New Signal Compensation Techniques For High Resolution SAR/ISAR Imaging

Synthetic aperture radar(SAR) and inverse synthetic aperture radar(ISAR) have the characteristics of working all time and all weather, long distance detection, wide swath and high resolution imaging, which shows the dramatically potential in the application of the microwave remote sensing. SAR and ISAR play an essential role in fields of national defense and economic. With the development of the information processing technique and the growing requirement of the radar application, radar images with high qualities are needed. In order to obtain a high-resolution and wide-swath radar image, it is necessary to explore the signal compensation techniques besides the imaging algorithm. In the most imaging algorithms for SAR and ISAR, the motional radar or targets are moving in an ideal track in the vacuum, and those motion parameters, such as the precision position, velocity and direction, are accurately known during the whole synthetic aperture time. While in reality, there exist the error and interference in the periods of radar signal transmitting, propagating, receiving and processing, which would degrade the final radar performance.In this dissertation, several key techniques for high resolution SAR and ISAR signal compensation have been studied, including the compensations of echo model error, the transmitting error and the motion error. Based on a new ananlysis of signal model for high resolution ISAR imaging, the signal compensation processing of high resolution ISAR is studied and the motion compensation research in high resolution SAR is developed. Finally, my work extends to the imaging algorithm of the new-type bistatic SAR imaging.The main content of the dissertation is summarized as follows:In the aspect of the echo model error compensation, an accurate ISAR echo model for fast moving targets is introduced by a comparison with the “stop-go” model. In such accurate echo model, the target’s motion during the time periods of pulse transmitting, propagating, and receiving has been taken into account. Some new problems have been brought on, which have been ignored in the “stop-go” model, for example the variedambiguity numbers and the drifted frequency center in the stepped frequency signal. These problems would lead to a range defocusing and a degraded resolution, which would be more and more salient with the increasing time bandwidth product(TBP) of the signal. To deal with these new problems in the accurate echo model, a stepped frequency synthetic preprocessing and synthesis algorithm for the fast moving target is presented, which is capable of obtaining a high resolution range profile(HRRP) and a well-focused ISAR image with large TBP.In the aspect of the transmitting error compensation, the research of the ionospheric effect on the radar echo is developed. As the random and dispersive nature of the ionosphere, it will severely affect the propagation and scattering of electromagnetic waves through it, especially in the spaceborne earth observation SAR and the ground-based space observation ISAR. The ionosphere model is constructed and the ionospheric effects are analyzed first. And then the way to evaluate the total electron content(TEC) from the received ISAR signal is explored based on the ionospheric contaminated phase. Finally, two estimation methods are proposed to compensate the ionospheric effect on SAR and ISAR signals: one is the method to estimate the TEC in the frequency domain, which can be applied to the spaceborne SAR and the ground-based ISAR. The other is the technique to estimate the TEC by the two-dimensional spectrum division, which is validated by the real experiment data from a ground-based ISAR. Furthermore, an estimation method based on minimum entropy principle is analyzed as well.In the aspect of the motion error compensation, the motion error compensation algorithms for the high resolution circular SAR and spotlight mode SAR in airborne platform are proposed, respectively. In the study of the circular SAR, the target’s instant slant range is derived in the presence of radar three-dimensional motion error, based on which the variance of slant range error with range and azimuth is analyzed in detail. By making use of the parameters of the inertial navigation system(INS), the expression of the variant slant range error is reached. Afterwards, the Chirp-Z transform is introduced to compensate the variant amplitude error and phase error. Finally, a well-focused scene image using range- and azimuth-dependent motion compensation algorithm for Circular SAR imaging is obtained. In the study of the spotlight SAR, based on the polar formatalgorithm and the Doppler rate estimation algorithm, a real-time imaging scheme for spotlight SAR based on Field Programmable Gate Arrays(FPGA) is proposed, in which the motion error phase is estimated from the echo data and compensated in azimuth wavenumber domain. The scheme is suitable for high resolution airborne spotlight mode SAR imaging without the high precision INS and it has been implemented in the hardware system with FPGA and DSP chips successfully. The real-time experiment result on the hardware system shows the validity and reliability of the proposed scheme.In the aspect of the new-type bistatic SAR imaging, the imaging algorithm for the hybrid bistatic SAR configuration is presented, in which the transmitter works in sliding spotlight mode and the receiver inverse sliding spotlight mode. Firstly, the two-dimensional point target spectrum with high order approximation is derived using the series reversion. Then the space-variant spectrum with range is obtained through approximating the analytical expression. The Doppler folding of the azimuth raw signal is effectively eliminated via a spectral analysis(SPECAN) process without any inefficient sub-aperture and interpolation operation, resulting in a well-focused full image in the Doppler domain. Simulation results confirm the validity of the imaging algorithm.