artificial satellite, object constructed by humans and placed in orbit around the earth or other celestial body (see also space probe). The satellite is lifted from the earth's surface by a rocket and, once placed in orbit, maintains its motion without further rocket propulsion. The first artificial satellite, Sputnik I, was launched on Oct. 4, 1957, by the USSR; a test payload of a radio beacon and a thermometer demonstrated the feasibility of orbiting a satellite. The first U.S. satellite, Explorer I, launched on Jan. 31, 1958, returned data that was instrumental in the discovery of the Van Allen radiation belts. During the first decade of space exploration, all of the satellites were launched from either the United States or USSR. Today, there are more than three dozen launch sites in use or under construction in more than a dozen countries.
If placed in an orbit high enough to escape the frictional effects of the earth's atmosphere, the motion of the satellite is controlled by the same laws of celestial mechanics that govern the motions of natural satellites, and it will remain in orbit indefinitely. At heights less than 200 mi (320 km) the drag produced by the atmosphere will slow the satellite down, causing it to descend into the denser portion of the atmosphere where it will burn up like a meteor. To attain orbital altitude and velocity, multistage rockets are used, with each stage falling away as its fuel is exhausted; the effect of reducing the total mass of the rocket while maintaining its thrust is to increase its speed, thus allowing it to achieve the required velocity of 5 mi per sec (8 km per sec). At this speed the rocket's forward momentum exactly balances its downward gravitational acceleration, resulting in orbit. Once above the lower atmosphere, the rocket bends to a nearly horizontal flight path, until it reaches the orbital height desired for the satellite.
Unless corrections are made, orbits are usually elliptical; perigee is the point on the orbit closest to the earth, and apogee is the point farthest from the earth. Besides this eccentricity an orbit of a satellite about the earth is characterized by its plane with respect to the earth. An equatorial orbit lies in the plane of the earth's orbit. A polar orbit lies in the plane passing through both the north and south poles. A satellite's period (the time to complete one revolution around the earth) is determined by its height above the earth; the higher the satellite, the longer the period. At a height of 200 mi (320 km), the period of a circular orbit is 90 min; at 500 mi (800 km), it increases to 100 min. At a height of 22,300 mi (36,000 km), a satellite has a period of exactly 24 hr, the time it takes the earth to rotate once on its axis; such an orbit is called geosynchronous. If the orbit is also equatorial, the satellite will remain stationary over one point on the earth's surface.
Tracking and Telemetry
Since more than 1,000 satellites are presently in orbit, identifying and maintaining contact requires precise tracking methods. Optical and radar tracking are most valuable during the launch; radio tracking is used once the satellite has achieved a stable orbit. Optical tracking uses special cameras to follow satellites illuminated either by the sun or laser beams. Radar tracking directs a pulse of microwaves at the satellite, and the reflected echo identifies both its direction and distance. Nearly all satellites carry radio transmitters that broadcast their positions to tracking antennas on the earth. In addition, the transmitters are used for telemetry, the relaying of information from the scientific instruments aboard the satellite.
Types of Satellites
Satellites can be divided into five principal types: research, communications, weather, navigational, and applications.
Research satellites measure fundamental properties of outer space, e.g., magnetic fields, the flux of cosmic rays and micrometeorites, and properties of celestial objects that are difficult or impossible to observe from the earth. Early research satellites included a series of orbiting observatories designed to study radiation from the sun, light and radio emissions from distant stars, and the earth's atmosphere. Notable research satellites have included the Hubble Space Telescope, the Compton Gamma-Ray Observatory, the Chandra X-ray Observatory, the Infrared Space Observatory, and the Solar and Heliospheric Observatory (see observatory, orbiting). Also contributing to scientific research were the experiments conducted by the astronauts and cosmonauts aboard the space stations launched by the United States (Skylab) and the Soviet Union (Salyut and Mir); in these stations researchers worked for months at a time on scientific or technical projects. The International Space Station, whose first permanent crew boarded in 2000, continues this work.
Communications satellites provide a worldwide linkup of radio, telephone, and television. The first communications satellite was Echo 1; launched in 1960, it was a large metallized balloon that reflected radio signals striking it. This passive mode of operation quickly gave way to the active or repeater mode, in which complex electronic equipment aboard the satellite receives a signal from the earth, amplifies it, and transmits it to another point on the earth. Relay 1 and Telstar 1, both launched in 1962, were the first active communications satellites; Telstar 1 relayed the first live television broadcast across the Atlantic Ocean. However, satellites in the Relay and Telstar program were not in geosynchronous orbits, which is the secret to continuous communications networks. Syncom 3, launched in 1964, was the first stationary earth satellite. It was used to telecast the 1964 Olympic Games in Tokyo to the United States, the first television program to cross the Pacific Ocean. In principle, three geosynchronous satellites located symmetrically in the plane of the earth's equator can provide complete coverage of the earth's surface. In practice, many more are used in order to increase the system's message-handling capacity. The first commercial geosynchronous satellite, Intelsat 1 (better known as Early Bird), was launched by COMSAT in 1965. A network of 29 Intelsat satellites in geosynchronous orbit now provides instantaneous communications throughout the world. In addition, numerous communications satellites have been orbited by commercial organizations and individual nations for a variety of telecommunications tasks.
Weather satellites, or meteorological satellites, provide continuous, up-to-date information about large-scale atmospheric conditions such as cloud cover and temperature profiles. Tiros 1, the first such satellite, was launched in 1960; it transmitted infrared television pictures of the earth's cloud cover and was able to detect the development of hurricanes and to chart their paths. The Tiros series was followed by the Nimbus series, which carried six cameras for more detailed scanning, and the Itos series, which was able to transmit night photographs. Other weather satellites include the Geostationary Operational Environmental Satellites (GOES), which send weather data and pictures that cover a section of the United States; China, Japan, India, and the European Space Agency have orbited similar craft. Current weather satellites can transmit visible or infrared photos, focus on a narrow or wide area, and maneuver in space to obtain maximum coverage.
Navigation satellites were developed primarily to satisfy the need for a navigation system that nuclear submarines could use to update their inertial navigation system. This led the U.S. navy to establish the Transit program in 1958; the system was declared operational in 1962 after the launch of Transit 5A. Transit satellites provided a constant signal by which aircraft and ships could determine their positions with great accuracy. In 1967 civilians were able to enjoy the benefits of Transit technology. However, the Transit system had an inherent limitation. The combination of the small number of Transit satellites and their polar orbits meant there were some areas of the globe that were not continuously covered—as a result, the users had to wait until a satellite was properly positioned before they could obtain navigational information. The limitations of the Transit system spurred the next advance in satellite navigation: the availability of 24-hour worldwide positioning information. The Navigation Satellite for Time and Ranging/Global Positioning Satellite System (Navstar/GPS) consists of 24 satellites approximately 11,000 miles above the surface of the earth in six different orbital planes. The GPS has several advantages over the Transit system: It provides greater accuracy in a shorter time; users can obtain information 24 hours a day; and users are always in view of at least five satellites, which yields highly accurate location information (a direct readout of position accurate to within a few yards) including altitude. In addition, because of technological improvements, the GPS system has user equipment that is smaller and less complex. The former Soviet Union established a Navstar equivalent system known as the Global Orbiting Navigation Satellite System (GLONASS). The Russian-operated GLONASS will use the same number of satellites and orbits similar to those of Navstar when complete. Many of the handheld GPS receivers can also use the GLONASS data if equipped with the proper processing software. Beidou is China's satellite-based navigation and global positioning system. It began operations is 2011 with 10 satellites, succeeding an experimental system that became operational in 2001, and is planned to utilize 35 satellites when completed in 2020.
Applications satellites are designed to test ways of improving satellite technology itself. Areas of concern include structure, instrumentation, controls, power supplies, and telemetry for future communications, meteorological, and navigation satellites.
Satellites also have been used for a number of military purposes, including infrared sensors that track missile launches; electronic sensors that eavesdrop on classified conversations; and optical and other sensors that aid military surveillance. Such reconnaissance satellites have subsequently proved to have civilian benefits, such as commercially available satellite photographs showing surface features and structures in great detail, and fire sensing in remote forested areas. The United States has launched a series of Landsat remote-imaging satellites to survey the earth's resources by means of special television cameras and radiometric scanners. The data from remote-imaging satellites has also been used in archaeological research. Russia and other nations have also launched such satellites; the French SPOT satellites provide higher-resolution photographs of the earth.
See M. V. Fox, Satellites (1996); S. A. Kallen, The Giant Leaps: The Race to Space (1996); M. Long, 1997 Phillips World Satellite Almanac (1997); A. Luther, Satellite Technology: An Introduction (2d ed. 1997).
Satellites, Non-Governmental High Resolution
Satellites, Non-Governmental High Resolution
High-resolution satellites, generally understood to be those with a spatial resolution of 2 meters (6.6 feet) or less, have the capability to provide forensic information from areas that are otherwise inaccessible to law enforcement officials.
Resolution is a measure of the ability of an image to depict detail. When used in reference to digital images such as those produced by remote sensing satellites, resolution generally refers to size of the pixels, or fundamental elements, comprising the image. A 2-meter resolution image consists of elements representing the average color or intensity of a 2x2 meter area of Earth's surface. Nothing smaller than 2x2 meters will be depicted as a distinct object. The smallest objects that can be clearly identified on an image, however, will be much larger than the resolution because many pixels are required to represent the characteristic shape or outline of an object. A 2-meter resolution satellite image might, therefore, show distinct images of buildings covering tens or hundreds of square meters, but not a small shed or automobile covering an area the size of a 2x2-meter pixel.
The best commercial satellites operating in 2005 had resolutions of 1 meter (3.3 feet) or less. However, intelligence satellites operated by the U.S. government were believed to have a resolution of about 2 centimeters (0.8 inches). Images with that resolution, however, have never been released for public use.
The first remote sensing satellites were built, launched, and operated by government agencies in the 1960s. In the interest of national security, images from these satellites were tightly controlled and generally inaccessible to civilian officials and forensic scientists. Imagery from the first Landsat satellites, launched by the United States in the 1970s, was publicly available but its low resolution (tens of meters) made it useful only for regional studies. After an attempt to privatize and eliminate government subsidies for the Landsat program in the 1980s, the United States passed the Land Remote Sensing Policy Act of 1992. This act emphasized the importance of satellite imagery, returned the Landsat program to government operation, mandated that its data be made available at cost, and included a provision for the licensing of commercially operated remote sensing satellites. At about the same time, the French government developed the SPOT (Satellite Pour l'Observation de la Terre) program and marketed its imagery through a subsidized corporation. Like Landsat imagery, however, SPOT imagery generally did not provide the resolution necessary for detailed forensic work.
The Landsat and SPOT satellites paved the way for a new generation of high-resolution commercial satellites that provide images detailed enough for forensic work. The commercial IKONOS satellite, launched by the multi-national Space Imaging consortium in 1999, orbits Earth at an altitude of 680 kilometers (422.5 miles) and provides panchromatic (black and white) images with 1-meter resolution. The EROS A1 satellite, built by the ImageSat International consortium in Israel and launched from Russia in 2000, provides 1.8-meter (6-foot) resolution. Its successor, the EROS B1, will have 0.70-meter (2.3-foot) resolution when operable in 2006. The highest resolution commercial satellite imagery available in 2005 came from the QuickBird satellite operated by the Colorado company DigitalGlobe. QuickBird produces 0.62-meter (2-foot) resolution panchromatic images and 2.4-meter (7.9-foot) resolution color images. The panchromatic images, in particular, are detailed enough to depict individual automobiles, pieces of machinery, or ground disturbance associated with illegal activities.
Panchromatic images have higher resolutions (smaller pixel size) than color, or multi-spectral, images. This is because digital imaging sensors have a fixed number of photosites that respond to light. When a panchromatic image is made, each photosite senses the total intensity of light. When a multi-spectral image is made, in contrast, the photosites must be divided among the spectral bands being depicted. Thus, a color image consisting of infrared, red, green, and blue bands would have one-fourth the resolution of a panchromatic image from a sensor with the same number of photosites.
One particularly high profile application of commercial high-resolution satellite imagery was the search for debris from the space shuttle Columbia, which exploded over Texas in 2003. The QuickBird satellite was immediately redirected to cover the accident area, and the resulting images showed areas of broken trees and highly reflected debris. The detailed satellite images allowed accident investigators to better document the extent of the debris field and recover pieces of the shuttle.
High-resolution commercial satellite imagery is also invaluable in the aftermath of natural disasters such as the 2004 Indian Ocean tsunami. There, it was used to help guide relief efforts and provided important information for researchers studying the effects of tsunamis.
Other applications of commercial satellite imagery in forensic science are less exotic. Government officials in Arizona, Georgia, and Minnesota have used satellite imagery to detect illegal cotton cultivation, logging, and pollution. Because satellites pass over any given location no more frequently than every few days, they are best suited for the characterization of slow processes such as growing crops or persistent problems such as air or water pollution. For the same reason, it is unlikely that satellite imagery will provide images that catch thieves, kidnappers, rapists, or murderers committing crimes.
Like photographs and videotapes, satellite images can be manipulated and must therefore be authenticated for use in court. Prosecutors or plaintiff's attorneys must establish that any processing or enhancement techniques used on the imagery were properly documented and followed accepted professional standards, whereas defendant's attorneys may question the authenticity of imagery used against their clients. Although some manipulation must be done in order to transform digital information into a visible image, it is critical to establish that the manipulation did not distort or otherwise misrepresent the area being depicted in the imagery.
see also Digital imaging; Geospatial Imagery; GIS.
Satellites, Non-Governmental High Resolution
Satellites, Non-Governmental High Resolution
█ WILLIAM C. HANEBERG
Satellite imagery at resolutions useful for military and intelligence purposes has historically been available only from satellites developed, launched, and operated by governments. As a result, access to and dissemination of the high-resolution satellite images was tightly controlled in the interest of national security. Since 1999, however, commercial satellites have made high-resolution images publicly available at a relatively low cost. In the United States, the Land Remote Sensing Policy Act of 1992, which was motivated in part by Russian willingness to sell declassified 2 m resolution military satellite imagery in the early 1990s, provided the legal foundation for private ownership of American remote sensing satellites. In 1994, the Clinton administration issued guidelines for the licensing of commercial remote sensing satellite operations.
The resolution of an image is the size of the smallest object that can be depicted, and the best images currently available from commercial satellites have resolutions ranging from 50 cm to 1 m. Imagery from the most recent intelligence satellites launched by the United States government, in contrast, is believed to have a resolution of about 2 cm. No images with this resolution, however, have been released to the public. Although there is no universally accepted definition of "high resolution," in part because its meaning changes as technology improves, at the time this article was written it was generally understood to mean resolutions of 2 m or less.
IKONOS, named after the Greek word for "image," was the first commercial satellite to provide images with 1 m resolution. Its products include 1 m panchromatic (black and white) and true color images in addition to 4 m multispectral imagery. Following a sun-synchronous orbit 680 km above Earth's surface, IKONOS passes over any given longitude at about 10:30 a.m. local time each day and revisits any given geographic location every three days. Space Imaging, the company that operates IKONOS
was founded by a consortium of firms from the United States, Japan, and South Korea. The satellite was launched in August 1999 after the first version was destroyed when its launch vehicle malfunctioned and crashed the previous spring.
Developed by an international consortium of companies based in Cyprus and known as ImageSat International (ISI), the EROS A1 satellite was built largely in Israel and launched in 2001 from a Russian facility in Siberia. It was the first high-resolution commercial satellite developed outside of the United States. The EROS A1 camera, which can provide 1.8 m resolution panchromatic images, is
based on technology originally developed for Israeli intelligence satellites. The successor the EROS A1, known as the EROS B1, is scheduled for launch in late 2004 and is expected to produce both panchromatic and multi-spectral color imagery with 0.87 m resolution.
The highest resolution commercial imaging satellite currently in operation is QuickBird, operated by the Colorado-based firm Digital Globe, which follows a sun-synchronous orbit 450 km above Earth. The first QuickBird was lost in space after a late 2000 launch from a Russian facility in Siberia. A replacement was successfully launched from Vandenberg Air Force Base, California, atop a Delta II launch vehicle in late 2001. QuickBird supplies 0.62 m resolution panchromatic images and 2.4 resolution multispectral color images.
Proponents of commercial high-resolution imaging satellites argue that their images will be useful for a variety of civil work that includes infrastructure monitoring, natural disaster recovery, endangered species habitat identification and monitoring, and natural resource exploration. Commercially available high-resolution images can also be used to monitor troop and equipment movement, observe construction activity, identify targets in inaccessible areas, and remotely assess battle damage. This has led the United States government to prohibit its licensees from obtaining or selling high-resolution imagery of Israeli territory in response to concerns raised by the government of Israel. It also reserves the right to restrict operations during times of national security emergencies. These restrictions do not apply, however, to commercial satellites operated by companies outside of the United States.
█ FURTHER READING:
Baker, J.C. "Commercial Observation Satellites: A Catalyst for Global Transparency." 2002. <http://www.imagingnotes.com/julaug01/global.htm> (12 April 2003).
"Digital Globe." <http://www.digitalglobe.com/> (12 April 2003).
"ImageSat International." 2003. <http://www.imagesatintl.com/> (12 April 2003).
"Space Imaging—Visual Products. Visible Results." 2003. <http://www.spaceimaging.com/> (12 April 2003).
LIDAR (Light Detection and Ranging)