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| SRTM Partners and Imaging Radar History at NASA and JPL |
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| The History of Imaging Radar at NASA and JPL NASA and JPL's orbital radar program began with the Seasat synthetic aperture radar in 1978. Seasat was a single-frequency, L-band (wavelength of 24 cm or 9-1/2 inches), single polarization, fixed look-angle radar designed for ocean studies. The first of the Shuttle Imaging Radars, called SIR-A, flew on the second Space Shuttle flight in 1981 and was also an L-band radar with a fixed look-angle. The second of that series, SIR-B, which flew on the Space Shuttle in 1984, used an L-band, single polarization radar with an adjustable look angle. The third mission, SIR-C/X-SAR, provided increased capability over Seasat, SIR-A, and SIR-B by being able to acquire images at three microwave wavelengths: L-band with quadruple polarization; C-band (6 centimeters or 2.4 inches) quadruple polarization; and X-band (3 centimeters or 1.2 inches)with a single polarization. SIR-C/X-SAR also had a variable look angle and could image at incidence angles between 20 and 65 degrees. SIR-C/X-SAR flew onboard the Shuttle in April and October of 1994, providing radar data for two seasons. Typical image sizes for SIR-C data products were 50 by 100 kilometers (30 by 60 miles), with resolution of between 10 and 25 meters (33 and 82.5 feet). Parallel to the development of spaceborne imaging radars, JPL has built and operated a series of airborne imaging radar systems. The laboratory currently maintains and operates an airborne SAR system, known as AIRSAR/TOPSAR, which flies on a NASA DC-8 jet. This system collects radar images at three radar wavelengths: C-band, L- band, and P-band (68 cm or 27 in.). It also collects interferometric radar data at C-band and L-band with pairs of antennas mounted on the fuselage. NASA's Office of Earth Sciences also is studying designs for future imaging radar missions that would be less expensive to fly and provide detailed coverage of regions that have not yet been imaged. Potential missions might include mapping freezing and thawing transition zones in polar regions, monitoring changes in the Earth's crust on a scale of millimeters, mapping topography areas that are prone to earthquakes and volcanic activity, and mapping vegetation cover to assess the rate of recovery from such stresses as deforestation, biomass burning, and other human activities and natural phenomena. The National Imagery and Mapping Agency (NIMA) The National Imagery and Mapping Agency (NIMA)is a member of the intelligence community and is a Department of Defense combat support agency. NIMA was created in 1996 to accelerate the fusion of geospatial information and imagery intelligence and to meet growing customer needs for a common, digital view of the mission space. NIMA is committed to delivering the imagery and geospatial information that gives national policymakers and military users information superiority in a rapidly changing global environment. With its headquarters in Bethesda, Md., NIMA operates major facilities in St. Louis, Mo., Washington, D.C., and Reston, Va., and services a wide array of customers throughout the world. Professionals in disciplines such as cartography, imagery analysis, the physical sciences, geodesy, and photogrammetry make up NIMA's combined military and civilian work force. Mission NIMA's mission is to provide timely, relevant, and accurate imagery, imagery intelligence, and geospatial information in support of U.S. national security objectives. Mapping in Three Dimensions Presently, the U.S. has a better global topographic map of Venus than it does of the Earth's surface. However, the SRTM mission will improve our ability to produce topographic information by collecting high-resolution elevation data over most of the Earth's land surface. To process the data, NIMA uses Digital Terrain Elevation Data (DTEDŽ), a process of evenly spaced points on the Earth's surface whose elevations have been recorded. These elevation data will provide an estimate of surface height every 30 meters, a density three times greater than the currently available height map. Digital Terrain Elevation Data (DTED) DTED is used by NIMA's military and civilian customers for a wide range of uses. Recently, NIMA provided the National Transportation Safety Board DTED converge to complete a video reconstruction of a major aviation accident. By providing a third dimension to normally flat map depiction, the DTED enabled investigators to view the terrain as the pilot saw it before the accident. After Hurricane Mitch, NIMA provided DTED to the U.S. Geological Survey in support of a White House-sponsored Central American Reconstruction Task Force. To aid in recovery operations and infrastructure reconstruction, DTED was used to show terrain elevation in the affected countries before and after the hurricane. Back home, DTED has been combined with satellite imagery to generate a variety of commercial remote sensing data and products through NASA's Global LandSat Mapping Project. The terrain data from SRTM is likely to spawn many uses for geospatial information that will save lives and enhance economic development around the world. SRTM data also can be used for hydrological studies that analyze ground water flow, optimizing the locations for cellular phone towers; earthquake and glacier activity monitoring; and terrain modeling around airports to provide for safer approach routes. Military uses of DTED include scene visualization, command-and-control, navigation and targeting. When draped over imagery in terrain modeling systems, DTED is used to produce simulated "fly throughs" that facilitate planning and enable aircrews to rehearse a mission before flying it. DTED also has been used to generate realistic radar and cockpit views in flight training simulators. The Army uses DTED to determine field of view from different advance points, develop lines of sight, and identify minimum heights to keep a target in sight. The Army also uses DTED to determine the slope of terrain for off-the-road mobility and in dam analyses. Terrain analysts can measure the capacity of a dam with DTED and predict the impact of flooding, including the velocity of released water. Products are currently produced with DTED on the Army's Combat Terrain Information System (CTIS). As the Army moves toward "dominant battle space awareness" under Joint Vision 2010, DTED will become a fundamental component of this vision. DLR (Deutsches Zentrum fuer Luft- und Raumfahrt, German Aerospace Center) Germany's contribution to this spectacular international concerted effort is the X-band radar interferometer with DLR as the project lead and responsible for system engineering, mission operation, calibration, data processing, archiving, distribution, and data utilization. Dornier Satellitensysteme, together with the Italian Space Agency (ASI), is responsible for the development of the X-SAR flight instrument. X-SAR operates within the X-band of the electromagnetic spectrum at a wavelength of 3.1 cm, equivalent to a frequency of 9.6 Gigahertz. DLR's Institute of Radio Frequency Technology was able to draw on its long experience with imaging synthetic aperture radar systems. Idea and conception for the construction of the X-SAR system originated at this institute. Several teams have participated in its realization. The scientific and technological supervision, as well as calibration and mission control, are with Institute of Radio Frequency Technology. The German X-SAR captures 50-kilometer swaths of the Earth's surface, mapping 40 percent of all orbits at a significantly high resolution. DLR's German Space Operation Center (GSOC) in Oberpfaffenhofen is involved in mission planning, operational system design and in the mission itself. Each minute of the precious remote sensing time is planned years in advance. During the mission, the team will work in NASA's mission and payload control center in Houston, Texas. According to the open policy on Earth observation data, which is one of the main goals of DLR, X-SAR data will not be classified but will be made available to the general public. DLR's German Remote Sensing Data Center (DFD), also located in Oberpfaffenhofen, will process the data acquired in the course of the mission and take care of the distribution. Users are from many disciplines. Hydrologists can exactly determine area of river flooding, geologists make out small changes in earthquake zones and around volcanoes, ecologists receive precise information in the extent and condition of vegetation, and climatologists are able to improve their forecast models. Also telecommunication, agriculture, air traffic, and marine navigation experts can profit from the SRTM information. Dornier Satellitensysteme GmbH plays a leading role in the area of Earth observation in Europe. The company is a main industrial contractor for the radar satellites ERS-1 and ERS-2 and heads the environmental mission Envistal-1. For many years DSS has successfully participated in the development of Earth observation devices. One focus was on high-resolution SAR instruments. Dornier Satellitensysteme also offers geo-information products through its own service. |
