Research On Techniques Of SAR-GMTI For Distributed Satellite SAR Systems With Multiple Phase Centers

Synthetic aperture radar (SAR) combined with ground moving target indication(GMTI) can provide high resolution images of the stationary scene as well as thedetection and relocation of ground moving targets simultaneously, which has beenwidely used in many civilian and military applications. Compared with single channelSAR systems, multichannel SAR systems have the advantage that the cluttersuppression can be achieved by using the spatial degrees of freedom in azimuth, thusarousing extensive attention of the scholars in various countries. This dissertationaddresses some key techniques of GMTI for distributed satellite SAR systems withmultiple phase centers, such as the determination of the along-track baseline, the hybridand time-varying baseline preprocessing, the system design to make the GMTI becompatible with high-resolution wide-swath (HRWS) imaging, the ambiguous cluttersuppression and robust target motion parameter estimation. The main work can besummarized as follows:1. The possible temporal and spatial relationships between different phase centersin multichannel SAR systems are analyzed. The signal model of moving targets inmulti-channel SAR systems with alternate transmission and reception is established.Based on the theoretical derivation, the necessary condition for the existence of thealong-track baseline is indicated. And we can draw a conclusion that the along-trackbaseline between different phase centers whose azimuth positions are different fromeach other at different time is equal to the azimuth spacing between them after usinginterpolation to make them appear at the same time. This conclusion provides the basisto determine the along-track baseline for various multichannel formations. The GMTIperformance comparison with different antenna partitions and alternating strategiesshows that an appropriate antenna partition and alternating strategy is important inimproving the GMTI performance.2. A preprocessing method to deal with the hybrid and time-varying baseline indistributed satellite SAR-GMTI systems is proposed. The time-varying baseline makesthe correlation of SAR images decreases; and the hybrid baseline and squint make itdifficult to estimate the along-track baseline accurately. For the purpose ofcompensating the time-varying baseline, based on the geometrical configuration ofdistributed-satellite SAR systems, the total least squares algorithm is applied to fit thetrack equations of the satellites. And a new virtual parallel track is constructed with theparameters of the track equations. An equivalent constant baseline is obtained by the compensation of the path difference between the new and the original tracks. Then, theformula for calculating the along-track baseline in the case of hybrid baseline and squintis deduced, and an along-track baseline determination method based on ephemeris dataand radar echoes is proposed. Simulation results illustrate that the performance ofground moving target indication can be improved by the time-varying baselinecompensation and accurate velocity estimation and relocation of moving targets can beobtained by using the proposed method to determinate the along-track baseline.3. Using multichannel in azimuth to suppress the Doppler ambiguities allows forHRWS SAR. If the degrees of freedom in azimuth are used for the clutter suppression,the GMTI can be achieved. Therefore, a spaceborne multichannel SAR system has thepotential to offer GMTI and HRWS imaging capabilities simultaneously. However, inthis case, the GMTI performance may suffer from the Doppler ambiguity caused by theundersampling. To investigate the impact of Doppler ambiguity on GMTI performance,the multichannel signal models of the clutter and the moving target with Dopplerambiguity are derived in complex image domain. And the system design considerationsare given. Taking into account the blind velocity, the spatial ambiguity and thesignal-to-noise ratio (SNR) loss, the influence of pulse repetition frequency (PRF) onthe GMTI performance is dicussed in detail. Then a simulation example of PRFselection to optimize the system performance is provided. According to the theoreticalanalysis and the simulation example, an important principle is obtained that for the PRFselection, it is necessary to avoid the ambiguous clutter notches falling into the radialvelocity interval of interest due to the spatial ambiguity. Finally, the theoreticalinvestigations are verified by real data experiments.4. A new ambiguous clutter suppression method based on using multiplecharacteristics to select training samples with azimuth multichannel SAR systems isproposed. To deal with the ambiguous clutter suppression issue for a heterogeneousregion which contains two adjacent terrains having large reflectivity difference, such asthe near-shore water regions, the multichannel steering vector of the ambiguous clutteris derived, and the multichannel signal model in the image domain is established. Basedon the derived steering vector and the established signal model, the statisticcharacteristics of the magnitude and interferometric phase histograms combined withthe edge detection are used to select training samples, and then the adaptive matchedfiltering technique is used suppress the ambiguous clutter. The validities of the proposedmethods are verified by the simulation results.5. In practice, inevitable image coregistration error and channel phase mismatch will significantly degrade the estimation performance of the target radial velocity in theGMTI processing with multichannel SAR systems, making it difficult to relocate themoving target accurately. To overcome the influence of these non-ideal factors, a newradial velocity estimation method using the subspace projection algorithm with adaptivedata reconstruction is proposed. Based on the joint-pixel signal model, the Wienerweight vector is used to reconstruct the multichannel data vector of the pixel containingthe moving target. Then, the subspace projection algorithm is adopted to deal with theradial velocity estimation with this reconstructed single―snapshot‖data. The validityand robustness are verified by both simulations and real SAR data experiments.