Repeat-pass Spaceborne SAR Interferometry For DEM Generation In Cloudy-Rainy Mountianous Area

Digital Elevation Model(DEM)as an important geospatial data is widely used in civil economy and national defense construction,as well as in the field of the humanity and physical science.There is no high-accuracy global DEM so far in our country.Accelerating acquisition of high-accuracy DEM over national territory-ocean coverage enven global is one of the requirements for “Outline of national medium and long term planning for basic surveying and mapping(2015-2030)”.Survey,topographic map scanning,aerial photogrammetry,light detection and ranging(Lidar)etc can obtain small-scale high-accuracy DEM with the weaknesses of time-labor consuming,not applicable to large-scale application especially overseas,vulnerable to impact of wether like cloudy,misty,rainy and snowy.The GPS can obtain large-scale high-accuracy DEM with the disadvantages of sparse samples and high costs which limit its application.Repeat-pass spaceborne Synthetic Aperture Radar Interferometry(InSAR)has become a research hotspot for the acquisition of large-scale DEM with the characteristics of active imaging,penetrating cloud and mist,day-night working.Moreover,GF-3 satellite developed by our country has come into service from 2017 and obtained the first batch high-resolution SAR images which can support data for our county to employ InSAR technique to acquire global high-accuracy DEM.To acquire higher accuracy DEM by means of In SAR,researchers at home and abroad have made lots of attempts.Their studies mainly focus on three aspects.Firstly,auxiliary DEM is used in phase unwrapping over rough regions to decrease the difficulty and error of phase unwrapping,considering neither atmospheric phase nor the detection of body of water which is complete incoherent.Secondly,multi-baseline InSAR technique is developed to reduce observation error based on redundant measurements,considering phase noise from atmosphere neither.Thirdly,with the development of time-series InSAR technique,researchers at home and abroad tend to focus on the improvement of surface deformation accuracy.However,there is a little study on the high-accuracy height acquisition of buildings in urban area,and little study on the corrections of auxiliary DEM to obtain higher accuracy DEM based on elevation residuals.To solve problems above,in this paper,repeat-pass spaceborne single-baseline InSAR and multi-baseline InSAR are studied to acquire high accuracy DEM over mountainous area with rough and vagetable terrain,cloudy and rainy weather in Kunming City,Yunnan Province.The correction of auxiliary DEM based on elevation residuals obtained from time-series InSAR technique is studied to acquire higher accuracy DEM over hilly area with rough and vegetable terrain,rainy and snowy weather in north of Huludao City,Liaoning province.The accuracy of InSAR DEM is evaluated by Digital Products of Fundamental Geographic Information 1:50000 DEM Level One.Main studies are as follows.Firstly,phase unwrapping of single-baseline InSAR technique base on auxiliary DEM is developed.The algorithm of restoring phase ambiguity based on auxiliary DEM is proposed.Phase unwrapping through differential phase is developped by baseline refinement base on fringe frequency of phase different interferometry,detection and elevation value of body of water and named “Difference and Compensate”.Single-baseline InSAR experiments are carried out by use of two ALOS-2 PALSAR-2 images(acquired on 20160701,20160729)over mountainous area in Kunming City.The results demonstrate that:(1)The accuracy of single-baseline InSAR DEMs can be up to the standard of Digital Products of Fundamental Geographic Information 1:100000 DEM Level Three;(2)Among three InSAR DEMs,the accuracy of InSAR DEM obtained from restoring phase ambiguity based on auxiliary DEM is best,while InSAR DEM obtained from “Difference and Compensate” is better;(3)The detection of body of water is of high accuracy.Secondly,the algorithm of phase accumulation base on multi-baseline is developed.The phase accumulation algorithm with weights base on flattened phase interferometry or differential phase is proposed.Multi-baseline InSAR experiments are carried out by use of four ALOS-2 PALSAR-2 images(acquired on 20160701,20160715,20160729,20160909,respectively)over mountainous area in Kunming City.The results demonstrate that:(1)The phase accumulation algorithm can reduce atmospheric phase significantly;(2)The accuracy of InSAR DEMs obtained from phase accumulation base on six interferograms can be up to the standard of Digital Products of Fundamental Geographic Information 1:50000 DEM Level Three and American DTED-1.Thirdly,the correction of auxiliary DEM based on elevation residuals obtained from time-series InSAR technique,is proposed.Time-series InSAR technique can effectively overcome time or baseline decorretion and effects of atmospheric phase by extracting high-density high-quality stable targets of which the surface deformation velocity and elevation residuals can be inversed.Then,elevation residual map can be achieved by spatially interpolating based on the elevation residuals of stable targets.Finally,the auxiliary DEM can be corrected by use of elevation residual map.The experiment of auxiliary DEM elevation correction based on time-series InSAR technique is carried out by use of 22 ALOS-1 PALSAR-1 images over hilly area in north of Huludao City.The stable targets are extracted as well as the elevation residuals are inversed and spatially interpolated by Kriging interpolation.The stable targets account for about 6.34% of the whole image pixels.Small area of 9 km×9 km,steep and vegetable terrain is selected and analyzed in detail.The results demonstrate that the elevation accuracy is increased by 15.6% after SRTM DEM correction.