Study Of ISAR Imaging Algorithm For Complex Motion Target

Inverse Synthetic Aperture Radar (ISAR) imaging has significant practicalvalue on both civilian and military for the characteristic of providinghigh-resolution radar images under the conditions of all-weather, all-time, longdistances, different frequency and different polarization, which greatly improves theability of radar to obtain information. In recent years, with the ISAR imagingtechnology continues to develop, ISAR imaging technology of steady targets isbecoming more and more mature, so more and more researchers are focusing onISAR imaging study of complex motion targets.According to the radar imaging theory, radar imaging obtains a high rangeresolution by transmitting large bandwidth signals and obtains a high azimuthresolution by the coherent accumulation of the Doppler signals caused by therelative movements between the scattering points on targets and the radar. However,when the target has complex motions, it’s difficult to get long time coherent signals,which means that the coherent imaging angle of rotation is too small, thus theimaging resolution of complex motion target will be limited. In this paper, a seriesof exploratory work on the ISAR imaging technology of complex motion targets isdone and a series of great results are achieved.Firstly, the basic principles of ISAR imaging are discussed, including themathematical model of ISAR imaging, linear frequency modulated(LFM) signalsand pulse compression technology, ISAR imaging resolution on the twodimensions-range and azimuth, basic ISAR imaging algorithms, motioncompensation techniques. Besides, a novel range alignment method-the minimumvariance method was proposed. At last, the simulated and measured data wereprocessed to validate the basic knowledge of ISAR imaging and the minimumvariance method discussed before.Furthermore, three ISAR imaging methods of complex motion targets arestudied, as follows:1) Cubic Phase Function (CPF) Parameter Estimation MethodIf the radar transmits linear frequency modulated (LFM) signals and the targethas complex motions, the echo signal will have a higher order phase, which willcause the radar images obtained using the traditional Range-Doppler (R-D) methodblurring. Cubic Phase Function (CPF) was proposed in recent years as a newtechnology to estimate the signal parameters. It can be used for the cubic phasesignal estimation and can be extended to estimate higher order phase signals, so itwill be very promising to use CPF for ISAR imaging. This paper presents a new ISAR imaging method based on CPF parameter estimation. With this method, theparameters of each signal component of the target echoes can be estimated, and thenthe estimated parameters can be used for target echoes demodulation to eliminatethe high order phase terms, then the focused ISAR image can be obtained throughthe two-dimensional Fourier transform of the demodulated signals. Simulation andmeasured data processing results show the effectiveness of this method.2) Compressive Sensing MethodCompressive Sensing technique is also a new signal processing techniqueproposed in recent years. It makes use of the sparsity of the signal for signalsampling and samples signal with a much lower rate than the traditional signalsampling theory through non-correlated measurements, which greatly reduces thedata rate, and the origin signal can be recovered from the signal samples with a highprobability. The non-correlated measurement process of Compressive Sensingtechnique can effectively reduce the raw data rate of the radar imaging system, thussolves the problems of collection, transmission and storage of large amount of data.Compressive Sensing technique is able to reconstruct the origin signal from a smallsignal sampling, so we consider applying it to the imaging of complex motiontargets to compensate for the shortcoming of lack of coherent signals caused by thecomplex target motions. The paper proposes a new ISAR imaging method ofcomplex motion targets based on Compressive Sensing technique and validates itthrough simulation and measured data processing.3) Distributed ISAR MethodDistributed ISAR imaging method is another new ISAR imaging techniqueemerging in recent years. The technique’s basic principle is to emit electromagneticwaves from imaging radars carried by a group of formation platforms, so as to gethigher azimuth resolution than single radar by joint processing of the echoes inorder and using the relative rotary motion between the target and the radar. Thefundamental reason is each platform is able to observe the single scatter of theimaged target from different range of viewing angles, by appropriately exploitingtogether the data collected by all platforms, the scatter is globally observed from amuch wider observation angle and therefore an ISAR image with a higher azimuthresolution can be achieved. So, distributed ISAR imaging technique can be appliedto a complex motion target imaging to compensate for the low azimuth resolutioncaused by the small coherent imaging angle. This paper presents a distributed ISARimaging technique of complex motion targets, and applies it to simulation andmeasured data processing, and the imaging results show that this method iseffective.After the above study, we solve the problems in the ISAR imaging of complexmotion targets with good results. The results show that all the three methods we propose have good practical value.