A modified secondary range compression algorithm at apogee in GRO SAR

It is known that computing the right range is very complex in geosynchronous orbit (GEO) synthetic aperture radar (SAR) for the factors that, firstly, the velocity and the slant angle at apogee fitted by the line trajectory model are the complex numbers and, secondly, "Stop-and-Go" assumption will brings about a range error. This paper explores the range issue and proposes a new range mode. It was found that the problem of complex number could be avoided by adopting the norm to express the range and the impacts of "Stop-and-Go" assumption might be considered in the modified range model. Based on the new range model, the expression of two-dimensional echo in frequency domain was deduced and the compensation function in the course of imaging algorithm was improved. Further, an accurate azimuth compression function was modified by means of a high-order chirp rate in azimuth. The result shows that a 40 km×40 km scene can be focused very well under the synthetic aperture time (SAT) of 100 s by the improved secondary range compression (SRC), and the peak side-lobe ratio (PSLR) and integrated side-lobe ratio (ISLR) of the edge points are very near to the theoretical value.