Measurements on the Absolute 2-D and 3-D Localization Accuracy of TerraSAR-X

Abstract

Abstract:The German TerraSAR-X radar satellites TSX-1 and TDX-1 are well-regarded for their unprecedented geolocation accuracy. However, to access their full potential, Synthetic Aperture Radar (SAR)-based location measurements have to be carefully corrected for effects that are well-known in the area of geodesy but were previously often neglected in the area of SAR, such as wave propagation and Earth dynamics. Our measurements indicate that in this way, when SAR is handled as a geodetic measurement instrument, absolute localization accuracy at better than centimeter level with respect to a given geodetic reference frame is obtained in 2-D and, when using stereo SAR techniques, also in 3-D. The TerraSAR-X measurement results presented in this study are based on a network of three globally distributed geodetic observatories. Each is equipped with one or two trihedral corner reflectors with accurately (<5 mm) known reference coordinates, used as a reference for the verification of the SAR measured coordinates. Because these observatories are located in distant parts of the world, they give us evidence on the worldwide reproducibility of the obtained results. In this paper we report the achieved results of measurements performed over 6 1/2 years (from July 2011 to January 2018) and refer to some first new application areas for geodetic SAR.

Mitigation of Tropospheric Delay in SAR and InSAR Using NWP Data: Its Validation and Application Examples

“…In this section, both SAR SPD and GPS SPD are used as reference for validation of the ECMWF integrated SPD. For this experiment, three trihedral CRs with 1.5 m edge length were installed at two test sites close to permanent GNSS stations: two at the Wettzell geodetic observatory (WTZR) in Germany, with WTZR 01 orientated for ascending (ASC) orbits and WTZR 02 for descending (DSC) orbits, and one at the Metsähovi observatory in Finland (METS) for DSC orbits [42][43][44]. High resolution SAR images from the two German satellites TerraSAR-X (TSX-1) and TanDEM-X (TDX-1) have been acquired regularly: 176 scenes from July 2012 to October 2015 at WTZR and 155 scenes from December 2013 to February 2016 at METS.…”

“…The maximum mean offsets (up to −27.6 mm) are observed in WTZR 01, which are mainly caused by the local ground subsidence that occurred after the installation of the CR [44]. (c) scatter plot of the range delays integrated using OP data (ECMWF SPD) and SAR SPD.…”

“…The absolute accuracy of the tropospheric delay calculated with the direct integration method using ECMWF data has been validated against both GPS and CR measurements, demonstrating an accuracy in the centimeter range (Section 3). Note that the accuracy estimates also contain errors in the reference data used (GPS ZPD [38] and SAR SPD [1,44]).…”

“…In the framework of the DLR@Uni Munich Aerospace Project "Geodetic Earth Observation", five CRs have been installed in three test sites [43,44]: two CRs at the geodetic observatory in Wettzell As discussed in Section 3.4, discrepancies between expected and measured absolute ranges are caused by geodetic and atmospheric effects [1,26,34]. Assuming that geodetic effects and ionospheric delay have been already compensated as described in [43,44], the remaining tropospheric delay can be compensated using either the IGS SPD (mapped ZPD) or the integrated ECMWF SPD using ERA-Interim data sets. A statistical analysis of the ranging residuals after these corrections has been performed.…”

Geocoding Error Correction for InSAR Point Clouds

“…The coordinate transformation residuals between object and sensor coordinate systems are compensated for by taking into account the plate tectonics effect and referring the observation to a reference epoch [26]. Finally, the geometric calibration constants in range and azimuth are updated based on the most recent studies concerning long-term corner reflector experiments [34]. For this purpose, all the aforementioned effects are initially mitigated using the described methods and then the median of offsets between the expected position of the corner reflector, surveyed with GNSS, and its corresponding position in a time series of SAR images is considered as new re-calibration constants [34].…”