Research On The Imaging Technology Of Spaceborne Multimode Synthetic Aperture Radar

Spaceborne synthetic aperture radar(SAR), as a high resolution microwave remote sensing method, is valuable in both military and civil applications. Nowadays, different applications bring more and more requirement in terms of resolution, swath width and polarization. To satisfy multiple requirements in a satellite, having several modes becomes a trend of the spaceborne SAR. Spaceborne multimode SAR means that the spaceborne SAR can work on Scan SAR, spotlight, terrain observation by progressive scans(TOPS)/inverse TOPS and sliding spotlight modes beside the stripmap mode. These modes can compromise the swath width and resolution through the management of the timing and beam pointing, which can greatly enrich the mapping capacity of the spaceborne SAR. At present, the research on single mode spaceborne SAR is relatively mature, but the research on spaceborne multimode SAR is usually in allusion to one or some similar modes. This thesis aims to research the multiple modes with an whole train of thought, through the generic modeling of the signal of the spaceborne multimode SAR, researching the generic imaging algorithm, and the generic methods of imaging performance analysis and parameter design, and tries to establish the theory framework of the spaceborne multimode SAR imaging technology. Based on this, subjecting to the problems at present and the developping trend in the future, it proposes 3 new working modes, the district TOPS, the full-polarization TOPS and the combined TOPS mode. The major research content of this thesis is:Chapter II researches the signal modeling and simulation of the spaceborne multimode SAR. Firstly, principles of each mode is introduced, and then based on the stripmap signal model; the generic signal model of the multimode spaceborne SAR is established. Based on the established signal model, the signal feature of each mode is analyzed, and the similarities and differences in the time-frequency relationship of the modes are summarized, and the requirements in imaging algorithm and the performance of each mode are preliminarily discussed. At last, spaceborne multimode simulator is designed and realized. The simulation examples of high resolution spotlight mode with point array targets and wide swath TOPS mode with distributed targets are provided. The research results of this chapter established the theory and simulation base of the research in imaging algorithm, performance analysis and parameter design.Chapter III researches the imaging algorithm of spaceborne multimode SAR. Firstly, the principle of the time/frequency domain extending technology are studied, and the method of the spaceborne multimode SAR imaging based on the time/frequency domain extending technology is provided. Secondly, the principle of the fractional Fouriour transform/ time-frequency transform is studied and the method of the spaceborne multimode SAR imaging based on the time-frequency transform technology is provided. Based on the comparison and analysis of the two technology, combining the frequency extending technology and the time-frequency technology, a highly efficient imaging algorithm of TOPS mode is presented. Comapred with the existing algorithms, it can improve the processing speed at more than 35%. Based on this algorithm, a generic multimode imaging algorithm is extended. The algorithm shares the kernel modules. By combining and adjusting part of the modules, it can process the data of the stripmap mode, Scan SAR mode, spotlight mode, TOPS/inverse TOPS mode and the sliding spotlight mode. The practical flow of the algorithm is provided, and the validity of the algorithm is validated by the point array targets and distributed targets simulation.In chapter IV, the imaging performance analysis and parameter design problems of spaceborne multimode SAR are researched. In the field of performance analysis, firstly, based on the research results of signal modeling and signal feature analysis in Chapter II, the resolution and mapping range of each mode is analyzed. Then radar equations of the point target and the distributed target are derived and the feature of the antenna pattern illumination and its influence on the signal-to-noise are analyzed. At last, the ambiguity feature of each mode is analyzed, and the calculation method of the ambiguity to signal ratio of each mode is derived. In the field of parameter design, firstly, subject to the beam scan in the azimuth direction in the spotlight mode, TOPS/inverse TOPS mode and sliding spotlight mode, the influence of the pulse repetition frequency(PRF) selection of electronically beam scan is studied. Then according to the feature of each mode, the spaceborne multimode SAR are divided to 2 kinds, the single swath non continuous modes and multiple subswath continuous modes. The generic parameter design method of each kind is studied, and an parameter design method of TOPS mode is presented.In Chapter V, subjecting to the problems at present and the developping trend in the future, some new working modes are presented. Firstly, through analysis the resolution and swath of the modes, the concept of "district TOPS", which can be used to mapping a district with high resolution and/or wide swath, are proposed. 4 realizations are provided and their parameter design methods are provided. Taking the simulation results of this thesis, though increasing the azimuth modules of the antenna, it can improve the azimuth resolution form 12 m to 7m, 38.33%, while keeping the swath width. Secondly, the range ambiguity to signal ratio(RASR) of the traditional full-polarization space borne SAR is analyzed, and points that the cross-polarization RASR performance is deteriorated because the cross-polarization signal is interrupted by the co-polarization ambiguities in the traditional full-polarization SAR systems. Then the full-polarization TOPS mode is proposed to improve the cross-polarization RASR performance. In the simulation of this thesis, the cross-polarization RASR can improve 0.9d B at the situation of / 5HH HVS S ?d B(such as the forest), and 14.2d B at the accasion of / 20 HH HVS S ?d B(such as the seas and oceans). At last, subject to solve the problem that mechanical beam scan system cannot swith its beam pointing immediately, a combined mode of TOPS mode and inverse TOPS mode is proposed. The new mode can realize the TOPS mode in mechanical scan system to obtain high quality image, and the results can be used in the mosaic mode.