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

Ulrich Balss,Christoph Gisinger,Michael Eineder

doi:10.3390/rs10040656

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.

Highlights

Space-borne Synthetic Aperture Radar (SAR) is mainly known for its ability to provide image observations of the Earth’s surface and the measurement of relative shifts making use of the carrier phase (i.e., SAR interferometry)— independent from weather and time of day
Due to the well-defined shape of their impulse-response and to their high radar cross section (RCS), the exact positions such artificial targets in SAR images are precisely detectable so that the location accuracy mainly depends on the geometric features of the object of research: the end to end SAR system consisting of SAR sensor and SAR processing system
A lesson learned from our long-term measurement series, is to be aware of weather influences affecting the performance of a corner reflector (CR) [21]

Summary

Introduction

Space-borne SAR is mainly known for its ability to provide image observations of the Earth’s surface and the measurement of relative shifts making use of the carrier phase (i.e., SAR interferometry)— independent from weather and time of day. The objective of this paper is to highlight the ability to provide absolute localization accuracy of bright, well-detectable radar targets at centimeter level [1,2,3,4] with respect to a given geodetic reference frame like International Terrestrial Reference Frame, release 2014 (ITRF2014) [5] Such a high accuracy—where TerraSAR-X/TanDEM-X lead worldwide among space borne SAR sensors [6]—can be achieved only if the SAR data are processed and calibrated with meticulous care and if they are corrected for well-known effects such as wave propagation and solid Earth dynamics, as it is done in geodesy [7]. The indirect annotation of the signal delays as part of the geometrical SAR range bias in units of length is common practice when generating SAR products, because it is convenient when correcting range measurements, but this leads to a mixing of different effects and complicates the usage of alternative atmospheric correction methods

Objectives

Methods

Results