| Synthetic Aperture Radar (SAR), which has the characteristic of all-weather, day/night and long range, can enhance radar's information acquisition capability, especially the battlefield awareness ability, and has great value in both civilian and military applications.In SAR, high geometric resolution in range is obtained via transmission of large bandwidth pulses. On the other hand, high resolution in azimuth is the result of an intensive coherent data processing operation aimed at synthesizing an antenna array. In recent years, SAR is widely used in various platforms, such as the minitype airplane, helicopter and Unmanned Aerial Vehicle. Due to atmospheric turbulence, the SAR platform trajectory is often subject to small deviations from the ideal straight line, resulting in an incorrect interpretation of the geometry of the SAR image or a loss of focus in the image. To account for such errors, flight parameters are measured onboard with Global Positioning Systems (GPS) and Inertial Navigation Units (INS). However, the accuracy of GPS and INS available in our country is relatively low. Therefore, the research on raw data based motion compensation is of great importance for high resolution airborne SAR imaging.The conventional motion compensation method is mainly applicable for the broad-side imaging case, and assume that the beamwidth in azimuth is narrow enough. In addition, topographic variations within the scene is neglected during motion compensation. However, in many applications, these assumptions can not be satisfied. For example, the SAR system may be operated in the squnit mode, the beamwidth of the radar may be wide and the topographic variations within the scene may be severe. So it is worth analysising the limitations of the conventional motion compensation method.Compared with the conventional monostatic SAR, the bistatic SAR uses a separate transmitter and receiver, flying on different platforms, which enables the exploitation of additional information contained in the bistatic reflectivity of targets. The bistatic SAR also has many other advantages like: flexibility, long range, reduced vulnerability for military applications, ability to use multilevel interferometry, etc. However, these increased advantages are paid for by an increased complexity in designing bistatic SAR systems. Besides technical problems such as the synchronisation of the oscillators, the bistatic imaging algorithm and motion compensation method have still not been sufficiently resolved.The primary contributions of this dissertation, which is devoted to the above aspects, are summarized as below:1. The motion compensation method for airborne broad-side looking SAR is studied. Based on the analysis of the motion error model of broad-side looking SAR, a novel method for raw data based motion parameters extraction and motion compensation is presented, which can reduce the requirement for the INS. This method contains the following procedures: first, the raw data is divided along the azimuth direction. Second, the Doppler rate of each range bin of the sub-aperture data is estimated. Third, the motion parameters of the aircraft are estimated by the Weighted Least-Square method. Finally, the motion errors in the line of sight direction and the aizmuth direction are corrected respectively using the estimated parameters.2. The geometric distortion problem in motion compensation is analysed, then a method is presented for geometric distortion correction.3. The limitations of the conventional motion compensation method in the case of topographic variations, wide beamwidth and large squint angle are analysed. Then, the improved methods are presented for these cases. Also, the effect of the remaining error after motion compensation on the final SAR image is analysed.4. The limitation of the dominant scatter selection method in phase gradient algorithm (PGA) algorithm is analysed, then an improved method is presented. This method can adaptively select the best isolated scatters for the phase error estimation. It is shown that the improved PGA algorithm outperforms the original one especially for the scene in which the bright scatters are not isolated.5. An extended chirp scaling (ECS) algorithm is presented for the bistatic spatial invariant configuration based on series reversion and numerical computation. In this method, the two dimentional point target spectrum is first computated by series reversion. Then, an analytic expression of the range migration factor is obtained by numerical approximation of the bistatic parameters. Finally, the range migration correction is implemented by the Chirp Scaling operation. This algorithm can be considered as an extension of the monostatic ECS algorithm to the bistatic case. The proposed algorithm has the advantages of wide application scope, high accuracy and relativly low computation load, since no interpolation is required during the whole processing chain.6. Based on the analysis of the signal model, a novel frequency scaling algorithm (FSA) for the azimuth-invariant bistatic SAR is presented. This algorithm can be considered as an extension of the monostatic FSA algorithm to the bistatic case. High resolution imaging can be achieved by using this method even in the long baseline case. In addition, this algorithm combine the numerical computation method with the Deramp technique, thus the so-called azimuth spectral folding effect in the spaceborne case can be overcomed.7. Based on the idea of nonlinear chirp scaling, an improved FSA is presented for the bistatic SAR with high squint angles. This algorithm compensates the range-variant secondary range compression factor, thus improving the focus ability of the algorithm in the high squint angle case.8. The general bistatic SAR imaging method is investigated, especially for the Hybrid spaceborne/airborne bistatic SAR. It is pointed out that the accuracy of the point target spectrum derived by Taylor series expansion around the point of stationary phase is determined by both the velocities and the squint angles of the transmitter and receiver. As the satellite has a larger velocity than the airborne platform, and there may be a large difference between the transmitter's and the receiver's squint angle, the point target spectrum derived by this method is not accurate sufficiently. Then, based on the method of stationary phase and the bistatic geometry, the two methods are proposed to compute the point target spectrum of the hybrid spaceborne /airborne bistatic SAR. One is based on analytical expression computation, while the other is based on numerical computation. The first method has the advantage of high accuracy and low computation load, while the second method is totally accurate and has a relatively larger computational load.9. The motion compensation method for the bistatic SAR is studied. Based on the motion error model of the bistatic SAR, a method for estimating and compensating the three dimentional motion errors of the the transmitter and receiver is presented. In this method, the sum of the motion errors of the transmitter and receiver is directly estimated from the raw data. Then, these estimated parameters is used for motion compensation. The forward velocity variations of the transmitter and receiver is compensated by using a phase compensation function.
|
PDF Full Text Request