Study On Spaceborne/airborne Hybrid Bistatic SAR Imaging Formation In Frequency Domain

Spaceborne/airborne hybrid bistatic SAR (SA-BSAR), in which the receiver mounted on the aircraft receives target's reflected signals illuminated by the spacecraft borne SAR transmitter in the low Earth orbit, is becoming prospective in the field of Earth remote sensing and mapping. However, there are essential velocity difference and dramatic geometric asymmetry between the transmitter platform and the receiver platform in SA-BSAR. These characteristics lead to the azimuth variance in the system's geometry structure and in the platforms'relative position relationship, and they will in turn affect the imaging theories and algorithms. Therefore, traditional imaging theories and technologies, developed for monostatic SAR and azimuth-invariant bistatic SAR cases, could not be applied to SA-BSAR any more. In this dissertation, studies on key imaging theories and technologies in SA-BSAR are carried out. The main content and contributions are presented as follows:1. Taking the essential velocity difference and dramatic geometric asymmetry between the system's transmitter platform and the receiver platform into consideration, the fundamental theories on SA-BSAR's azimuth-variant imaging model and key imaging parameters are developed. These works will lay foundation on the system's image formation processing techniques, imaging performance analysis and spatial attitude optimization scheme.2. The existing methods to achieve the azimuth-variant BSAR two-dimensional spectrum's analytical expression can not be applicable to SABSAR. In this dissertation, an'Air-Phase'method to achieve analytical solution for SA-BSAR system impulse response is proposed. To overcome the difficulty of resolving analytical solution for the system's stationary phase point, the two-dimensional spectrum's phase is approximated by utilizing the characteristics of SA-BSAR. SA-BSAR signal model in two-dimensional frequency domain is developed by'Air-Phase'method and this model will lay foundation on designing frequency-domain imaging algorithms for SA-BSAR.3. Due to the essential velocity difference between SA-BSAR's platforms, the range cell migration (RCM) of SA-BSAR system echo signal is two-dimensional space-variant. The two-dimensional space-variant RCM model is derived both in two-dimensional frequency domain and range-Doppler domain. And the space-variant RCM's effect on imaging result is studied through simulation. These study results give insight into image formation processing in frequency domain for SA-BSAR.4. To handle high-precision imaging in narrow-coverage case, two kinds of space-variant RCM compensation methods, which are based on the one-order approximation of the two-dimensional space-variant RCM model in two dimensional frequency domain and in range-Doppler domain, are presented respectively. In the two compensation methods, Scaled Fourier Transform and Chirp Scaling are utilized, respectively. Accordingly, two high precision and computationally efficient frequency-domain imaging algorithms for SA-BSAR narrow-coverage case are developed.5. To realize high-precision imaging in wide-coverage case, a space-variant RCM compensation method, which is based on the two-dimensional space-variant RCM model, is proposed. Scaled Fourier Transform with variable scaling factor and partition of a whole imaging scene are utilized to compensate the space-variant RCM in wide-coverage case. A high precision and computationally efficient frequency-domain imaging algorithm based on this RCM compensation method is developed.6. With respect to the system geometry in the presence of motion error in SA-BSAR, quantitative analysis of the received data spectrum and effects on imaging result are carried out. To limit the resolution loss resulting from motion error, an effective motion compensation strategy is integrated in the imaging algorithm for narrow-coverage case.