Research On Spaceborne Synthetic Aperture Radar Real-Time Imaging Processing Algorithms

With the increasing application requirements,spaceborne synthetic aperture radar(SAR)often needs to perform some urgent earth observation tasks with high timeliness requirements.Therefore,in order to quickly obtain the imaging results of the spaceborne SAR,this dissertation studies the real-time imaging processing algorithms for spaceborne SAR,and proposes a sequential real-time imaging idea that the sub-aperture data recording and sub-aperture real-time imaging processing are synchronized.Thus,after the echo data have been recorded,the final imaging result can be obtained as long as waiting for the imaging processing time of the last sub-aperture data,which effectively reduces the generation delay of the final imaging result.By extending the proposed sequential real-time imaging idea to the spaceborne multi-mode SAR imaging processing,high-resolution spaceborne spotlight SAR imaging processing,spaceborne highly squinted stripmap SAR imaging processing and spaceborne azimuth multi-channel SAR imaging processing,the real-time imaging processing algorithms for spaceborne SAR studied in this dissertation can realize the quick imaging of single sub-aperture delay from the single imaging mode to the unification of multiple imaging modes,from the low resolution to the high resolution,from the broadside configuration to the highly squinted configuration and from the single channel to the multiple channels.1.For the real-time imaging of the spaceborne multi-mode SAR,a real-time unified imaging algorithm based on azimuth sub-aperture complex-valued image combining and scaling is proposed.The proposed algorithm can use a same algorithm flowchart to perform real-time unified processing for the stripmap,sliding spotlight and terrain observation by progressive scans(TOPS)data.In the proposed algorithm,the sub-aperture complex-valued image with partial resolution is obtained by the range pulse compression and azimuth processing,and then the pixel numbers of each sub-aperture complex-valued image for coherent combining are calculated.Finally,these sub-aperture complex-valued images are coherently combined by pixels shifting to obtain the full-resolution image of the whole echo data.The proposed algorithm combines the sub-aperture sequential real-time imaging idea into the spaceborne multi-mode SAR imaging processing,which effectively realizes the quick imaging for the unification of multiple imaging modes.The advantages of the proposed algorithm can be clearly seen through the computational complexity analysis,the point targets simulation analysis and the real data imaging analysis.2.For the real-time imaging of the high-resolution spaceborne spotlight SAR,a highresolution real-time imaging algorithm via sub-aperture coherent superposition in image domain under curved orbit condition is proposed.In the proposed algorithm,the echo data are first divided into sub-apertures to avoid the azimuth spectrum aliasing.And then,according to the new sub-aperture range model under the curved orbit condition,the azimuth space-variance of the equivalent velocity caused by the curved orbit can be eliminated by the azimuth time scale transformation and higher-order phase compensation.After the range processing,the phase transition function is introduced to convert the azimuth hyperbolic phase to a standard quadratic phase and eliminate the range space-variance of the equivalent velocity at the same time.Then,the dechirp function is applied for the sub-aperture signals to obtain the partial-resolution sub-aperture complex-valued images.Finally,these subaperture complex-valued images are coherently superposed in the image domain without shifting operation to obtain the full-resolution image.The proposed algorithm combines the sub-aperture sequential real-time imaging idea with the complex signal characteristics under the curved orbit conditions,which effectively realizes the quick imaging for the highresolution spaceborne spotlight SAR.The advantages of the proposed algorithm can be clearly seen through the computational complexity analysis,the point targets simulation analysis and the surface targets simulation analysis.3.For the real-time imaging of the spaceborne highly squinted stripmap SAR,a highly squinted real-time imaging algorithm based on sub-aperture azimuth frequency scale transformation is proposed.In the proposed algorithm,the tilted frequency spectrum of the sub-aperture signal is converted into the orthogonal frequency spectrum by using the subaperture linear range walk correction to weaken the two-dimensional coupling of the signal.Then,the sub-aperture signal spectrum of the spaceborne highly squinted stripmap SAR is equivalent to that of the spaceborne broadside stripmap SAR by using the sub-aperture azimuth frequency scale transformation to eliminate the azimuth space-variance caused by the sub-aperture linear range walk correction.After that,the chirp scaling algorithm and dechirp operation are performed to complete the range and azimuth processing,and thus the partial-resolution sub-aperture complex-valued images are obtained.Finally,these subaperture complex-valued images are coherently combined to obtain the full-resolution SAR image.The proposed algorithm combines the sub-aperture sequential real-time imaging idea with the special highly squinted configuration,which effectively realizes the quick imaging for the spaceborne highly squinted stripmap SAR.The effectiveness of the proposed algorithm can be verified by the computational complexity analysis and point targets simulation analysis.4.For the real-time imaging of the spaceborne azimuth multi-channel SAR,a multi-channel real-time imaging algorithm based on sub-aperture azimuth spectrum reconstruction is proposed.In the proposed algorithm,the azimuth spectrum ambiguity of the sub-aperture signal is eliminated by the sub-aperture azimuth spectrum reconstruction.And then,the range cell migration correction,range pulse compression and sub-aperture azimuth focusing are performed for the unambiguous sub-aperture signals to obtain the partial-resolution subaperture complex-valued images.Finally,these sub-aperture complex-valued images are coherently combined to obtain the full-resolution imaging result.The proposed algorithm combines the sub-aperture sequential real-time imaging idea with the complex spectrum characteristics of the multi-channel signal,which effectively realizes the quick imaging for the spaceborne azimuth multi-channel SAR.The effectiveness of the proposed algorithm can be verified by the computational complexity analysis and point targets simulation analysis.