Side-Looking Airborne Radar (SLAR)

The Side-Looking Airborne Radar (SLAR) is an image-producing system that derives its name from the fact that the radar beam is transmitted from the side of the aircraft during data acquisition. SLAR is an active sensor; the system provides its own source of illumination in the form of microwave energy. Consequently, imagery can be obtained either day or night. Since microwave energy penetrates most clouds, SLAR can be used to prepare image maps of cloud-covered areas. Data are X-band synthetic aperture radar (horizontally transmitted, horizontally received) with the exception of some test sites. Since the radar signal is transmitted at a depressional angle below the horizontal plane in which the aircraft is flying, the signal strikes the terrain at an oblique angle, and the surficial expression of the geologic structure may thus be enhanced. The topographic expression of some surface features, such as subtle faults and folds, may be more clearly seen on radar imagery than on conventional aerial photographs or satellite images. SLAR projects were contracted on a yearly basis and multiple contractors were involved in processing and delivering data to the USGS. The SLAR project areas corresponded to one or more 1:250,000-scale topographic maps. Each 1:250,000-scale area is generally 1 by 2 degrees for the conterminous U.S. and 1 by 3 degrees for Alaska. The 1:100,000-scale mosaic products are enlarged from one quarter (NE, SE, SW, or NW) of the 1:250,000-scale mosaics. The source data for the SLAR Mosaics were acquired between 1980 and 1994. Near Range has shorter shadow, better detail, shows vegetation and flat lands. Far Range has more exaggerated shadow. It shows elevations of land and relief better than near range. SLAR products available from the USGS include radar mosaics of most of the project areas. Coverage exists over 25% of conterminous United States and covers more than 10% of Alaska.