Study On New Technologies For SAR/ISAR Detection And Imaging Of Moving Target

As an all-day/night, all-weather and far range active observation system, radar imaging technique plays an important role in both military and civilian applications, and thus it is the landmark in the development of radar. Synthetic aperture radar(SAR) detection and imaging of moving targets is widely used in the battlefield reconnaissance, disaster assistance, and so on. Inverse synthetic aperture radar is mainly used for air, space and ocean targets imaging. Both of them can enhance the information acquisition capability and improve the target identification performance to a great extent.The primary contributions of this dissertation, which are devoted to the above two aspects, are summarized as follows:The echo energy of weak ships is weak and the motion parameters of ships are intractable to be estimated accurately, both of them resulting in the difficulty of SAR detection and imaging of these targets. In order to solve this problem, a sandglass transformation is proposed. It can integrate the LFM signals coherently as well as suppress the cross terms of Cohen class time-frequency distribution without loss of resolution under the condition of knowing no motion parameters of targets. Hence, the sandglass transformation based method is effective to detect and generate high resolution images of weak ships. Numerical and experimental results confirm the effectiveness of the proposed method.Quadratic frequency modulated(QFM) signal exists widely in nature and is adopted in many radar systems and navigation systems. However, due to its complex form, the detection and parameter estimation of them are difficult. We first analyse the similarity between the WVD of LFM signals and TCD of QFM signals, and then introduce the sandglass transformation into QFM's detection and parameters estimation. In addition, the difference between sandglass transformation of LFM signals and that of QFM signals is discussed. Simulation results confirm the effectiveness of the proposed method.For SAR imaging of multiple ships, different ships move with different velocities in different directions, i.e. different scatterers have different Doppler histories, which means that we can not obtain a high quality SAR image by only compensating a uniform phase term. In practice, the echo energy of ocean is weaker compared with that of ships. Therefore, the ships can be regarded sparse relative to radar imaging area, and thus a new approach based on compressed sensing (CS) is presented for SAR imaging of multiple ships. We convert the multiple ships imaging into a problem of sparse signal reconstruction with certain orthogonal basis, and hence the sparse reconstruction of CS can be fulfilled and a theoretical upper bound of the cross-range resolution is obtained. Real data results verify the effectiveness of the CS imaging framework.For inversed synthetic aperture radar (ISAR) imaging of remote maneuvering targets via linearly stepped-frequency modulated (LSFM) waveform, a digital stretch technique and a phase coherence compensation method are proposed to achieve high resolution range profile (HRRP) synthesis. By using this digital stretch technique, the sampling rate can be reduced dramatically and the coherence of the echo can be easily recovered. Since the motion estimation error is inevitable for conventional parameter estimation methods, the residual Doppler will contribute to the phase discontinuity at the juncture between the neighboring sub-pulses, which may blur the HRRP to some extent. By using the phase coherence compensation method, the residual Doppler can be compensated easily, namely, the phase difference is eliminated. Thus, a good HRRP can be obtained by means of time domain sub-pulses synthesis technique. Finally, a well-focused ISAR image can be achieved after the conventional ISAR imaging algorithm. Experimental results confirm the effectiveness of the proposed methods.In order to achieve a 3D image of target by using InISAR technique, a high quality ISAR image of target must be obtained firstly. However, since the phase history of maneuvering target is non-linear, the ISAR image may be blurred seriously by using Fourier transform, while the time-frequency distribution class methods may result in the loss of resolution or disruption of original phase or introduction of extra cross terms. In order to achieve a high quality ISAR image of maneuvering target(isolated scatterers), a super resolution ISAR imaging algorithm based on sparsity of target is presented. The presented method can not only achieve high quality super resolution image(isolated scatterers) of target, but also preserve the original phase information of target. Then, the high quality 3D image of target can be obtained by using conventional InISAR 3D imaging algorithm. The effectiveness of the proposed method is validated by both numerical and experimental results. According to the problems of real data obtained firstly in our country, we suggest several research aspects to be worthy of special attention in the future. In addition, we further study the super resolution ISAR imaging and InISAR 3D imaging under the condition of sparse frequency-short aperture length. The simulation results confirm the effectiveness of the proposed method. Based on the characteristics of ISAR imaging, an ISAR imaging real-time processing system is designed. Which is designed composed of hardware block, data communication and algorithm realization. The design conceptions of the system are listed as follows: The hardware block adopts the technique of distributed parallel processing, which is in accordance with the processing of ISAR imaging. The design of data communication adopts serial to parallel conversion and switch module associated with link port, which makes the data transmission from one node to any other with high efficiency and flexibility. A real-time processing algorithm is designed according to the experimental parameters and target characteristics, and the task allocation of the real-time signal processing on the digital signal processing board is completed on the base of computational burden analysis. Finally, Experimental results and analysis validate the effectiveness of the real-time ISAR imaging system.