Reseach On High Resolution Wide Swath Synthetic Aperture Radar Imaging Mode And Algorithm

Synthetic aperture radar(SAR) can provide the ground images in all-day and all-weather conditions. In the applications such as the global resource surveillance, the wide-range mapping et al., the high resolution and wide swath mapping abilities are required to SAR simultaneously. These requirements make the high resolution wide swath(HRWS) imaging become an important research direction in the future SAR research field. But the conventional SAR platforms and imaging modes have their own limits for HRWS imaging. For the conventional spaceborne platform and imaging modes, high resolution imaging and wide swath imaging put contradictory requirements to the azimuth sampling frequency. For the airborne platform, the low flight height results in narrow swath. Developing novel platforms, proposing novel imaging modes and researching the corresponding signal processing methods become the main research directions for HRWS imaging.The research of this dissertation is carried out around solving the HRWS imaging problems and attempts to solve them through different technique paths. It is described as follows.1. A near space slow platform HRWS imaging mode based on azimuth beam switching is proposed. The flight height of the near space platform can ensure the wide swath imaging. The velocity of the platform is about several tens meters per second,which can decrease the azimuth sampling frequency of high resolution imaging. Then the internal contradiction of the spaceborne platform is solved. But if the traditional stripmap mode is employed, the imaging efficiency will be very low. In this paper, a novel imaging mode based on the azimuth beam switching is proposed to solve the problem. The basic idea of the proposed imaging mode can be described as follows. The pulse repetition frequency(PRF) is improved several times, which will result in redundant azimuth sampling positions along the traveling track. The required imaging azimuth swath is divided into sub-swaths with the same number. In the redundant azimuth positions the imaging beams are switched to illustrate different azimuth swaths.The effects of the azimuth sampling frequency increasing times to the squint angels of the azimuth sub-swath, the image qualities and the imaging efficiency are researched.2. An HRWS Scan SAR imaging method based on compressive sensing isresearched. Traditional scan mode can obtain wide swath images of the ground, while the azimuth resolution is low because all sub-swaths share the same synthetic aperture.Based on the compressive sensing technology, on the premise of the sparsity of the observed scenario, the complete azimuth echoes are reconstructed from the uncompleted azimuth samples of the sub-swaths to solve the low azimuth resolution problem. The signal model of the sparse representation of the Scan SAR azimuth echoes is built. A random sampling scheme to the azimuth echoes is proposed.3. An HRWS spaceborne imaging mode that has the short term revisit ability is proposed. The short term revisit imaging ability has a good application prospect in the ground changes surveillance. It can be achieved through transmitting two azimuth sub-beams(an afterward and a forward beam) to illuminate the observed scenario simultaneously. The echoes of the two transmitting beams are aliased in fast time domain. And the Doppler band width is twice of the traditional stripmap mode. In order to obtain wide swath imaging, the receiving antennas are configured as the displaced phase center multiple azimuth beam(DPC MAB) to decrease the required azimuth sampling frequency. The decreased azimuth sampling frequency results that the echoes of the two transmitting beams are totally aliased in two dimensional frequency domains.A reconstruction algorithm named multiple beam multiple channel reconstruction algorithm(MBMCA) based on the digital beamforming(DBF) technology is proposed to separate and reconstruct the totally aliased echoes. A spaceborne system design example is provided. The validities of the proposed imaging mode and signal processing method are verified by the numerical simulations.4. An azimuth wide scenario spotlight SAR imaging mode based on azimuth sub-scenario stitching method is proposed. The proposed mode overcomes the shortcoming of spotlight mode(narrow azimuth scenario size) through dividing the azimuth observed scenario into several sub-scenarios, imaging the sub-scenarios respectively and sub-scenario images’ stitching. With the increasing of the azimuth scenario size and the resolution, the squint angles of the sub-scenarios increase dramatically. The research focuses on solving the focusing problem of the high squint spotlight SAR with de-chirp receiving method. An extended frequency scaling algorithm(EFSA) based on range walk correction in time domain is proposed. The processing steps of EFSA are derived in details.