Study On Spaceborne Multi-channel High Resolution And Wide Swath SAR Imaging

Spaceborne synthetic aperture radar (SAR) has been receiving more and moreattention because of its cloud-penetrating and day and night operational capabilities.Nowadays, SAR is widely used in fields of military reconnaissance, civil constructionand science research. However, traditional spaceborne single channel SAR systemsuffers from a tradeoff between the achievable resolution and swath width, i.e. theminimum antenna constraint. Fine azimuth resolution requires high pulse repetitionfrequency (PRF), while low PRF is utilized for wide swath. Fortunately, incorporatedwith digital beam-forming (DBF) processing, multi-channel spaceborne SAR systemsare able to overcome this limitation and yield high resolution and wide swath (HRWS)images.In this dissertation, some of key techniques for multi-channel spaceborne HRWSSAR system have been studied. The whole dissertation is composed of two main parts.In the first part, multi-channel spaceborne SAR system in azimuth is studied, which isone of the most typical systems for high resolution and wide swath SAR imaging. Insuch SAR systems, the low PRF, usually much lower than the instantaneous Dopplerbandwidth, is employed to avoid the range ambiguities and implement wide swath,which results in the ambiguous Doppler spectrum, and DBF is utilized to suppressDoppler ambiguity yielding HRWS SAR images. With the improvement of bothresolution and swath, the amount of raw data is increased greatly, imposing higherrequirement for satellite storage and transimission link. For such problems, multipleelevation beam technique for HRWS spaceborne SAR systems is studied. Thistechnique can substantially reduce the amount of data to be recorded and stored on thesatellite without deteriorating other performances, and provide a probable scheme forfuture spaceborne HRWS SAR imaging.The main work of the dissertation is summarized as follows:1. Azimuth multi-channel spaceborne HRWS SAR imaging techniquesIn Chapter2and Chapter3, the signal model of multi-channel spaceborne HRWSSAR in azimuth is analyzed, followed by two typical imaging methods. For the specialcharacteristics of azimuth multi-channel spaceborne HRWS SAR system, the followingworks have been done.A common echo model based on three dimensional coordinates for spacebornemulti-channel SAR systems is built with the consideration of both along-track and across-track baselines between transmitter and receivers, as well as the effective phasecenter (EPC) phase compensation equation. Almost all the methods for the azimuthmulti-channel SAR imaging assume that the echoes received by each channel can beregarded as that received by the reference channel with an along-trackbaseline-dependent time delay after certain phase compensation. The phasecompensation, however, has not been given in a general case. Besides, neither theacross-track baseline between transmitter and receivers is considered, which is of greatsignificance to distributed high resolution InSAR systems, nor the sensor orbitinformation after focusing is given, which is the basis for the interferometry and targetlocation. In this dissertation, a common EPC phase compensation method based onthree dimensional coordinates of transimitter and receivers is given, considering thephase terms arising from both the along-track and across-track baselines are given, andthe residual phase error is analyzed too. The experiements show that when cross-trackbaseline between the reference channel and other receivers exists, the compensatedphase is varied with range swath and target elevation. If the phase error caused by spacevariance cannot be ignored, the data can be compensated with the assistance of coarsedigital elevation model (DEM) after range compression. The computer simulationconfirms the accuracy of the method.Regarding the multi-channel spaceborne SAR system in azimuth, theperformance of the space time adaptive processing (STAP) approach applied to HRWSSAR imaging is investigated. The multi-channel transfer function method and theSTAP-based approaches are two typical algorithms for suppressing Doppler ambiguities.The performance of the reconstruction algorithm has been well analyzed in variousliteratures, and demonstrated by the ground-based, airborne and spaceborne campaigns.While, there is no literature discussing the performance of STAP-based approach indetail. In this dissertation, the phase preservation of STAP is confirmed by analyzingthe position of imaging EPC of output data after Doppler suppression. It shows thatafter Doppler ambiguity suppression, the echo can be regarded as unambiguity ones thatobtained by the reference channel with higher PRF, and the radar positioncorresponding to each azimuth sample time is determined by the reference receiver. Thephase preservation of SAR imaging guarantees the following interferometry processingand target locating etc. Besides, two important parameters, signal to noise ratio (SNR)scaling and azimuth ambiguity to signal ratio (AASR), are evaluated, and comparedwith the multi-channel reconstruction method. The derivations of SNR scaling andAASR here are more legible and comprehensible than those introduced in other literatures. The numerical analysis is confirmed by the simulated results, which showsthat the STAP-based method has a better performance than other methods when PRFdeviates from the uniform sampling.The channel errors and their effects are analyzed for multi-channel SAR systems inazimuth. Adopting DBF technique to suppress Doppler ambiguity requires that thecharacteristics of each channel are identical. In practice, however, for someenvironmental factors, the channel errors are unavoidable. The mismatch amongchannels will degrade the performance of DBF. In addition, the limited precision of themeasurement will also cause errors. The channel error factors are analyzed in thisdissertation and then decomposed into channel gain, phase and along-track positionerrors according to their effects on DBF. The influence of channel errors on HRWSimaging is analyzed in detail and confirmed by computer simulation experiments. Theresults show that the effect of channel phase error is great, the effect of the channel gainerrors can be compensated by simple channel balancing, and the influence of thealong-track position errors is little. The STAP-based method has a better ambiguitysuppression performance than the reconstruction approach because of its capability ofplacing nulls in the directions of the interferences.Two novel methods are proposed to estimate channel errors for multi-channelHRWS SAR systems in azimuth. In practice, the channel errors are inevitable andshould be compensated in order to improve the performace of HRWS SAR imaging.Therefore, two channel error estimation methods for azimuth multi-channel SARsystem are presented: the Signal Subspace Comparison Method (SSCM) and AntennaPattern Method (APM). SSCM is based on the fact that the space spanned by the signaleigenvectors is equal to that by the practical steering vectors. The signal subspace isobtained by eigen-decomposing the echo covariance matrix, and then compared with thetheoretical signal subspace that derived from the system parameters to obtain the phaseerror. This method has great advantages of light computational load and high accuracy.Furthermore, it has no requirement that the SAR systems must operate in rightside-looking mode. The APM incorporates with the antenna patterns to estimate thechannel errors directly without matrix decomposition and inversion processing, which isvery efficient, but only suitable for uniform distributed scenes. Both the theoreticalanalysis and experiments demonstrate the effectiveness and efficiency of these twomethods.2. Elevation multi-channel spaceborne HRWS SAR imaging techniques In Chapter4, the novel HRWS SAR imaging technique is proposed. With theimprovement of resolution and range swath, the amount of data is increased greatly andsubsequently impose strict requirement on satellite storage and transmission link. Inorder to solve such problem, the detailed system design scheme and processing methodbased on multiple elevation beam technique for HRWS imaging is presented as well asits system performance such as range ambiguity to signal ratio (RASR). The wideimage swath is illuminated by a sequence of narrow and high-gain antenna beams. Thewhole antenna plane transmits a narrow beam to illuminate a far-range and subsequentlyproceed to the near range, and all the channels receive the radar echoes simultaneously.As a result, the echoes from different subswaths will overlap in the receivers, therebythe amount of data to be recorded and stored on the satellite will be reduced. All theechoes from different subswaths can be separated from each other by DBF and yieldingthe wide swath SAR image. For terrain with strong topographic variance, the subbeamscan be separated with the assistance of coarse DEM. The experiment shows that theerror caused by DEM accuracy is neglectable. Finally, the simulation data confirms thevalidity of the method.