Synthetic Aperture Radar
This article was initially published in Today's Engineer on March 2009
Synthetic Aperture Radar, which was developed in the 1950s as a military reconnaissance tool, was a solution to the 1940s need for an all-weather, 24-hour aerial remote surveillance device. In the late 1940s, the United States Army was looking for an aerial reconnaissance tool which could see through clouds and would not depend on the hours of daylight. Radar — given its ability to penetrate clouds and fog, and its non-dependence on the wavelengths of visible light — seemed the logical choice. The obstacle was that, in order to achieve a high enough resolution to be useful, the radar antenna would have needed to have been about the size of a football field, far too large for a reconnaissance aircraft to carry.
Carl Wiley, working at Goodyear, Arizona, (which later became Goodyear Aerospace, and eventually Lockheed Martin Corporation) in 1951, suggested the principle that — because each object in the radar beam has a slightly different speed relative to the antenna — each object will have its own doppler shift. A precise frequency analysis of the radar reflections will thus allow the construction of a detailed image. A radar antenna a meter or so wide can be made to acquire an image which otherwise would have required a much larger one. Approximately one year after Wiley, researchers at the University of Illinois independently developed the same idea, as well as developing beam-sharpening and autofocus concepts. During the summer of 1953, the University of Michigan’s “Project Wolverine” laid the plans which would result in the development of a practical SAR. The processing demands stretched the limits of the analog processors of the day, however, Emmett Leith, one of the pioneers of holography, believed that optical processing of the data could satisfy the requirement. The method worked. In 1957, airborne synthetic aperture radar was yielding dramatic results, and the University of Michigan system had proven itself.
In 1974, the National Oceanic and Atmospheric Administration and engineers from Jet Propulsion Laboratories began exploring the possibilities for oceanic observations using a satellite carrying a synthetic aperture radar. SAR’s wavelengths make it sensitive to small surface roughness changes, meaning that it is ideal for monitoring surface wave patterns and currents. SAR can measure displacement accuracy to within several millimeters. The June 1978 launch of Seasat was the first civilian application of synthetic aperture radar, and it provided a powerful new tool to scientists studying the earth. Prior to Seasat, civilian image acquisition of the earth was via Landsat cameras, using visible light and providing resolutions in the tens of meters. Seasat operated until October of 1978, when it was disabled by a massive short circuit in its power system. Since that time, many SARs have flown on board the Space Shuttle.
Synthetic Aperture Radar has become one of the most valuable tools for remote sensing of the earth and its environment. With resolutions down to one .3 meter or less from 55 km away, and .11 meters from 25 km, it is used for sea ice observation, measuring glacier variations, wind pattern data collection, rainfall, erosion, warning of storm surges, vegetation structure, disaster management, identification of potential landslide areas, and drought prediction. The pressure of magma building up beneath volcanoes often causes them to rise or distort their surfaces. Synthetic aperture radar is capable of detecting such minute variations, and for that reason it has become a useful tool monitoring volcanoes and their lava flows as a way of warning of impending eruptions, as well as mitigating the post-eruption hazards.
Forests play an enormously important role in climate interactions, and the health of forests is related to the health of soil and to the quality of the watershed. Forests also typically cover vast areas, often inaccessible ones, thus airborne or satellite radar offers a practical means for detailed study. Foliage reflects varying radar returns depending on water content and density. Forestry mapping and management, including prediction of and monitoring of forest fires.
Damage assessment during floods has been greatly improved by SAR. Because radar can penetrate the cloud cover which accompanies severe weather, it is often the only reliable source for accurate information on how much land has been inundated, the cresting and current flows of rivers, the extent of crop destruction, and damage to roads, bridges, and buildings. By using SAR information, disaster relief organizations can save lives by choosing proper evacuation routes, and distribute food and medical aid more quickly.
Because of its ability to measure minute differences in the surface of the ground, SAR has revealed much new information about ground subsidence (which in itself is often a marker for water table variations) and the role it can play in natural disasters. Spaceborne SAR has shown that the land under parts of New Orleans underwent rapid subsidence in the three years preceding the 2005 Hurricane Katrina.
