Synthetic Aperture Radar (SAR) image data provide information different from that of optical sensors operating in the visible and infrared regions of the electromagnetic spectrum. SAR data consist of high-resolution reflected returns of radar-frequency energy from terrain that has been illuminated by a directed beam of pulses generated by the sensor. The radar returns from the terrain are mainly determined by the physical characteristics of the surface features (such as surface roughness, geometric structure, and orientation), the electrical characteristics (dielectric constant, moisture content, and conductivity), and the radar frequency of the sensor. By supplying its own source of illumination, the SAR sensor can acquire data day or night without regard to cloud cover. Elachi (1988) provides a technical overview of radar wave-surface interactions and their applications to land, water, and ice phenomena in Chapter 2 of Spaceborne Radar Remote Sensing. Most other remote sensing textbooks also provide introductory material on SAR system properties and image data applications.
Synthetic aperture radar (SAR) satellite systems currently in operation include the European Space Agency's (ESA) European Remote Sensing Satellite 1 (ERS-1), launched July 1991, and the Japanese Earth Resources satellite (JERS-1), launched February 1992. Contacts are provided for ERS-1 data and JERS-1 data. The ERS-1 sensor operates in the C-band frequency (approx. 5.6 cm wavelength) and JERS-1 operates in the L-band frequency (approx. 23 cm wavelength). Both sensors have a nominal spatial resolution of approximately 30 m. The Canadian Space Agency plans to launch its RADARSAT in 1995.
The SAR systems are now beginning to provide SAR image data on a long-term, sustained basis. The ERS-1 satellite, with a projected lifespan of three years, will be followed by an ERS-2 satellite planned to continue SAR data acquisition into the late 1990s, when advanced SAR sensors are expected to become operational as part of the Earth Observing System (EOS).
The current level of experience in operational use of SAR data is very limited compared to the use of visible and infrared data acquired by the multispectral satellite sensors. Several major characteristics of SAR data taken together, however, may promote more extensive evaluation and use of SAR data for land-use and land-cover information. These characteristics include 1) the unique information of surface roughness, physical structure, and electrical conduction properties; 2) the high spatial resolution; 3) the 24-hour, all-weather data-acquisition capability; and 4) the now-realizable long-term continuity of the data that enables repetitive (seasonal) coverage of major global land regions. Evaluations of SAR image data are addressing its potentially promising capabilities to detect and monitor 1) wildfire burn scars in boreal forests, as reported by Kasischke et al. (1992) in "Initial Observations on Using SAR to Monitor Wildfire Scars in Boreal Forests"; 2) deforestation patterns; 3) forest ecosystem characteristics (Dobson et. al. 1992); 4) tidal inundation in coastal wetlands; 5) land-use mapping and crop monitoring, as described by Lichtenegger (1992) in "ERS-1: Landuse Mapping and Crop Monitoring"; and 6) urban areas, rural habitations, and other man-made features on the landscape. Figure 1 is an ERS-1 image acquired over a portion of the Brazilian Amazon. Figure 2 illustrates dramatic differences between images of ERS-1 and JERS-1 data.