Study On Airborne High-resolution SAR Imaging Technology

This paper addresses the author's research work on the airborne high-resolution synthetic aperture radar(SAR). The research has been focused on the SAR imaging algorithms and motion compensation. Through a comparison between the simulation result and the real data, a method by combination of the motion compensation with the frequency scaling algorithm(FSA) is put forward. This method is proven viable for achieving a high-resolution image generation.According to pulse compression theory and SAR principle, the paper firstly gives an analysis on the principle of high-resolution SAR imaging, then, notes that SAR range resolution is dependent of signal bandwidth, while SAR azimuth resolution is dependent of the bandwidth of the LFM-featured Doppler signal generated from the relative movement between aircraft carrying the radar and the fixed ground target, thus, high azimuth resolution can be realized through pulse compression.The SAR imaging processing includes range compression and azimuth compression. The range compression processing is independent of azimuth direction. It is one-dimensional processing, whereas, the azimuth compression is dependent of range migration. It is two-dimensional correlation processing. How to solve the effect of the range migration in the azimuth compression processing is a major concern of SAR imaging algorithm. Different SAR imaging algorithm results in different image quality and different processing computation volume. Based on an analysis and comparison of the advantages and disadvantages of several conventional airborne SAR imaging algorithms, an in-depth research for the RMA(range migration algorithm) and the FSA algorithms has been carried out. The simulation results and real images by using of the two algorithms are given.The SAR imaging algorithms are ideally analyzed with the presumption that the aircraft carrying the radar moves in a straight line and at constant speed, however, in practice, various movement deviations exist. The effect of the deviations on the imaging should not be neglected. A quantitative analysis about the effect of each kind of phase error on the imaging of a point target is given in the paper. First, an evaluation about phase error is carried out through simulation. Then, the effect of aircraft movement deviations is analyzed. Generally, there are two methods for motion compensation. One is to make use of the measurements from GPS, inertia measurement unit(IMU), inertia navigation system(INS), etc. This is an open-loop motion compensation. Another method is based on echo data. However, so far, the measurements of flight movement from GPS and IMU/INS have no enough accuracy and data renewal rate for achieving a high resolution imaging. Accordingly, the paper has beenfocused on the motion compensation through the clutter lock and auto-focusing techniques both of which are based on echo data. The paper also gives their common processing methods, and presents their advantages and disadvantages through simulation and comparison.In conclusion, a method by combination of motion compensation with FSA is put forward. An evaluation using real data shows that this method can effectively correct the variant error which, however, can not be corrected by any conventional auto-focusing method.