Abstract
In Sentinel-1 TOPS mode, the antenna sweeps in the azimuth direction for the purpose of illuminating the targets with the entire azimuth antenna pattern (AAP). This azimuth sweeping introduces an extra high-frequency Doppler term into the impulse response function (IRF), which poses a more strict coregistration accuracy for the interferometric purpose. A 1/1000 pixel coregistration accuracy is required for the interferometric phase error to be negligible, and the enhanced spectral diversity (ESD) method is applied for achieving such accuracy. However, since ESD derives miscoregistration from cross-interferometric phase, and phase is always wrapped to [ − π , π ) , an initial coregistration method with enough accuracy is required to resolve the phase ambiguity in ESD. The mainstream for initial coregistration that meets this requirement is the geometrical approach, which accuracy mainly depends on the accuracy of orbits. In this article, the authors propose to investigate the feasibility of using the conventional coregistration approach, namely the cross-correlation-and-rigid-transformation, as the initial coregistration method. The aim is to quantify the coregistration accuracy for cross-correlation-and-rigid-transformation using the Cramér-Rao lower bound (CRLB) and determine whether this method could eventually help to resolve the phase ambiguities of ESD. In addition, we studied the feasibility and robustness of the cross-correlation plus ESD under different conditions. For validation, we checked whether the cross-correlation plus ESD approach could reach the same coregistration accuracy as geometrical plus ESD approach. In general, for large areas with enough coherence and little topography variance, the cross-correlation method could be used as an alternative to the geometrical approach. The interferogram from the two different approaches (with ESD applied afterward) shows a negligible difference under such circumstances.
Highlights
Coregistration is one of the important steps in SAR interferometry (InSAR)
The difference between the ScanSAR and the Terrain Observation by Progressive Scans (TOPS) mode is that, in the TOPS mode, the antenna “sweeps” in the azimuth direction in each subswath so that all targets are scanned by the entire azimuth antenna pattern (AAP), and the scalloping effect is minimized when comparing with the ScanSAR mode [2]
The focus is on two points: 1. The first one is to evaluate the coregistration accuracy of the cross-correlation method by checking the cross-interferogram phase in the enhanced spectral diversity (ESD) step
Summary
Coregistration is one of the important steps in SAR interferometry (InSAR). Coregistration means that the single look complex (SLC) images must be “aligned” well so that the they could be compared pixel-wise for differences, where the differences reveal ground information such as height or movement. The difference between the ScanSAR and the TOPS mode is that, in the TOPS mode, the antenna “sweeps” in the azimuth direction in each subswath so that all targets are scanned by the entire azimuth antenna pattern (AAP), and the scalloping effect is minimized when comparing with the ScanSAR mode [2]. This new acquisition mode was firstly tested on the German satellite TerraSAR-X for validation [3,4,5]. In the Discussion Section, we will present another case in the California Central Valley that shows a very similar ground feature
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