Study Of High-resolution SAR And New Type SAR Imaging

Synthetic Aperture Radar (SAR) imaging is a powerful and well established microwaveremote sensing technique which can be used for long distance imaging and enables highresolution measurements of the earth surface independent of weather conditions and sunlightillumination. It is producing high resolution and high quality images that SAR imagingpursuits. Compared with low resolution SAR, high resolution SAR data needs more acurrateimaging formation model and more precise imaging processing. This dissertation is devotedto imaging for High Resolution SAR on High-Speed Platform, imaging for wide field andhigh resolution spaceborne spotlight SAR, fast imaging for circular SAR, imaging and motionparameters estimation for moving targets in high resolution SAR system with wide swath. Themain research results are as follows:1. Echo Model Analyses and Imaging Algorithm for High Resolution SAR on HighSpeed Platform“Stop-go-stop” approximation of radar carrier simplies the processing of SAR imaging,which is widely used in the SAR systems. There exist two kinds of neglected motion in thegeometric configuration of SAR based on the “stop-go-stop” approximation: the motionoccurring between transmission and reception of a pulse and during the interval transmissionand reception of a pulse, so that the imaging processing based on the “stop-go-stop”approximation is not accurate. With the increasing requirements on high resolution imaging,the errors brought by the “stop-go-stop” approximation may be intolerable for SAR imaging.The errors caused by the “stop-go-stop” approximation are introduced, including the quadraticphase error in range, the range cell migration error, the cubic phase error in azimuth and thelocation deviation of the targets, and then the application scope for the “stop-go-stop”approximation is presented correspondingly. A reference system of the spaceborne spotlightSAR with0.2m azimuth resolution is given for simulation to illustrate the intolerable errors.In order to make sure the coverage performance of the system and reduce the pressure todata transmission system, the pulse repetition frequency (PRF) should not be too high, whichmakes it difficult to exceed the Doppler bandwidth and will induce azimuth spectrum aliasing.In order to use the precise imaging processing of the stripmap SAR, the azimuth spectrum ofthe data should not be aliasing. Based on the accurate echo model, a spotlight imagingalgorithm for the high resolution SAR on high-speed platform is presented. The proposedalgorithm takes advantage of the “two-step SAR focusing strategy” and the rangefrequency-dependent reference function is adopted in the azimuth preprocessing. The simulations for the azimuth preprocessing illustrate the advantage of using the rangefrequency-dependent reference function in the azimuth preprocessing for high resolution SARon the high-speed platform. Point target simulations show the performance of the proposedimaging algorithm.2. A modifiedω-k algorithm for wide field and high resolution spaceborne SAR imagingAfter considering the compensation of the errors brought by equivalent-squint rangemodel and the errors brought by the “stop-go-stop” approximation, this dissertation producesa modifiedω-k algorithm for wide field and high resolution spaceborne spotlight SAR. Thevariation of the equivalent-velocity and the equivalent-squint angle along the range directioncannot be taken account into the Stolt mapping of the traditionalω-k algorithm. Before theproposed algorithm is derived, the error of the range migration correction (RMC) by using theuniform equivalent-velocity in the Stolt mapping is analyzed, and the necessity for modifyingthe traditional Stolt mapping is presented. In order to process the wide field and highresolution spaceborne SAR data precisely, by utilizing the relationship between the range andthe equivalent velocity, the Stolt mapping of the classical ω-k algorithm is modified. Themodified Stolt mapping takes the range-dependence of the equivalent velocity into account, sothat the range cell migration is corrected accurately. Using the modified Stolt mapping, themodifiedω-k algorithm is presented, which guarantees the uniform image quality along rangeand is validated by the simulation.3. Fast imaging processing of circular SARBecause of the special track of circular SAR, the imaging algorithms of SAR in lineartrack cannot be used in processing the circular SAR data directly. However, the responsefunctions of the circular SAR systems can remain the consistency by changing the sameaspect angle both of the target and the starting position of the radar. Because of thischaracteristic, circular SAR can be processed in the azimuth frequency domain by using thefast imaging algorithm of SAR in linear track for reference. In this dissertation, the exactsignal spectrum expressions of circular spotlight SAR and circular looking around SAR arederived. By using the reasonable approximations of the exact signal spectrum expressions, thefast imaging algorithms are presented. The simulations show the good performance of theproposed algorithms.4. Motion parameters estimation and imaging for moving targets in SAR system with alow PRF sampling rateBased on the CS theory, a novel method is proposed for the high resolution imaging andthe motion parameters estimating of the ground moving targets in the SAR system with lowPRF sampling rate. Firstly, the clutter cancellation is used to acquire the information of the moving targets in the range compression data domain. Secondly, after the azimuth dechirpprocessing, the range-walk of each target envelope is corrected by the Keystone transform andthe sparse range-walk correction filtering. Thirdly, the redundant base matrix is constructedaccording to the azimuth signal expression, and then, based on the CS theory, the motionparameters estimation and imaging for the sparse targets can be resolved by finding thesparsest solution of the optimization equation. The simulations show the performance of theproposed algorithm.