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SAR concept of "synthetic aperture" comes from processing techniques that can simulate a much larger antenna size. The SAR instrument used by the Shuttle Radar Topography Mission (SRTM) consist of a 197 foot long mast with a 28 ft receiver, the biggest such object ever deployed in space. With this long distance between receiver and transmitter the triangulation of the signals is potentially much more accurate.
The spaceborne SAR flew aboard two space shuttle flights in April and October, 1994. The unique perspective of space and the advanced capabilities of this Spaceborne Imaging Radar-C and X-Band Synthetic Aperture Radar (SIR-C/X-SAR) Mission have allowed scientists to explore our planet in a way that has not yet been possible. Data from SIR-C/X-SAR and other international SAR systems such as the European Space Agency's Remote Sensing Satellites (ERS-1/2), Japan's Earth Resources Satellite (JERS-1), and Canada's Radarsat are currently being used for studies of ecology, hydrology, oceanography, geology, ice sheets, glaciers, icebergs and for archeological exploration.
Ecological applications of SAR include land cover classification, above-ground woody plant biomass measurement, wetland inundation delineation, crop identification and monitoring. SAR data have also been used for malaria risk mapping in tropical regions by providing imagery that shows the spatial and temporal distribution of flooding.
Hydrologic applications of SAR include measuring snow condition and snow pack thickness, which is used for estimating runoff and water supply. Oceanographers are using SAR data to study surface and internal waves, wave/current interactions, arctic sea-ice motions and for monitoring shipping lanes for icebergs. SAR data are also being used by geologists in studies of past climates and volcanic/earthquake hazards.
Seasat image of wave patterns (not
clouds!) off the coast of Southern California
The longer L-band and P-band radar wavelengths have the useful capability of being able to see beneath the surface. SIR-C/X-SAR obtained data of the Sahara Desert that showed braided channels and paleodrainages beneath the surface, this was a complete surprise to the scientists and engineers involved because although is was theoretically possible they did not anticipate being able to see more than several meters into the surface. The important property of this part of the world is that there is no moisture (very low dielectric), even at depth, so the waves can propagate through the sand, bounce off the solid bedrock and then back out again.
These images are from a region of North Africa's Sahara Desert near Kufra Oasis in southeast Libya.
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Volcano, earthquake, floods, fires, landslides and other natural hazard investigations benefit greatly from a unique capability of SAR called "interferometry" which uses coordinated measurements obtained at slightly different angles acquired in sequence along a flight line. These measurements combined to produce distance and surface heights. Combining one interferometric data pair with a third measurement, obtained at a different time, allows for very precise measurements to be made. This technique, known as differential interferometry, has enormous potential for mapping changes in topography on the order of a centimeter and for mapping subtle shifts in earth's crust in areas of active faulting.
