Research On Challenge Technologies Of MIMO-SAR

Synthetic aperture radar (SAR) has already become the advanced science and technology in the field of remote sensing, and received wide attention by scholars of many countries, which plays an important role in military scout and national economy. As two strong application demands for SAR, the high-resolution and wide-swath imaging as well as the moving target detection and imaging is always a hot topic in the field of radar imaging. However, there exists a tight tradeoff between azimuth high-resolution and wide-swath for the conventional single-channel SAR, which is also difficult to achieve ground moving target indication (GMTI) emerged in the strong background clutter effectively. Therefore, some multi-channel SAR systems with single-transmitting and multi-receiving antennas, as well as corresponding algorithms are proposed in succession, where system degrees of freedom (DOF) is determined by receiving channel number. However, the antenna size and number of receiving channels are restricted by moving platform size and load, besides there exists strong demands on multifunction design for the advanced SAR, so novel thinking, theory and techniques are needed to be proposed urgently.MIMO (multi-input multi-output) radar is a novel system proposed in recent years. Much more equivalent channels and DOF than the employed antennas can be obtained by transmitting multiple waveforms simultaneously that may be correlated or uncorrelated with each other. Therefore, the diversity gains are offered by MIMO radar in detection, estimation, imaging and etc., so that the radar performance may be significantly improved.MIMO-SAR, as a novel imaging radar, is an innovative concept proposed recently in the field of radar sensor technology, which integrates the advantages of MIMO radar and SAR. The system DOF can make further increase by utilizing the multiple transmitting elements and receiving elements, and thus MIMO-SAR provides an effective way to improve the performance of conventional SAR. However, as a new branch of radar imaging, researches on MIMO-SAR are starting, and many key problems are needed to be studied and improved deeply. Around this guideline, our research work can be described as followsAiming at the demands on high-resolution, good correlation properties and low probability of intercept for orthogonal waveforms of MIMO-SAR, two novel kinds of orthogonal wideband hybrid-coding waveform set are proposed, namely polyphase signed-chirp (PSC) coding waveforms and polyphase discrete frequency (PDF) coding waveforms. Compared with conventional orthogonal polyphase coding (PPC) waveforms, the higher resolution and better correlation properties can be obtained by PSC waveforms. Compared with conventional orthogonal discrete frequency coding (DFC) waveforms, the PDF waveforms have better correlation properties. In addition, according to the designed rule for uniform equivalent spatial samplings and maximum system DOF during a transmitting and receiving (T/R) duration, an optimal uniform linear array configuration for MIMO-SAR is also proposed.Aiming at the equivalent phase center error, the analytic expressions are derived quantitatively, and then the influence on the azimuth imaging quality is studied. It is shown that the periodical quadratic phase error is a key factor in the equivalent phase center error, and it can bring about the"spurious peaks"effect on azimuth image for MIMO-SAR. In addition, based on evaluation of SAR image quality, the equivalent phase center error bounds in MIMO-SAR are proposed, which may provide reference to system design and successive signal processing for MIMO-SAR.Aiming at the demand on high-resolution and wide-swath imaging for static scene, two imaging schemes for frequency divided MIMO-SAR and code divided MIMO-SAR are researched, respectively. In order to improve separation capacity of the mixed receiving echoes and successive imaging quality, the optimal receiving filter is considered and used for code divided MIMO-SAR. The imaging indexes are compared between MIMO-SAR and multiple phase centers SAR (MPC-SAR), which show that the tradeoff between azimuth high-resolution and wide-swath can be solved by MIMO-SAR more effectively than the existing MPC-SAR. In addition, the equivalent spatial samplings DOF tradeoff schemes for MIMO-SAR are proposed to realize the multifunctional design.Aiming at the demand on moving target detection and imaging, a novel equivalent channel signal model of moving target for MIMO-SAR is proposed firstly. Influenced by radial velocity of moving target, a periodically modulated signal will be introduced into the signal model of MIMO-SAR, and then there will also appear the special"spurious peaks"effect in azimuth image. In order to suppress the"spurious peaks"and background clutter, three signal processing methods for MIMO-SAR are proposed based on displaced phase center antenna (DPCA), along track interferometry (ATI) and clutter suppurssion interferometry (CSI) techniques, respectively. The first method will suppress strong background clutter in the rawdata domain, and the"spurious peaks"are eliminated by compensating the error signal caused by radial velocity of moving target. The second method will eliminate strong background clutter in the image domain, and a new way to suppress"spurious peaks"using clutter is carried out, where the magnitudes and interferometry phases of"spurious peaks"can be weakened by modulating system parameters and using strong background clutter, respectively. The third method will also suppress strong background clutter in the rawdata domain, and the integration of detection, location, velocity estimation and imaging for moving target will be achieved, where location precision can be improved by using the infermetric phases of"spurious peaks".