Study On Waveform Diversity Techniques For Range Ambiguity Suppression In High-Resolution And Wide-Swath SAR Imaging

Synthetic aperture radar(SAR)is capable of imaging a large scene on the Earth's surface in all weather conditions including the sunlight illumination,and is of great value for applications in the field of modern remote sensing.The capability of high-resolution and wide-swath(HRWS)imaging is highly desired in future SAR systems,which can satisfy the increasing demands for SAR systems that can simultaneously provide detail images and wide unambiguous swath coverage in many applications,such as wide area surveillance,land and sea traffic observation,and disaster management.However,conventional SAR systems,limited by the ‘minimum antenna area constraint',only can increase the imaging swath at the expense of a degraded azimuth resolution.To overcome this limitation,one must solve the range ambiguity caused by a high pulse repetition frequency(PRF),or the azimuth ambiguity resulted from a low PRF,as the swath coverage increases.The range ambiguity not only limits the imaging swath,but also will cause ghosts appearing in the obtained SAR images,which degrades the images' quality.Thus,it is significant to resolve the range ambiguities for eliminating the undesired echoes and realizing the HRWS imagingThis dissertation focuses on studying the methods of the range ambiguity suppression by using the waveform diversity techniques for the HRWS-SAR imaging systems.Main accomplishments include:1.On the issue of the range ambiguity suppression in the single-channel SAR systems,it is proven that the pulse phase coding(PCC)technique can discriminate the range ambiguity.The suppression performance of the range ambiguities for the PPC technique is analyzed in detail.It is found that the PPC technique,by coding the phases of the transmitted pulses along the slow time,equivalently shifts the spectra of the range-ambiguous echoes by PRF/2 in the Doppler frequency domain,so that the range-ambiguous echoes can be suppressed by a Doppler filtering.Because the PPC technique always makes the optimum frequency shift(PRF/2)for the undesired spectra from different range-ambiguous regions,it is better than the previous azimuth phase coding(APC)method.Moreover,the PPC technique has much simpler coding scheme compared with the APC method,which makes it easier to be implemented in practice.To further improve the performance of range ambiguity suppression for the PPC technique,a two-pulse cancellation(TPC)method is proposed.The proposed method,followed by an azimuth compression,can provide better performance in the range ambiguity suppression compared with the original PPC technique if the PRF is chosen suitably.Simulation results have verified the capability of the range ambiguity suppression of the PPC technique,and the effectiveness of the proposed method.2.The practical limitation of realizing HRWS imaging with a single-channel SAR by using the PPC technique is analyzed.If the SAR systems choose a high PRF,the SAR image swath can be improved by separating the desired echoes from two ambiguous range regions,or suppressing the undesired echoes from the adjacent range regions,with the PPC technique.However,this improvement for the imaging swath will be limited if a lower PRF is chosen.Simulation results combined with system design analysis demonstrate the capability and limitation of the PPC technique for increasing the imaging swath in practice.3.Under the multiple input and multiple output SAR systems,a new method of realizing the HRWS-SAR imaging based on a space-pulse phase coding(SPPC)technique is proposed.The SPPC technique is to perform the phase coding in both spatial channels and slow time pulses,and the coding scheme used is learned from the element-pulse coding(EPC)technique which is proposed to suppress deceptive mainlobe jamming in this laboratory.By properly designing the coding scheme,it is possible to separate the echoes from multiple ambiguous range regions from each other in the spatial domain.For each particular range region,an optimized receive beampattern can be applied to extract the desired echoes from the presumed range region and suppress the undesired echoes from other range-ambiguous regions.Besides,it is capable of further mitigating the residual range-ambiguous echoes by optimally designing the coding scheme and the transmit beampattern.In the final,a bank of imaging processors can be implemented to obtain the high-resolution images of these range regions,which can be synthesized into a whole HRWS SAR image of the scene of interest.Simulation results have demonstrated the effectiveness of the proposed method.4.To improve the coding flexibility for the SPPC-based HRWS-SAR imaging,two coding schemes,referred to as the uniform-cyclic coding and the uniform-acyclic coding,respectively,are proposed for discriminating the range ambiguities.By choosing a proper phase gradient factor,both coding schemes can be used to resolve the range ambiguities.The uniform-cyclic coding scheme presents a little better performance for the range ambiguity suppression,but the uniform-acyclic coding scheme can preserve the coherency of the range-ambiguous clutters,which may be desired in some applications such as SAR-based ground moving target indication(SAR-GMTI).Simulation results have demonstrated the effectiveness of the proposed coding schemes.5.On the issue that the MIMO-SAR is limited for applications because of the nonexistence of the ideal orthogonal waveforms,the feasibility of applying the MIMO-SAR systems in some scenes based on the current non-ideal orthogonal waveforms is verified.It is analyzed that both the MIMO system and transmitting the orthogonal waveforms is necessity for the SPPC-based HRWS-SAR imaging.However,the ideal orthogonal waveforms in the same frequency band do not exist in practice,and the interference among waveforms may lead to a degradation in the imaging quality of the MIMO-SAR,which makes the applicable scenes for the MIMO-SAR limited.The effect of the non-ideal orthogonal waveforms on the imaging quality of the MIMO-SAR is analyzed through simulation experiments with two kinds of current waveforms.Simulation results demonstrate the feasibility of applying the MIMO-SAR in the scenes where most of the scatterers are rather weak.