysis is the recently developed automated computer-aided application which is in increasing use.
Object-Based Image Analysis (OBIA) is a sub-discipline of GIScience devoted to partitioning remote sensing (RS) imagery into meaningful image-objects, and assessing their characteristics through spatial, spectral and temporal scale.
Old data from remote sensing is often valuable because it may provide the only long-term data for a large extent of geography. At the same time, the data is often complex to interpret, and bulky to store. Modern systems tend to store the data digitally, often with lossless compression. The difficulty with this approach is that the data is fragile, the format may be archaic, and the data may be easy to falsify. One of the best systems for archiving data series is as computer-generated machine-readable ultrafiche, usually in typefonts such as OCR-B, or as digitized half-tone images. Ultrafiches survive well in standard libraries, with lifetimes of several centuries. They can be created, copied, filed and retrieved by automated systems. They are about as compact as archival magnetic media, and yet can be read by human beings with minimal, standardized equipment.
Beyond the primitive methods of remote sensing our earliest ancestors used (ex.: standing on a high cliff or tree to view the landscape), the modern discipline arose with the development of flight. The balloonist G. Tournachon (alias Nadar) made photographs of Paris from his balloon in 1858. The first tactical use was during the civil war. Messenger pigeons, kites, rockets and unmanned balloons were also used for early images. With the exception of balloons, these first, individual images were not particularly useful for map making or for scientific purposes.
Systematic aerial photography was developed for military surveillance and reconnaissance purposes beginning in World War I and reaching a climax during the Cold War with the use of modified combat aircraft such as the P-51, P-38, RB-66, F4-C and the SR-71 or specifically designed collection platforms such as the U2/TR-1, A-5 and the OV-1 series both in overhead and stand-off collection. A more recent development is that of increasingly smaller sensor pods such as those used by law enforcement and the military, in both manned and unmanned platforms. The advantage of this approach is that this requires minimal modification to a given airframe. Later imaging technologies would include Infra-red, conventional, doppler and synthetic aperture radar
The development of artificial satellites in the latter half of the 20th century allowed remote sensing to progress to a global
scale as of the end of the cold war. Instrumentation aboard various Earth observing and weather satellites such as
Landsat, the Nimbus and more recent missions
such as RADARSAT and UARS provided global measurements of various
data for civil, research, and military purposes. Space probes to other planets have also provided the opportunity to conduct
remote sensing studies in extra-terrestrial environments, synthetic aperture radar aboard the
Recent developments include, beginning in the 1960s and 1970s with the development of image processing of satellite imagery. Several research groups in Silicon Valley including NASA Ames Research Center, GTE and ESL Inc. developed Fourier transform techniques leading to the first notable enhancement of imagery data.
The introduction of online web services for easy access to remote sensing data in the 21st century (mainly low/medium-resolution images), like Google Earth, has made remote sensing more familiar to the big public and has popularized the science.
According to a NOAA Sponsored Research by Global Marketing Insights, Inc. the most used software among Asian academic groups involved in remote sensing are as follows: ESRI 30%; ERDAS 25%; RSI ENVI 17%; MapInfo 17%; ERMapper 11%. Among Western Academic respondents as follows: ESRI 39%, ERDAS 27%, MapInfo 9%, AutoDesk 7%, RSI ENVI 17%.
Associations and Societies
Spatial Agencies
Remote Sensing Centres
Remote Sensing: Universities
Satellites (missions, platforms and sensors)
Tutorials
Other relevant sites
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