Radar imaging problems of scanning modes in SAR Kondor-E space vehicle

Abstract

The article examines experience practice in radar imagining (RI) in one of the normal operation modes (scanning mode - SM ) of onboard synthetic aperture radar (SAR)  in space vehicle (SV) Kondor-E. This mode RI stands out for low linear resolution (20 … 30 m). It enables to apply rather simple radio hologram processing algorithms. It may be noted that one of important features of this mode compared to other normal modes is using ScanSAR mode during scanning, i.e. comprehensive RI is formed from separate frames by joining them. It means that a comprehensive partial frame is synthesized during one (rather small) accumulation interval. The algorithm of harmonic analysis can be applied to solve the mentioned tasks.  It is rather effective from the computing point of view and, despite inherent shortcomings, it can be used successfully in this mode. RI separate frames could be amplitude-modulated due to irregularity of antenna beam pattern in respect to range and azimuth. To reduce this effect the both coordinates overlapping is provided in SAR operation. Summing of the overlapping RI areas would be obvious plus point in this problem solution. However, it turns out to be uneasy due to the inaccuracy of the antenna beam adjustment in respect to azimuth and due to RI pixel discrepancies in respect to range. And therefore, RI separate frames have to be sewed together end-to-end and special RI contrast alignment algorithms have to be applied. Other feature of the scanning mode is the significantly varied pixel pitch in respect to range and azimuth. Scale nonlinearity in RI generating has to be as well considered in particular. Special emphasis has to be put on the problem of joining partial frames in skewed scanning mode. The article gives RI examples obtained when processing radio holograms of SV Kondor-E SAR confirming SM mode specified features and methods of taking them into account.