Analyzing TOPS Coregistration Models And Their Influence On Coherence

Synthetic Aperture Radar Interferometry(InSAR)is a useful tool in remote sensing,allowing for Digital Surface Model(DSM)generation in meter to decimeter precision and for surface motion estimations in millimeter precision.Recently,SAR interferometry with Terrain Observation by Progressive Scanning(TOPS)is becoming a hot research topic.TOPS is an advanced ScanSAR mode,achieving large swath-width and solving its scalloping problem.However,TOPS interferometry requires a rather complex procedure for image coregistration.This is explained by the particularities of the TOPS mode,where large variation of the Doppler centroid within each burst demand a very high azimuth coregistration accuracy.An accuracy of around 1/10th of a pixel,as it is usually recommended for interferometric data,could result in large azimuth phase discontinuities in each TOPS burst.Therefore,an overall azimuth coregistration accuracy better than 1/1000th of a pixel is required for TOPS data in order to avoid phase discontinuities.By analyzing different co-registration approaches(standard,geometric,and Enhanced Spectral Diversity(ESD)),the impact of each one on the coherence is shown.The standard approach is based on finding the maximum of the cross-correlation or keeping the complex coherence as high as possible.Its limitation is that the offsets are computed trough a finite number of matching points and then approximated using an interpolation method,thus,the accuracy is related to the interpolation factor.The geometric approach is based on external sources of information which are Digital Elevation Model(DEM)and precise orbit parameters.Its accuracy is limited by the accuracy of the precise orbit parameters.The third approach,which is the improvement of Spectral Diversity(SD)technique suitable for TOPS mode,is based on the spectral properties of the overlapping regions where we have a large spectral separation.This approach achieves the required accuracy;however,it may not achieve good performance(but achieves the required accuracy)if we have strong deformations in overlapping regions or if these areas contain fields that cover non-stable targets.In addition,a new approach has been developed in order to improve the ESD approach.The new approach is based on the improvement of the two low-resolution interferograms before generating the differential interferogram.This improvement can be reached by the correction of two SAR images using Spectral Diversity(SD)technique before applying ESD.This approach has been tested and shows better result at the cost of an increased runtime.This is explained by the additional processing step,SD,which is runtime consuming.A solution for that is proposed as future research.It can be given by change of process sequence for coregistration step.The basic idea is to consider each burst as a complex SAR image;then,we process each corresponding master and slave bursts separately at the same time(run-parallel).This new process sequence would reduce runtime for the TOPS coregistration step,however,demanding more temporary disk and memory space due to the parallel implementation.