Image Algorithm Research And Azimuth Ambiguity Analysis Of Spotlight Synthetic Aperture Radar

Spotlight Synthetic Aperture Radar is a kind of special work mode, People show more and more interest in it. It achieves longer synthetic aperture time by steering the radar antenna beam to always illuminate the same area on the ground (spot), thus acquire an image of small area with high geometrical resolution in azimuth direction which strip-map SAR can not achieve.Usually in Space borne Spotlight SAR, the azimuth bandwidth is much larger than the pulse repetition frequency (PRF), these will cause serious spectral folding phenomenon. So we can't use the strip-map conventional processing methods to focus the image. The "two-step" method which appeared in recent years firstly using deramping operation in the azimuth direction which has fixed Doppler frequency modulation rate, thus removed spectral folding phenomenon, kept the space-varying characteristic of received data and achieved azimuth coarse focusing. After that, strip-map conventional processing methods were adopted to carry on the follow-up processing and obtain precise focusing. This article carries on the further study of "two-step" method, according to different SAR models, Chirp-Scaling algorithm were modified and used to obtain precise focusing.Because there are essential differences between strip-map SAR and spotlight SAR, the traditional method to analyze the azimuth ambiguity in strip-map SAR does not match the situation in spotlight mode. The article introduces the azimuth ambiguity calculation function of spotlight SAR in details through analyzing the origin of the azimuth ambiguity and echo signal characteristics.PRF selection is very important in spotlight SAR system design, the articleintroduces the limitations of PRF selection and pointed that, we can optimize PRF selection according to the azimuth ambiguity function curve, this research has important meaning doubtless in actual work.