John Walter Antony

Hierarchical Bayesian Data Analysis in Radiometric SAR System Calibration: A Case Study on Transponder Calibration with RADARSAT-2 Data

A synthetic aperture radar (SAR) system requires external absolute calibration so that radiometric measurements can be exploited in numerous scientific and commercial applications. Besides estimating a calibration factor, metrological standards also demand the derivation of a respective calibration uncertainty. This uncertainty is currently not systematically determined. Here for the first time it is proposed to use hierarchical modeling and Bayesian statistics as a consistent method for handling and analyzing the hierarchical data typically acquired during external calibration campaigns. Through the use of Markov chain Monte Carlo simulations, a joint posterior probability can be conveniently derived from measurement data despite the necessary grouping of data samples. The applicability of the method is demonstrated through a case study: The radar reflectivity of DLR’s new C-band Kalibri transponder is derived through a series of RADARSAT-2 acquisitions and a comparison with reference point targets (corner reflectors). The systematic derivation of calibration uncertainties is seen as an important step toward traceable radiometric calibration of synthetic aperture radars.

Results of the TanDEM-X Baseline Calibration

The TanDEM-X mission based on two satellites provides a radar interferometer in space with the goal to derive a global Digital Elevation Model (DEM) with never achieved quality for a global coverage: a global DEM with a relative height accuracy of 2 m and 10 m absolute. In order to achieve this mission goal, the radial and cross-track distance between both satellites, the so called baseline has to be known with high accuracy. Only then, systematic baseline errors can be detected and compensated for, i.e., an accurate calibration of the global DEM can be ensured. The paper describes the procedure and the results of calibrating the baseline, verifying the outstanding accuracy of this calibration procedure.

In-orbit calibration of the TanDEM-X system

In addition to the first satellite TSX already in-flight since 2007 [1], the second satellite TDX of the TanDEM-X system could be successfully launched in 2010 [2]. The primary object of the TanDEM-X mission is to generate a highly accurate digital elevation model (DEM) with never achieved accuracy on global scale. But in addition to this DEM acquisition based on a bistatic satellite constellation, nominal TerraSAR-X operation shall be available anymore, i.e. the bistatic TanDEM-X mission and the monostatic TerraSAR-X mission have to be operated in parallel with both satellites. Consequently the second satellite TDX had to achieve the same accuracy and performance as those of the first satellite TSX. Based on a short overview of the different calibration procedures the paper discusses the calibration results achieved for the whole TanDEM-X system, successfully in-flight since June 2010.

Tests of the TanDEM-X DEM calibration performance

The TanDEM-X mission based on two satellites provides a radar interferometer in space with the goal to derive a global Digital Elevation Model (DEM) with never achieved quality for a global coverage: a global DEM with a relative height accuracy of 2m and 10m absolute. In order to achieve this mission goal, the distance between both satellites, the so called baseline has to be known extreme precisely. Only then, systematic baseline errors can be detected and compensated for, i.e. an accurate calibration of the global DEM can be ensured. The paper describes the procedure and the results of calibrating the baseline, verifying the outstanding accuracy of this calibration procedure.

The DLR Spaceborne SAR Calibration Center

Abstract A necessary activity for any SAR system is its calibration to establish the relation between radar measurements and geophysical parameters. During this process, all essential parameters of a SAR image are linked to their geophysical quantities. This includes the geolocation of the SAR image, its backscattering characteristics (in amplitude and in phase) and polarimetric information. The Microwaves and Radar Institute of the DLR has gained extensive experience in these calibration procedures during the last decades and has developed special methods and dedicated reference targets for spaceborne SAR system calibration. Through examples of calibration results obtained for different spaceborne SAR mission, the capabilities of the DLR SAR Calibration Center are presented.

The DLR SAR calibration center

A SAR system has to be calibrated for quantitative measurement. During this process all essential parameters of a SAR image are linked to their geophysical quantity. This includes the location of the image pixel, its backscattering characteristics (in amplitude and in phase) and polarimetric information. The Microwave and Radar Institute of the DLR has gained a lot of experience in these calibration procedures during the last decades and has developed special methods and dedicated reference targets for spaceborne SAR system calibration. In example of calibration results obtained for different spaceborne SAR systems the DLR SAR Calibration Center is presented.