Two dimensional processing for airborne synthetic aperture radar

The use of airborne synthetic aperture radar suffers from the difficulties raised by the nonlinearities of the carrier trajectory. An original method for motion compensation allowing the use of two dimensional processing for image synthesis is introduced. It provides a more elegant and more efficient solution to range migration and range versus Doppler coupling, compared to the classical range/Doppler processing. Trials with the RAMSES airborne SAR of the ONERA confirm the interest of the new method. We recall and analyse the principle of motion compensation both with range/Doppler processing and with adapted pre-integration. The classical algorithm (specan) uses the variations of distance between the aircraft and a target point in the middle of each focus band in order to compute phase shifts and range migrations. The problem is that the replica is generally not time invariant when the flight path is not perfectly linear with uniform velocity. Typically, along one minute of recorded signal, the airplane may deviate transversally from the straight line of some 60 meters and longitudinally from uniform speed of about 25m. Therefore, the Doppler processing must be performed in the time domain, leading to an increase in computation time with respect to frequency domain processing. Adequate adapted pre-integration may alloy the use of two dimensional processing by (at least on a data block) simulating a linear uniform motion of the center of phase. This enforces the requirement for the Fast Fourier correlation that the replica is the same for the whole data block. We explain the organization of the adapted pre-integrated two dimensional processing used with deramped signal. The aliasing problem occuring with the integration along a curve in a discretely sampled range compressed data, is no more a concern since the range migration is implicit in the two dimensional processing. As a result, even when range migration is important the images are of better quality compared to those obtained by classical range/Doppler processing.