Study And Experiments On Key Techniques Of Airborne Azimuth Multichannel SAR Coherent Imaging

Nowadays,the high resolution and wide swath(HRWS)is the hottest major development direction of the spaceborne synthetic aperture radar(SAR)systems.And the most commonly way to achieve HRWS SAR imaging is the azimuth multi-channel SAR.In fact,the azimuth multichannel SAR faces with many problems in actual processing,such as Doppler ambiguity suppression and channel balancing.In general,the researches based on airborne SAR systems are implemented and verified before the spaceborne SAR systems are brounght into application.Different from spaceborne azimuth multi-channel SAR processing,airborne azimuth multi-channel SAR processing also faces with the problem of motion compensation.Besides,the impact of motion compensation on channel consistency needs to be considered.In addition,compared with the spaceborne SAR system,many error sources have more complicated and larger impacts on the systemtic performance in the airborne SAR system due to the influence of non-nominal motion and small slant range.As a result,some negligible errors in the spaceborne system are on the airborne platform.It also becomes non-negligible.Therefore,it is of great importance to research in detail in the airborne azimuth multi-channel SAR/InSAR data processing,which has significant value in scientific and engineering application.This dissertation is dedicated to improve the efficiency and accuracy of airborne azimuth multi-channel SAR processing,and to improve the airborne azimuth multi-channel SAR processing flow.The major contributions are summarized as follows.(1)Summary of the developments and researches in azimuth multichannel SAR and analysis on siginal and error modelThe developments of azimuth multi-channel SAR systems over the years are reviewed,and the studies on azimuth multi-channel SAR processing techniques are summarized.The signal model in the airborne azimuth multi-channel SAR is introduced in Chapter 2.The effect of the motion errors on the channel consistency is analyzed,and the main sources of channel errors and their effects are discussed in detail.Then,the general form of multi-channel SAR channel error model is given.(2)Channel error calbiration techniques in the azimuth multi-channel S ARChannel errors are a key factor that affects the performance of azimuth multi-channel SAR systems,so the calibration of channel errors is an indispensable procedure for azimuth multi-channel SAR processing.In Chapter 3,the following research work is mainly focused on channel calibration in airborne azimuth multi-channel SAR systems:A robust estimation algorithm based on range spectrum analysis multi-channel HRWS SAR is proposed to estimate range sampling errors.The algorithm firstly obtains the interferometric phases between the range spectrums of adjacent channels,and then achieves the constant phase terms and the linear phase terms coefficient(corresponding to channel sampling errors)via phase unwrapping and weighted polynomial fitting.In addition,based on the theory of spatial cross-correlation coefficients,the baseband Doppler centroid and channel phase mismatches can be simultaneously estimated from constant-phase term estimates.The advantage of this algorithm is that it overcomes the effects of phase wrapping and hopping,and it can obtain the baseband Doppler centroid and channel amplitude and phase mismatched while achieving the robust estimation of channel range sampling errors.Experimental results based on the airborne recorded data verify the effectiveness of the proposed algorithm.Compared with the traditional methods based on polynomial fitting method or the image coregistration,the proposed algorithm has evident advantages in the precision and robustness.To resolve the phase inconsistency among channels,a simple and effective multi-channel HRWS SAR channel phase mismatch estimation algorithm is proposed based on the theory of clutter cancellation.Theoretical analysis shows that the echo signal of the reference channel can be constructed from the signals of other channels.If phase mismatches among the channels do not exist and there is no noise,the reconstructed signal should be exactly the same as the actual signal of the reference channel.Therefore,the channel phase mismatches can be estimated by minimizing the difference between the two signals.This algorithm requires a redundant receive channel in a multi-channel SAR system,which can be satisfied in many cases.Results based on simulated and measured data verify the effectiveness of the proposed algorithm.The comparison experiments show that this algorithm has the advantages of low computational load and high precision over the adaptively weighted least square(AWLS)algorithm.A brand-new channel azimuth baseline error estimation algorithm based on Doppler spectrum optimization is proposed.Theoretical analysis and simulated experiment both show that the effect of channel azimuth baseline error on the Doppler reconstruction performance cannot be overlooked in the airborne case,even those errors are in the millimeter-level.Similar to channel phase mismatches,channel azimuth baseline errors also degrades the performance of the Doppler reconstruction,resulting in a more even distribution of signal energy along the azimuth frequency.Inspired by the above fact,a channel azimuth baseline error estimation algorithm based on Doppler spectrum optimization is proposed.The proposed algorithm can estimate channel azimuth baseline errors by maximizing the mean power ratio between the signals inside PDB and those outside PDB.Experimental results based on the airborne recorded data verify the effectiveness of the proposed algorithm.(3)Doppler parameter estimation techniques for azimuth multi-channel SARDoppler centroid and Doppler chirping rate are the important parameters in SAR imaging,and in the azimuth multi-channel SAR processing,the Doppler centroid also affects the accuracy of the phase mismatch estiamtion.However,due to the characteristices of azimuth multi-channel SAR systems,traditional Doppler parameter estimation algorithms are no longer applicable.In order to accurately estimate Doppler parameters,the following researches are carried out in this dissertation:By analyzing the cross-correlation function between the adjacent channels,we discover that there exists a linear relationship between the Doppler centroid and the channel phase mismatches.As channel mismatches would degrade the performance of the Doppler spectrum reconstruction,a conclusion can be drawn that the deviation of the Doppler centroid also affects the Doppler reconstruction spectrum.Based on the above analysis,a Doppler centroid estimation algorithm based on Doppler spectrum optimization is proposed.In the proposed algorithm,according to the interferometric phase information of adjacent channels,the channel phase error can be mathmatically represented by the Doppler centroid.Therefore,the Doppler centroid could be estimated by maximizing the energy in the Doppler bandwidth in the reconstructed Doppler spectrum.The experimental results show that compared with the spatial cross-correlation coeficient(SCCC)method,the proposed algorithm has the same accuracy and performs more robustly in the undersampling case.Based on the analysis on the general form of range migration,the curve of range migration shows the potential in the Doppler parameter estimation.The differential signal along the range direction has more and more even distribution when Doppler centroid ambiguity number used in range walk correction deviates from the accurate value.And The variation of the instant slant range with the azimuth time is approximatively quadratic.Based on these two facts,a simple and effective Doppler centroid and chirping rate estimation algorithm is proposed,which is comprised of two main steps.In the first step,the baseband Doppler centroid could be estimated by employing the existing SCCC method,and the Doppler centroid ambiguity can be resolved by seacrching the Doppler ambiguity number which makes the differential signal along the range direction has the maximum variance.In the second step,after the range walk correction,several strong and isolated point targets are selected based on the maximum contrast criteria and their range migration curves are extracted by data preprocessing and interpolation.Then,a weighted polynomial fitting is adopted to obtain an accurate Doppler chirping rate.This algorithm is simple and easy to implement.Experimental results verify the effectiveness of the algorithm.(4)Airborne azimuth multi-channel SAR/InSAR coherent imagingAzimuth multi-channel SAR can achieve high resolution wide swath images,however actual applications such as terrain mapping is implemented by interferometric SAR and polarimetric interferometric SAR.In order to further conduct InSAR/PolSAR/PolInSAR researches,it is necessary to take the performance of phase preservation into fully consideration in the SAR imaging,including not only the phase relationship between the echoes of the receiving channels of the same antenna but also the interference-preserving property between the primary and secondary antennas.In Chapter 5,we firstly propose a simple and efficient method to rapidly calculate the slant range errors in the motion compensation procedure of azimuth multi-channel SAR system.Then we propose a two-step precise calibration algorithm to calibrate the channel phase mismatches that vary with the azimuth time,the elevation and the slant range.In the end,a complete processing strategy for airborne azimuth multi-channel SAR/InSAR imaging is proposed.The key steps of airborne azimuth multi-channel SAR processing,including effective motion compensation,and precise channel mismatch calibration,are fully considerd and discussed in detail in this processing strategy.Results on the measured data prove that this novel algorithm can effectively maintain the phase relationship between the receiving channels while achieving the well-focused SAR images,and has a good performance in interferometric phase preservation.As shown by interferometric results,there is no doubt that the proposed processing flow has little influence on coherence and negligible impact on interference performance.