Study On Theory And Technology For Accurate Polarimetry Using Phased Array Radar

The capability of the polarimetric information processing and the application effects are directly restricted by the accuracy of the acquired polarimetric information of targets.Due to the capabilities of fast beam scanning and flexible signal processing,phased array radars with accurate polarimetry capability will comprehensively improve the performances of detection,tracking,imaging,recognition and anti-interference in the areas of weather observation,air surveillance,air defense and anti missile.The feature that the polarizations of phased array antenna varies in beam scanning angles will have serious impacts on the accurate polarimetry,which is one of the urgent bottleneck problems.Meanwhile,the polarimetric waveforms and corresponding signal processing algorithms will also have impacts on the accurate polarimetry,which is another urgent bottleneck problem.This thesis,focusing on the two bottleneck problems,has made in-depth study on the polarimetric bias correction and polarimetric waveform design.The main contributions of the thesis are as follows:1.Correction of the radiation field non-orthogonality of the polarimetric phased array radar in the beam direction.Aiming at the fact that on the condition of narrow beam(beamwidth is about 1?)the polarimetric bias mainly comes from the the radiation field non-orthogonality in the beam direction,the boresight correction method is developed.This method is an improvement and extension of the previous projection matrix method,which takes the mutual coupling between array elements,the antenna pattern measurement errors of phased array radar and the amplitude and phase imbalances of H/V channels into account and perfects the basic theory of accurate polarimetry using phased array radars.According to the theoretical analysis and simulations,the impacts of the antenna pattern measurement errors of phased array radar,the amplitude and phase imbalances of H/V channels and the beam expansion during scanning on the accurate polarimetry have been quantitatively analyzed,which explicitly shows the accuracy requirements of phased array radar system under the given polarimetry accuracy.2.Correction of the radiation field non-orthogonality of the polarimetric phased array radar within the entire beam.For distributed targets,strictly speaking,the polarimetry will be influenced by the radiation field non-orthogonality within the entire beam.When the beamwidth is wide(> 5?)or the beamwidth is narrow and cross-polar elements of the polarimetric scattering matrix of distributed targets is small(over 20 dB compared with the co-polar elements),the impacts of the radiation field non-orthogonality within the entire beam is more obvious and the boresight correction method is unable to completely correct the polarimetric bias.Therefore,the correction method that is based on the synthetic integral of phased array antenna patterns is developed,which is called the array pattern-based correction method.The simulations show that this method can effectively improve the measurement accuracy of linear depolarization ratio for distributed targets,such as the rainfall.3.Polarimetric waveform design.Aiming at instantaneously full polarimetry,a new nonlinear frequency modulated waveform with very low sidelobe level is developed.Theoretically,with a 0.24 dB loss of signal-to-noise ratio,the designed peak sidelobe level can achieve-70 dB.According to hardware experiment,with a 0.24 dB loss of signal-to-noise ratio,the designed peak sidelobe level is lower than-60 dB.In X-band,with a target velocity of 50 m/s,the theoretical peak sidelobe level of the designed waveform is lower than-40 dB,which shows the good Doppler tolerance of the designed waveform.Based on the nonlinear frequency modulated waveform,the orthogonal nonlinear frequency modulated waveforms are developed.The designed orthogonal waveforms have controllable waveform isolation and peak sidelobe level of under-60 dB.When the target velocity is less than 50 m/s,the orthogonal waveforms still have good auto-correlation and crosscorrelation characteristics.Hence,the designed waveform can provide firm supports for accurate polarimetry.This thesis,focusing on the bottleneck problems in the area of accurate polarimetry,has made study on the two levels of theory and technology.The contributions of the thesis are of great significance for polarimetric phased array radar system analysis,polarimetric bias correction and polarimetric waveform design in both theory and technology.