Study On Error Analysis And Compensation For Multi-Channel Airborne SAR-GMTI Systems

The multi-channel airborne SAR-GMTI systems can detect not only fast moving targets outside the mainlobe clutter spectrum but also slowly moving targets inside using the multi-channel techniques. Simultaneously, the velocity of the moving targets can be evaluated accurately, and the targets can be refocused and relocated accurately on the high definition SAR image. Consequently, it makes all day/night, all-weather, strategic and battlefield reconnaissance be possible, and has caused considerable concern by all the military powers in the world. To detect the slowly moving targets inside the mainlobe clutter spectrum, the multi-channel techniques, such as DPCA, ATI, STAP, etc, can be used for clutter suppression, but they require that the frequency response between channels, time-delay of the returns, space position, airspace response of the antennas, etc, must be matched highly. In actual systems, the mismatch of these is inevitable and causes degradation of the detection performance of the system, even deterioration. Therefore, how to compensate these errors becomes an extremely urgent problem for the improvement of the system performance.Supported by the experiment project of an institute, we combine with the engineering practice tightly and carry out researches into error compensation for the multi-channel airborne SAR-GMTI systems by means of theory analysis, formula derivation and real data validation. Several key achievements are obtained and mainly summarized as:(1) Firstly, the clutter model with all the errors is derived out, and the relationship between the correlation characteristic of the clutter in the channels and the frequency response error, time-delay error of the return, space position error and airspace response error of the antennas is analyzed. Then the origin of these errors and the compensation approaches in the data field is studied in the doctoral dissertation, which greatly raise the detection performance for the moving targets in the multi-channel SAR-GMTI system.(2) To deal with the problem that the frequency response error degrades the performance for clutter suppression, an adaptive blind channel equalization method based on the echo signal is presented according to the characteristic the ground clutter for down-looking multi-channel airborne radars, in which the calibration signals is not needed. It can evaluate and compensate not only the frequency response error between channels, but also the time-delay errors induced by the receivers and returns.(3) When compensating for the azimuth position error, an all-pass time-delay filters is used to reduce the loss of the target signal outside the mainlobe clutter spectrum, and the cancellation characteristic about the target signal after compensation is analyzed in emphasis. Theory analysis and experiments show, the cancellation curve of the moving targets is related to not only the distance between channels, aircraft's velocity and wavelength, but also the pulse repetition frequency (PRF). As a result, the optimized selection for the PRF is important to solve the problem of blind velocity.(4) While strong scattering moving targets exist in samples, the performance of adaptive clutter suppression is degraded greatly and the signal of moving targets is cancelled. To deal with this problem, a robust dual-channel adaptive DPCA method for clutter suppression is proposed based on the criterion of maximum signal-to-noise ratio. In this method, the amplitude and phase for the adaptive weight is evaluated separatedly, which avoid the influence on each other.(5) For the multi-point minus one-point and joint pixel methods, the information on the adjacent pixels associated with the reference pixel is sufficiently used for suppressing the clutter, which can compensate all the system errors effectively at the same time. The relationship with them is derived out and the performance is compared by the measured data, which shows that although more information is used for the joint pixel method, the detection performance for the moving targets is worse than that for the multi-point minus one-point method if the signal of the moving targets cannot be constrained effectively.