A Study Of SAR Imaging And GMTI For HRWS SAR System With Multi-Channel Azimuth

Synthetic Aperture Radar (SAR) is a kind of2D high resolution imaging radar，which is based on the principle of synthetic aperture and pulse compression technologyas the theoretical basis. SAR is a great achievement of modern radar with thecharacteristics of all-weather, all day and night, and long range. Radar imagingtechnique can enhance information acquisition ability of SAR, and provides a hithertounknown opportunity for target indication. SAR therefore exhibits great value in allmilitary, defense and civilian applications. As an advanced imaging system, SAR canprovide the two-dimensional image of a scene with good spatial resolution and highradiation measurement precision. The study of GMTI is developed based on SARimaging technology. Since traditional SAR radars only simply take the―picture‖of astationary scene, however, moving targets will be displayed on the SAR image of astationary scene as blurred images. A moving target with a radial velocity will exhibitdisplaced from its true position on the SAR image, and the motion in the azimuthdirection will cause its image to be dispersed along azimuth. In practical applications,especially in military applications, it is always expected to achieve Ground MovingTarget Indication (GMTI) by using SAR sensors and to relocate ground moving targetson the SAR image of the scene. So GMTI is the basic function must have for SARsystem. The technique is called SAR-GMTI. Since along-track multichannel SAR canprovide more degrees of freedom than single channel one. For the realization of GMTIfunction, the radar array along-track liner distribution is the optimal configuration.Along-track multichannel SAR has the powerful ability to suppress the clutter, todistinguish moving targets from the clutter coming from land or sea and to accuratelyestimate the motion parameters. Thus it has been used extensively for air-to-groundsurveillance and reconnaissance.In recent years, many countries in the world have been making great efforts todevelop spaceborne/airborne Ground Moving Target Indication (GMTI) systems basedon multi-channel SAR, research on new SAR-GMTI theories and explore highlyefficient detecting and locating technology. Based on the extensive investigation of theSAR-GMTI developments over the world, this dissertation presents a detailed researchand analysis on SAR-GMTI algorithms and error estimation, with the emphasis onimplementing SAR-GMTI function for a side-looking multi-channel SAR, includingchannel phase errors estimation, subband error estimation for splicing into a largerimage of SAR imaging, GMTI and location, and parameters estimation. Based on the spaceborne multi-channel HRWS SAR systems as the object of study,the main content of this dissertation is summarized as follows.1. Combined with GMTI, multichannel SAR system can complete moving targetindication and parameters estimation at the same time for large area imaging. In themoving target indication, the required characteristics of each channel must becompletely consistent. However, in practical situations, characteristics of each channelcannot be completely consistent because of various reasons. These non ideal factors willseriously affect the moving target indication effect. In order to solve this problem, twomethods are proposed to estimate channel phase errors for multi-channel SAR andGMTI system. The first method is based on the linear relationship between the cluttereigenvectors and its steering vectors. For the second method, the received echoes areresampled in azimuth, and the channel errors are obtained based on the fact that thespace spanned by the signal eigenvectors referred to as the signal subspace is equal tothat by the practical steering vectors. The simulated and ground-based real datademonstrate the validity of the two methods, and show that the second method hasbetter performance.2. SAR system can obtain high range resolution image by transmitting linear FMsignal. As the resolution requirements increasing, the bandwidth of signal also increases.The common solution for transmitting a large bandwidth signal is to transmit severaldifferent carrier frequency subband signals, then compose each subband signals bymeans of digital signal processing to obtain the large bandwidth signal. In practicalsituations, each subband signal is not consistent due to the unavoidable existence ofvarious errors in system hardware, and then the result will influence the quality of thesynthesized signal. So it is necessary to estimate and compensate for each subbandsignals. In this paper, a novel method is proposed to estimate the sub-band errorsdepending on the overlap spectra between sub-band signals, and compensates themismatch between sub-band signals when we reconstructing the spectrum of a wideband signal by stepped-frequency chirp signals. This method estimates theamplitude-phase error and the time delay error between sub-band signals by fitting theinterferometric phase of the overlap spectra components. Besides, the window foreliminating the ghost images caused by the overlap spectra is proposed.3. The development trend of SAR imaging is to simultaneously obtain the HighResolution Wide Swath (HRWS) SAR image and moving target indication. However,spaceborne single-channel SAR system is restricted by the minimum antenna area. In order to meet the qualification of wide swath with none range ambiguity, the pulserepetition frequency can not be too high. But in order to meet the qualification of highrange resolution, the system must operate at high pulse repetition frequency. So it isdifficult to simultaneously obtain wide swath and high range resolution. In view of theabove problems, this dissertation discusses in detail how to use the multi-channel SARsystem for moving target detection in the presence of Doppler ambiguity case. A newmethod based on the space-time spectrum of GMTI for spaceborne multi-channelHRWS SAR system is presented, which is according to the space-time spectrumrelationship between moving target and clutter to contruct azimuth matching function.Firstly, the method utilizes beamforming for clutter suppression after azimuthmatching filtering, and then estimates moving target direction by fitting for the slantrange of the moving target. Secondly, focusing for the clutter suppressed data isperformed to obtain ambiguous images of the moving target, then all ambiguousmoving targets are obtained by Constant False Alarm Rate (CFAR) detectiontechnology. Finally, this method detects real targets according to the spatialrelationships of fuzzy images and motion direction estimated.4. The development trend of SAR is to simultaneously obtain high resolution inazimuth and High Resolution Wide Swath (HRWS). Based on the GMTI technology, amotion parameter estimation method for spaceborne multi-channel HRWS SAR systemsis presented. Firstly, the space time adaptive processing is adopted for cluttersuppression, then the motion trail and true azimuth position of moving target can beobtained after range compression for clutter suppression data. Secondly, focusing isprocessed. The real target is decided from ambiguous images by fuzzy Dopplerrelationship, and the azimuth shift of moving target caused by speed along the line ofthe sight direction is obtained. Finally, the motion velocity along the line of the sightdirection of moving target is obtained. The advantages of the method are its lowcomputational burden and high accuracy.