Background (Encyclopedia of Global Resources)
Remote sensing is the use of technology to extract information from objects or areas distant from the observer. It is the act of gathering information about a subject of interest without being in contact with the subject. This technology often collects energy beyond the sensitivity of human eyes and ears, utilizing the entire range of energy of the electromagnetic spectrum, particles, or fields. Remote-sensing technology is a successful tool for the discovery, inventory, and management of resources, both natural and human-made. These techniques make possible the collection of data beyond the range of human senses. Moreover, the large-scale perspective remote sensing affords accelerates our ability to map and identify change over time.
Several areas in the sciences utilize remote-sensing techniques. Astronomy has perhaps the longest history of gathering information from a distance, but there are other fields that do so, such as geophysics using seismic studies to decode the interior of the Earth; sonar for probing the ocean floor; and medicine using X rays and CAT scans.
Remote sensing developed from several origins, both scientific and technological. Technologically, the telescope, invented in the 1600’s, and photography, invented in the 1840’s, were significant advances in our ability to remotely sense our environment. Then, the discovery of energy(frequencies) beyond the familiar visible light, in the 1860’s, by...
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Early Earth Resources Satellites (Encyclopedia of Global Resources)
Remote sensing as a stand-alone discipline had a parallel development with the space race of the 1950’s and 1960’s. The advantages of a perspective from space became apparent with the National Aeronautics and Space Administration’s (NASA’s) April 1, 1960, launch of TIROS 1 (the Television Infrared Observations Satellite), whose mission was to observe weather patterns. The advantages to meteorologists and weather forecasting were obvious, but there was other information to be garnered from these types of images.
On July 23, 1972, the first of a series of satellites, Earth Resources Technology Satellite 1 (ERTS-1), was launched with the specific mission of remotely sensing the Earth’s surface. The success of this mission encouraged the launch of a succession of similar satellites with a different name: the Landsat series. This ongoing effort has given the scientific community decades of continuous coverage of the Earth’s surface.
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Passive vs. Active Sensors (Encyclopedia of Global Resources)
These satellites, and others from private companies and governments other than that of the United States, carry a variety of imaging scanners that look down on the Earth and transmit digital images to various ground stations. There are two types of sensors: active and passive. Active sensors that transmit to the target and collect bounces back via “radio detection and ranging” (radar) are examples. Passive sensors collect energy that is reflected from the target; for example, a camera collects light. The majority of remote-sensing satellites utilize multiple passive sensors. These are designed to record reflected energy at different electromagnetic frequencies, usually in both the visible and infrared portion of the spectrum. The advantage to combining different frequencies is that more contrast can be discerned between targets that are similar. For example, the green leaf of a corn plant can be distinguished from the green leaf of a watermelon—from space. This discrimination is based on the reflected energy, or the object’s spectral signature.
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Spectral Signatures (Encyclopedia of Global Resources)
The key to interpreting a satellite image is in understanding the spectral signatures of the various objects in the image. Passive scanners receive the energy from the Sun that is reflected from the target (for example, green plants). After it has passed through the atmosphere and interacted with the object, it passes through the atmosphere again and is collected by the satellite’s sensors. The atmosphere acts as a filter of some frequencies, and the object itself both absorbs and reflects frequencies. The signal (energy) that is reflected into space is different from the energy that left the Sun. This altered signal is specific to the object and is its “spectral signature.” Nonliving objects—rocks, soil, water—tend to have more stable signatures, but plants can vary depending on species, age, and health.
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Thematic Mapping (Encyclopedia of Global Resources)
By identifying the spectral signature of a target, all objects of the same signature can be mapped as the same, or the same theme. This is known as thematic mapping. For example, a cornfield will have a characteristic spectral signature that is different from an adjacent field of potatoes. Therefore, by identifying the spectral signature of the corn, all similar signatures within the image can be mapped as “corn.” These images are often color-coded to enhance discrimination between targets. This is known as “false color” imagery because the colors are assigned to enhance contrast and are not descriptive of the object as seen in sunlight. As the data are recorded in a digital format, all of the pixels (picture elements) that compose the image can be counted and the extent or area of any signature can be measured. For example, an image or images of the Midwest can be collected, the spectral signature of a crop (such as corn or wheat), can be identified, and computer software can calculate the area of crop on the ground, its health, and its stage of development.
This underscores the advantage of this technology. These types of thematic maps can be made over regional areas with computer speed. Geologists using the perspective of space can map large geologic structures, identify mineral deposits and potential fossil-fuel deposits, and inventory surface water resources. Further, in conjunction with land-use planners,...
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The Terra System (Encyclopedia of Global Resources)
The mission of NASA’s Terra system (formerly EOS AM-1), launched in December 18, 1999, is to research multidisciplinary problems directed primarily by researchers from the United States, Japan, and Canada. Terra’s broad goals are to investigate Earth systems; interactions among the atmosphere, hydrosphere, biosphere, and lithosphere; and changes in the global climate system. Terra has a design life of approximately fifteen years (2000-2015). The five main sensors it carries are:
•the Advanced Spaceborne Thermal Emissions and Reflection Radiometer (ASTER), which provides high-resolution imagery over fifteen spectral windows; it can develop thematic maps of surface temperature (reflectance) and elevation;
•the Clouds and the Earth’s Radiant Energy System (CERES), which measures solar-reflected and Earth-emitted radiation from the Earth’s surface to the top of the atmosphere;
•the Multi-Angle Imaging Spectroradiometer (MISR), which is composed of nine cameras using four different spectral windows; it primarily measures changes of atmospheric energy over time;
•the Moderate-Resolution Imaging Spectroradiometer (MODIS), which captures imagery over thirty-six spectral windows and maps the entire Earth in a one- or two-day period; and
•the Measurements of Pollution in the Troposphere (MOPITT), which (as its name implies) monitors pollution changes in the lower level of the atmosphere,...
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Perspective (Encyclopedia of Global Resources)
The science and technology of remote sensing are developing at a critical time for the citizens of the twenty-first century. Global population, approaching seven billion in 2009, was expected to be nine or ten billion by the middle of the twenty-first century. At the same time, a long-term trend toward increased socioeconomic status for nations such as China and India, combined with population, require ever-more resources to support an interlocking global economy. The ability to explore, inventory, and manage natural resources to maintain this growth is greatly enhanced. However, remote sensing goes beyond the search for raw materials to sustain humanity; it can help manage human resources as well. The data assists the understanding of the Earth’s changing climate, specifically rainfall patterns. This has a direct impact on where people can live and future migration patterns. By observing the growth of cities and communities over time, these data can assist in the most efficient land-use planning. The “big picture” perspective from space coupled with the speed of computers to interpret this imagery is a timely and welcome tool for the twenty-first century.
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Further Reading (Encyclopedia of Global Resources)
Campbell, James B. Introduction to Remote Sensing. 4th ed. New York: Guilford Press, 2007.
Jensen, John R. Remote Sensing of the Environment: An Earth Resource Perspective. 2d ed. Upper Saddle River, N.J.: Prentice-Hall, 2007.
Ustin, Susan L., ed. Manual of Remote Sensing: Remote Sensing for Natural Resource Management and Environmental Monitoring. 3d ed. New York: Wiley, 2004.
Geoscience and Remote Sensing Society. http://www.grss-ieee.org/
National Aeronautics and Space Administration. Tutorial on Remote Sensing. http://rst.gsfc.nasa.gov/
National Aeronautics and Space Administration Earth Observatory. Remote Sensing: Introduction and History. http://earthobservatory.nasa.gov/Features/RemoteSensing/remote.php
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Remote Sensing (World of Earth Science)
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
Remote Sensing (World of Forensic Science)
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.