Target motion compensation in synthetic aperture radar.

摘要

By collecting data as it moves along a straight line path and then coherently processing this data the synthetic aperture radar (SAR) obtains an angular resolution equivalent to that of a very long antenna. Improvements in resolution of a hundredfold or more over conventional radars are possible in SAR systems. One of the major difficulties in using such systems is the uncertainty in target position that results when targets have a radial velocity component relative to the radar. The SAR processing causes such targets to appear at the correct range but at an erroneous azimuth position. This research is directed toward the investigation of this phenomenon and the development of a signal processing method to reduce or eliminate it. Three different approaches are studied. The first approach is to make use of three antennas on the radar system as a means of determining the velocity components for each target present and providing a basis for shifting them back to their correct azimuth positions after SAR processing. The compensation is carried out in the time domain and is continuous along the flight line. The second approach is to use a 'doppler-insensitive filter' to compensate the phase of incoming signals of the moving target. After compensating, these received signals look like signals reflected from a stationary target. It is a time-consuming process, but has good performance. The third approach is to convert incoming signals to frequency domain. From this spectrum, the central frequency can be calculated. Meanwhile, the velocity of the target on the ground can be estimated from this central frequency. Using this estimated velocity to design the reference function which is used to correlate the incoming signals, the target location can be found. This method is similar to the conventional SAR signal processing except of using different reference function. An evaluation is carried out both analytically and by computer simulation. The comparisons between theoretical results and the values from the simulation are prepared.