Research On Imaging Algorithms For Monostatic System And Bistatic Systemof Geosynchronous Sar

Spaceborne synthetic aperture radar(SAR) with all-day and all-weather observed capability, is widely used in geological exploration, resource survey, disaster management, and military reconnaissance fields. For spaceborne SAR, resolution and swath width are the important indicators of its evaluating spaceborne SAR performance as in the normal pulse operated system, azimuth resolution and swath width of Low Earth Orbit SAR(LEOSAR) is restricted by each other. Since raising orbital altitude can effectively mitigate the above strictly restricting dilemma. Geosynchronous Earth Orbit SAR(GEOSAR) has gained more and more attention. GEOSAR has superior advantages over LEOSAR including the wider swath, shorter revisit period, higher anti-strike and anti-destroy ability, and it can monitor a specific area in longer time. However, the obvious drawbacks of GEOSAR such as higher transmitting power and higher engineering cost have restricted its hardware implementation in industry. On the other hand, the bistatic construction with GEOSAR transmitter working in strip mode and Unmanned Aerial Vehicle SAR(UAVSAR) receiver working in the multi modes(GEO-UAV Bi SAR) can well solve the problem and realize the high resolution image or wide swath image in the any dangerous areas.As GEOSAR entails the high altitude, elliptical orbiting track, longer synthetic aperture time and wider swath, the orbiting characteristics of the GEOSAR are different from other spaceborne radars. This paper systematically analyzes the system model, point target two dimensional frequency spectrum, and further studies its imaging algorithm of the monostatic system and bistatic system for GEOSAR. The main work is summarized as follows.1. Study on the motion characteristics and system parameters design of GEOSAR. The motion characteristics and Doppler properties of GEOSAR are different from the other LEOSAR, and these are the basis of following imaging research. Hence, based on the geometrical structure of GEOSAR, its motion characteristics and Doppler properties are analyzed, based on which, the radar system parameters are further studied, such as two dimensional resolution, signal bandwidth, pulse repetition frequency and average transmitting power.2. Study on imaging algorithm of GEOSAR. Considering the curve track of GEOSAR in the long synthetic aperture time, the linear range model doesn’t satisfy to the accuracy approximation requirements of the real slant range history; and since the movement of radar platform in the wave propagated delay time can not be ignored, GEOSAR doesn’t meet the so called “stop-go-stop” assumed condition. Hence, the range model based on non “stop-go-stop” assumption is proposed by using Taylor expansion in this paper. The feasibility and applicability of this range model is further discussed. Considering the fact that the radar has the wider swath, we apply the method of series reversion to deduce the point target two dimensional frequency spectrum with range variant, and CS imaging algorithm and SPECAN imaging algorithm based on non “stop-go-stop” range model is proposed. The former can realize to image in the large scene, and the latter has the high calculation efficiency.3. Study on the effects atmosphere on GEOSAR performance and its suppressions methods. Since GEOSAR mainly works on L band, in the long synthetic aperture time, the time variant of ionosphere and troposphere seriously affects the radar performance. To solve this problem, based on non “stop-go-stop” assumption, the range error model caused by ionosphere and troposphere is studied and the signal model polluted by atmosphere is further deduced. Moreover, the influences and suppressions of the ionosphere and troposphere on the imaging qualities are theoretically analyzed. Then, the simulation is carried out to analyze the influences of the ionosphere and troposphere on the imaging qualities through the global ionospheric grid datas and atmospheric meteorological informations from International GNSS Service(IGS), verify the effectiveness of suppression method is verified.4. Study on the imaging algorithm of Bistatic SAR with GEOSAR in the strip mode and UAVSAR in the strip mode(GEO-(ST)UAV Bi SAR). Since the motion characteristics of the transmitter and the receiver are different to the other spaceborne-airborne radars, a novel imaging algorithm based on the study of geometrical configuration and two dimensional resolution, is proposed. Hence, considering the motion characteristics of transmitter and receiver, we studied the geometrical configuration of this bistatic radar based on the beam scanning velocity of two platforms. Effects of geometrical configuration on the synthetic aperture time and two dimensional resolution are analyzed to obtain the optimized geometrical configurations. Then, we infer the bistatic range model through using Taylor expansion. The point target two dimensional frequency spectrum with range variant is gained by using the method of series reversion. Based on this spectrum, the CS imaging algorithm for GEO-(ST)UAV Bi SAR is presented.5. Study on the imaging algorithm and motion compensation of Bistatic SAR with GEOSAR in the strip mode and UAVSAR in the mode of steering antenna,(GEO-(SA)UAV Bi SAR). UAVSAR works in the mode of steering antenna, for example, spotlight mode, sliding spotlight mode and Terrain Observation by Progressive Scans(TOPS) mode. In the condition of small pulse repetition frequency, steering antenna causes azimuth frequency spectrum aliasing. To solve this problem, we introduce dechirp method to eliminate azimuth frequency spectrum aliasing, and propose CS imaging method for GEO-(SA)UAV Bi SAR. Then, in the condition of two platforms moving unsteadily, we studied the influences of motion errors on the radar performance. The Doppler informations are obtained by using the Doppler parameter estimation method, and the accurate motion compensation is realized. Finally, the bistatic imaging method of the non-stationary GEO-(SA)UAV Bi SAR is studied.