Research On The Methords Of High-resolution Airborne In Sar Phase Unwrapping

Phase unwrapping is one of the key step in InSAR data processing,its precision directly affects accuracies of DEM generation or surface deformation monitoring.The phase extracted from a complex SAR interferogram is wrapped since it represents a measure,modulo 2? and multiples of 2? must be added or subtracted.Ideally,when the sampling rate of the image satisfies the Nyquist sampling theorem,phase unwrapping can be achieved by simple two-dimensional integration.However,there are phase inconsistencies caused by overlay,shadow,etc.in the actual data.When the integration path passes through the above-mentioned area,the phase error will spread,which leads to unwrapping errors.In order to solve the above problems,dozens of phase unwrapping algorithms are proposed.The statistical cost network flow algorithm(SNAPHU)is a kind of network flow methods,which is widely used in InSAR data processing.Based on the SNAPHU algorithm,this paper tries to find a phase unwrapping algorithm for airborne SAR interferometry.While applying the SNAPHU to high resolution airborne InSAR data,if the interferogram has phase inconsistencies caused by lines of trees,the unwrapped result will appear large areas of phase jump along inconsistencies correspondingly.SNAPHU is a global optimization algorithm,which has limitations in dealing with local phase inconsistencies.Local phase inconsistencies can be optimized by local algorithm.Phase inconsistencies can be described by residues.Local data can be optimized by residues compensation.This paper proposes an airborne unwrapping algorithm combining the residue degradation with the statistical-cost network-flow,which has advantages of both the local optimum and the global optimum.Residues are degraded to non-residues according to values of residues and neighbouring pixels,and the local data are optimized.Then,apply the modified SNAPHU algorithm to the degraded image.The proposed algorithm can be divided into three steps:(1)residues degradation.Firstly,flatten the filtered interferogram.Then apply the residue degradation to the flattened interferogram.The residue degradation process contains residues detection and residues compensation.Phase inconsistencies lead to the presence of residues.Phase inconsistencies means the absolute phase difference between two neighbouring pixels exceeding ?.Therefore,in order to degenerate the residue,firstly locate the pair of pixels in the residue whose phase are not consistent,and then set a compensation constant C to compensate to the residue.Since the compensation changes the value of phase and also the phase difference between the neighbouring pixels,the residue can be degraded by multiple iterations.To balance the accuracy and efficiency,set a threshold N for remaining residues.To control the degree of degradation,when the number of residues is no more than N,the compensation stops.(2)phase unwrapping of airborne statistical-cost network-flow(SNAPHU)algorithm;In the airborne system,because the height of the platform is far less than the Earth's radius,the incidence angle 4)calculated by the original geometric model of SNAPHU is close to zero,which is obviously not correct.At the same time with a narrow mapping bandwidth in airborne system,the earth curvature can be ignored to simplify the model,so according to calibration parameters of airborne InSAR system,modify parameters and the geometric mode in statisticalcost network-flow(SNAPHU)algorithm.And the modified SNAPHU algorithm is employed for phase unwrapping.(3)median filter.Applying a median filter to the unwrapped result,which effectively reduces phase noise and the edge information is well preserved.So,applied a 5×5 median filter to the unwrapped image to get the final result.The efficiency and accuracy of proposed method is tested and validated by using the single pass dual antenna airborne InSAR data covering Jiangyou,Sichuan areas in 2011.Using SNAPHU directly,unwrapping results have a significant error region,there is a large area of the phase jumping.But after residues degradation,error unwrapped regions are significantly shrunk and phase jumping regions are effectively corrected.And after 5 × 5 median filter,the partial phase noise is removed,and the unwrapping result is smoother.Performances of improved methods are evaluated by the number of discontinuous points in azimuth and range directions.The less the number of discontinuous point,the better the anti-phase distortion performance,and the higher the unwrapping quality.The residues degradation effectively reduces the number of discontinuous points in the interferogram.The number of discontinuous points in either direction of improved airborne SNAPHU algorithm is smaller than that of the simple airborne SNAPHU algorithm.It is shown that the anti-phase distortion performance and the quality of the unwrapping results are improved obviously.The residue degradation process effectively solves the problem of large unwrapped phase jump,and becomes more robust to noise.This algorithm combines the advantages of local optimization and global optimization,which has good performance on the whole and effectively solves the problem of large unwrapped phase jump.Moreover,median filter can effectively reduce the noise,and becomes more robust to noise.