Research On Low Frequency SAR/GMTI Techniques With Single/Dual-Channel Data

The low frequency Synthetic Aperture Radar (SAR) working at VHF/UHF bandcan imaging targets under foliages, which is an important way for the detection andsurveillance of conceal targets. The low frequency SAR system with the capability ofGround Moving Target Indication (GMTI) can detect both static and moving targets inthe open or under foliages, which has become one of hot points of SAR research field.Comparing with conventional high frequency SAR/GMTI system, low frequencySAR/GMTI system has lower Signal to Clutter Ratio (SCR) and is common to use thearray antenna with larger size and higher cost. At present, there is little research on thesingle or dual-channel low frequency SAR/GMTI system, which has smaller size andlower cost. To promote the practical application of low frequency system, sometheoretical and technical problems of single or dual-channel low frequency SAR/GMTIsystem are studied with the real data in this dissertation. The main contents of thisdissertation are summarized as follows:1. The characteristics of moving target and clutter in low frequency SAR arestudied. The conclusions in this part provide both a theoretical basis and a basis ofassumptions for the following research in this dissertation. Firstly, based on the derivedtwo-dimensional spectrum of moving target in SAR image, the unified derivation ofmoving target imaging algorithms is deduced and the characteristic of moving target atdifferent azimuth sub-look images is studied. The research results show the potential toimprove SCR of moving target in practical system. Secondly, based on thelow-frequency SAR/GMTI measured data, the clutter statistical distribution model inlow-frequency system is studied, the interference pattern of the G distribution modelfamily is perfected, and the electromagnetic characteristics varying with azimuthsub-look of the trunk and the internal clutter motion of foliage clutter are verified.2. The moving target detection algorithms based on the wide accumulation angleare proposed. Heavy computation load and the associated difficulties of moving targetin a sequence of sub-look images are the problems of the practical processing with theexisting low-frequency moving target detection algorithms. Taking full advantage of thewide accumulated angle in low-frequency system, two moving target detectionalgorithms based on multi-resolution images and the difference of sub-look images areproposed respectively. A novel moving target fast detection flow with low false alarm isproposed in later. These proposed detection algorithms have the good capability ofclutter suppression and can enhance the SCR of moving target, which are suitable forthe practical application in the airborne low frequency SAR/GMTI system.3. The moving target positioning methods based on the part of Doppler informationare proposed. Firstly, a precise positioning method is proposed to locate slow moving targets in low SCR. It can solve the problem of azimuth position uncertainty and lay thefoundation for the practical system to achieve the high-precision calibration of themoving target in SAR images. Secondly, two velocity estimation methods are proposedto solve the ambiguity of Doppler spectrum and anti-peak interference for fast movingtargets. They can improve the potential performance of surveillance of air targets inlow-frequency SAR/GMTI system.4. The Orthogonal Frequency Division Multiplexing (OFDM) signals in thesingle-channel low-frequency SAR/GMTI system is studied. Firstly, the ambiguityfunction of OFDM SAR is derived, which shows that the signal encoding has littleinfluence on imaging resolution. Secondly, the impact of the OFDM encoding term inthe actual SAR imaging processing is studied, and two algorithms based on a weightedwindow and the encoding compensation are proposed respectively, to suppress thesidelobe in range and to improve the azimuth compression performance of OFDM SAR.The proposed latter algorithm can achieve the similar imaging performance with that ofthe traditional linear frequency modulation signal, which solves the theoretical problemsof OFDM SAR imaging. Thirdly, a model of clutter suppression and moving targetdetection is built to overcome the low SCR and the multipath effect from the trunk inlow frequency system. This model achieves high performance of clutter suppression inthe range Doppler domain by single channel SAR. The detection method has a lowcomputation load and is suitable for real-time processing, which can be used in rapiddetection and tracking of ground moving targets. The performance of detection is betterthan that of the linear frequency modulation signal.5. The pre-processing techniques for the airborne low frequency SAR/GMTIsystemare are studied. Firstly, a Radio Frequency Interference (RFI) suppressionmethod in the range-frequency azimuth-time domain of interferogram is proposed forthe RFI of along track interferogram in the dual-channel low-frequency SAR/GMTIsystem. This suppression method can eliminate the weak RFI in the interferogram fromthe real data, avoid the extra error from independent operation between channels andmake up for the drawback of existing RFI suppression methods. Secondly, combiningwith the motion error compensation techniques, the proposed RFI suppression methodand the characteristics of errors between low-frequency channels, a completepre-processing flow is proposed for the airborne dual-channel low-frequencySAR/GMTI system. This flow can greatly improve the coherence of along trackinterferogram in dual-channel system and help Along Track Interferometry (ATI)achieve good detection results.The satisfied research results in this dissertation have been testified by the flyingexperiment data from an airbone low frequency SAR/GMTI system, They haveimportant references value to advance the practical application and development oflow-frequency SAR/GMTI system. The proposed methods can be used not only in multiple channel low frequency SAR/GMTI system, but also as the references to thedesign of high frequency SAR/GMTI system.