A Study Of HRWS Imaging And GMTI With Array Radar

Radar is capable of sensing the environment and detecting objects with electromagnetic wave,and it features all-time,all-weather and long range work so as to be non-substitutable in information sensing for military and civil applications.SAR has been widely applied in many field,such as remote sensing,ocean observation,disaster prevention,and reconnaissance.By combining SAR imaging and ground moving target indication(GMTI),we can observe the stationary scene and detect moving targets simultaneously,which is of great value for applications such as traffic monitoring and reconnaissance.With the increasing requirements of various applications on SAR system,such as global mapping and weak slow-moving target detection,high resolution wide swath(HRWS)imaging and effective GMTI have become important development direction of the SAR technology.However,for high-speed platform radar,it is difficult to achieve high-resolution and wide-swath imaging simultaneously due to the criterion of minimum antenna size.That is,the range and azimuth ambiguous energy are difficult to be effectively suppressed,which would seriously restrict the realization of HRWS imaging and GMTI.By introducing multi-channel phased array antenna into SAR systems,it is possible to preliminarily improve the ability of HRWS imaging and weak slow-moving target detection.This dissertation aims at the problems: 1.the range and azimuth ambiguities are difficult to be suppressed for HRWS imaging;2.the GMTI detection performance is seriously affected by strong azimuth ambiguities.The design flexibility of the transmit diversity array in space,frequency,time and waveform dimensions,as well as the signal feature of the ambiguities in image domain,are fully exploited to suppress the range and azimuth ambiguities in HRWS imaging and GMTI applications.To address the above problems,the dissertation focuses on the following aspects:1.Due to the constraints of Doppler and range ambiguities,spaceborne SAR imaging suffers a serious contradiction between high azimuth resolution and wide swath.Aiming at this problem,a range ambiguity resolution approach for HRWS imaging using MIMO frequency diversity array(MIMO-FDA)is proposed.Utilizing the extra degree-of –freedom in range dimension,MIMO-FDA is capable of distinguishing the range ambiguous echoes in spatial frequency.Then,the unambiguous image and be construct by performing traditional SAR imaging processing on the reconstructed unambiguous data to achieve HRWS imaging.Simulation experiments verify the effectiveness of the proposed method.2.Spaceborne SAR spotlight imaging are severely limited on azimuthal coverage.To solve this problem,a transmit diversity array(TDA)-SAR system is designed,and a HRWS imaging scheme is proposed accordingly.TDA transmits the space-time coding waveform,which possesses an omnidirectional radiation pattern,and the transmit beampattern can be synthesized and arbitrarily steered via signal processing on receive.With additional receive beamforming,multiple two-way beams can be formed to cover different azimuth regions.Then,traditional spotlight SAR processing can be performed for each beam output to achieve HRWS imaging of the entire scene.3.Azimuth ambiguities are major concerns for spaceborne SAR-GMTI,because they are not able to be effectively suppressed considering that spatial channel number and pulse repetition frequency(PRF)are severely restricted.To solve this problem,two methods are proposed for detection and parameter estimation stages,respectively.One is a CFAR sliding window size design method,and the other is a false alarm recognition and rejection method based on signal feature of the azimuth ambiguity in image domain.The former method gives explicit expression of the smearing size for moving targets and azimuth ambiguities,so that the protection and sample windows are matched to moving target and azimuth ambiguities,respectively.Thus,moving targets are effectively preserved and the azimuth ambiguities are smoothed out.The latter method deals with the preliminary CFAR detection result by performing refocusing test in clutter canceled image,so as to identify azimuth ambiguities from real moving targets.Simulation analysis and measured data verify the effectiveness of the proposed method.