Recent Advances in Synthetic Aperture Radar Enhancement and Information Extraction
Dušan Gleich, Zarko Cucej · Sciyo eBooks · 2010
The Synthetic Aperture Radar systems are all weather, day and night monitoring systems, which use the electromagnetic radiation for image retrieval.SAR is one of the most advanced engineering inventions systems in the last decade.Specific radar systems are imaging radars, such as side looking SAR and SAR.Practical restriction to the length of the antenna resulted in very coarse resolution in the flight direction.Using a fixed antenna, illuminating a strip or swath to the sensor's ground track, resulted in the concept of stripping mapping.Modern phased-array antennas are able to perform even more sophisticated data collections strategies, as ScanSAR, spotlight SAR, but the strip map mode is the most applied mode on current satellites [1].The concept of using frequency (phase) information in the radar signal's along-track spectrum to discriminate two scatters within the antenna beam goes back to 1951 (Carl Wiley).The key factor is coherent radar,, where the phase and amplitude are received and preserved for later processing., but long antenna was required.The early SAR systems were based on optical processing of the measured echoed signal using the Fresnel approximation for image formation and are known as range-Doppler Imaging or polar format processing.The experience on airborne SAR systems in 60's and 70's culminated in L-band SAR system Seasat, a satellite launched in June 1978, primarily for ocean studies, the live time was 100 days, but the imaginary was spectacular, highlighting if geologic information and ocean topography information.Since 1981 Shuttle missions carried SAR systems.The first instrument was the Shuttle Imaging radar SIR laboratory and operated for 2.5 days.An improved version of SIRA orbited the Earth in 1984 and was able to steer the antenna mechanically to enable different angels.Cosmos 1870 was the first S-band SAR satellite of former Soviet Union, launched in 1987 and orbited at a height of 270 km and operated for 2 years, ALMAZ-1 was the second satellite launched in 1991 and operated for 1.5 years.First European Remote Sensing Satellite ERS-1 was operational in 1991 and operated until March 2000.Japan started spaceborne SAR program in 1992 with JERS in 1992, SIR-C/X-SAR was developed by JPL, DLR and ASI operated with C, L and X band.Canadian Space Agency lunched Radarsat in 1995.A SRTM (Shuttle Radar Topography mission) was carried out between 11 and 23 February 2000.In last decade many other satellites with SAR were lunched: Radarsat-2, ENVISAT, TerraSAR-X, Tandem-X, ALOS, Cosmo-Skymed, SAR lupe and forth coming constellation of Sentinel satellites. Principles of SARThe central idea of SAR processing is based upon matched filtering of the received signal in both the range and azimuth directions.Matched filtering is possible because the acquired SAR data are modulated in these directions with appropriate phase functions.The modulation in range is provided by the phase encoding of transmitted pulse, while the modulation in azimuth is created by the motion in the signal.The point targets are arrayed in a Cartesian type Coordinate system space defined by range, azimuth, and altitude as analogs of x, y and z directions.The altitude direction is omitted in the two-dimensional simulation.The platform in this simulation is an antenna attached to a plane traveling at an orbital velocity, along the azimuth direction and at the midpoint in the flight, the distance to the target equals the range of closest approach or minimum range to target.As an satellite platform is used in the simulation, the curvature of the earth is considered negligible and the orbital velocity is approximately equal to the platform velocity.The transmitted radar signal, x(t), is assumed to be a chirp pulse (linearly frequency modulated signal) given by www.intechopen.com