Research On Imaging Algorithms Of Multi-mode Spaceborne Synthetic Aperture Radar

As an important microwave remote sensing method of aerospace,the spaceborne synthetic aperture radar(SAR)has significant application in both military and national economy.The extensive application of spaceborne SAR has put forward more and more demands on performance indicators such as imaging resolution and mapping bandwidth.Thus,advanced spaceborne SAR systems often require a variety of working modes in nowadays.The multi-mode means in addition to stripmap SAR,the system can also work in wide swath mode like ScanSAR and TOPSAR,and high resolution mode like spotlight SAR and sliding spotlight SAR,therefore the earth observation capability of spaceborne SAR can be greatly enhanced.Besides,the classical imaging algorithms also need to be improved to match the update modes.Based on backgrounds above,the signal processing schemes of several hot SAR imaging modes were studied,which are as follows:1.The high resolution imaging algorithm of spaceborne sliding spotlight SAR was studied.Firstly,the relationship between azimuth time and frequency was analyzed.Then in order to directly apply the classical imaging algorithms to the sliding spotlight mode,the full aperture data was partitioned appropriately to sub-aperture to make sure the azimuth signal is unambiguous.To solve the problem of invalid traditional hyperbolic distance model for high resolution spaceborne SAR imaging,the ?k algorithm was studied which combined with one-step motion compensation along the line-of-sight direction,the imaging error from the aspect of motion compensation and imaging algorithms was minimized by using this algorithm.Finally,the mosaicking and azimuth compression of the sub-aperture data was completed by using BAS principle,and the detailed processing flow was given at the end of the chapter.2.The generalized polar format algorithm(GPFA)was applied to spaceborne TOPSAR.The special scanning mode of TOPS leads to azimuth aliasing both in time domain and frequency domain when imaging,so it is necessary to improve the traditonal PFA.Firstly the azimuth deramping function based on the center of rotation was used to reduce the doppler bandwidth to ensure the correct processing of the two dimensional interpolation.Then the space-invariant phase compensation was derivated and carried out.Finally,the azimuth scaling principle was adopted to solve the time domain aliasing problem.The simulation results show the validity of GPFA applied to TOPSAR.3.A wide swath imaging method of spaceborne circular-scanning SAR was proposed.As the antenna scans conically,it also scans periodically in each strip to get wider swath.The strip residence time distribution rule and imaging parameters of this mode were discussed.The imaging was completed in the sub-aperture and the geometric distortion correction based on the mapping relationship between the real geographical position and the imaging point was performed.Finally,the complete focused image can be obtained by image mosaicking.The imaging results show that under the premise of high image quality,the swath gets significantly increased.