The Raw Signal Modeling And Sar Imaging For A Sliding Spotlight Spaceborne Sar

Spaceborne Synthetic Aperture Radar (SAR) is located on a satellite. When the satellite moves along with the orbit, the radar will send out and receive with a specific frequency and form a virtual long aperture by antenna moving from one array element to another. Recently, western countries have successfully developed Spaceborne SAR systems and the military has been equipped with them.Nowadays, the most commonly used working modes in Spaceborne SAR are stripmap mode and spotlight mode. The stripmap mode can perform large area imaging with limited resolution while the spotlight mode can reach high resolution with narrow imaging area. This paper introduces Sliding Spotlight mode, which control the resolution in azimuth through controlling the ground moving speed of the antenna radiation area. The sliding spotlight mode can perform larger imaging area than the spotlight mode and can have higher resolution than the stripmap mode with the antenna of the same size. So the sliding spotlight mode can have a balance between high resolution and large area imaging.This paper firstly emphasizes the imaging principle of sliding spotlight mode under the spaceborne situation, derives the raw signal function of point target and analyzes its resolution and swath width. Then the paper briefly introduces spotlight mode and stripmap mode and analyzes their resolution and swath width. After that, compare these three modes and summarize their relationship and the advantages of sliding spotlight mode.In raw signal modeling, the paper has made an orbit model to derive the calculation methods for center slant range and slant range at any time between the satellite and the target under the spaceborne situation. The paper also modify the velocity, the center slant range and the squint angle considering the earth rotation and the satellite's yaw and pitch, which has made the raw signal model more suitable for the real situation.This paper analyzes three imaging algorithms including Chirp Scaling, Nonlinear Chirp Scaling and Frequency Scaling and compares their applications and features. Then the paper puts forward an improved algorithm with a chirp scaling factor of quadratic phase to complete range compression to meet the requirement of sliding spotlight imaging with high resolution and large area. Furthermore, compress the data in azimuth in order to avoid the pulse repetition frequency over high and make the imaging algorithm more suitable for the real situation.In simulation, the paper adds system ambiguity calculation model to derive the pulse repetition frequency to avoid it being over high and entail the upper and lower limit of the pulse repetition frequency so that the results of simulation are reliable and stable.Finally, this paper does the simulation under side-looking with or without the earth rotation and the satellite's yaw and pitch to verify the raw signal model and imaging algorithm. Then the paper simulates the imaging algorithm under different squint angles for the range and azimuth with or without being windowed to verify the reliability and stability of the algorithm under high squint angles. Thus, the simulation results have proved that the raw signal model and imaging algorithm in this paper for sliding spotlight Spaceborne SAR are reliable and effective.