Björn Döring

Final TerraSAR-X Calibration Results Based on Novel Efficient Methods

TerraSAR-X is a satellite mission for scientific and commercial applications operating a highly flexible X-band synthetic aperture radar (SAR) instrument with a multitude of different operation modes. As product quality is of crucial importance, the success or failure of the mission depends essentially on the method of calibrating TerraSAR-X in an efficient way during commissioning the entire system in a restricted time. Only then, product quality and the correct in-orbit operation of the entire SAR system can be ensured. This paper describes the in-orbit calibration method for TerraSAR-X and dedicated activities performed during the commissioning phase as well as final results derived from all calibration procedures.

Assessment of Atmospheric Propagation Effects in SAR Images

TerraSAR-X, the first civil German synthetic aperture radar (SAR) satellite, was successfully launched on June 15, 2007. After 4.5 days, the first processed image was obtained. The overall quality of the image was outstanding; however, suspicious features could be identified which showed precipitation-related signatures. These rain-cell signatures are thoroughly investigated, and the physical background of the related propagation effects is provided. In addition, rain-cell signatures from former missions like SIR-C/X and the Shuttle Radar Topography Mission are provided for comparison. During the commissioning phase of TerraSAR-X, a total of 12 000 scenes were investigated for potential propagation effects, and about 100 scenes revealed atmospheric effects to a visible extent. Some of the particularly interesting events were selected and are discussed in greater detail. An interesting case of data acquisition over New York will be presented, which shows typical rain-cell signatures, and the SAR image will be compared with weather-radar data acquired nearly simultaneously (within the same minute). By comparing the images, it can be clearly seen that reflectivities in the weather-radar image of 50 dBZ may cause visible artifacts in SAR images. Furthermore, in this paper, we discuss the influence of the atmosphere (troposphere) on the external calibration of TerraSAR-X. By acquiring simultaneous weather-radar data over the test site and the SAR acquisition, it was possible to flag affected SAR images and to exclude them from the procedure to derive the absolute calibration constant. Thus, it was possible to decrease the 1 sigma uncertainty of the absolute calibration factor by 0.15 dB.

TerraSAR-X Calibration Results

TerraSAR-X is a satellite mission for scientific and commercial applications operating a highly flexible X-band SAR instrument with a multitude of different operation modes. As product quality is of crucial importance, the success or failure of the mission depends essentially on the method of calibrating TerraSAR-X in an efficient way during commissioning the entire system in a restricted time. Only then, product quality and the correct operation of the SAR system can be ensured. The paper describes the method of calibrating TerraSAR-X and final results derived from all calibration procedures.

Final results of the efficient TerraSAR-X calibration method

TerraSAR-X is a satellite mission for scientific and commercial applications operating a highly flexible X-band SAR instrument with a multitude of different operation modes. As product quality is of crucial importance, the success or failure of the mission depends essentially on the method of calibrating TerraSAR-X in an efficient way during commissioning the entire system in a restricted time. Only then, product quality and the correct in-orbit operation of the entire SAR system can be ensured. The paper describes both the in-orbit calibration method for TerraSAR-X and dedicated activities performed during the commissioning phase as well as final results derived from all calibration procedures.

Independent Verification of the Sentinel-1A System Calibration

In the frame of the COPERNICUS program, the main objective of the Sentinel-1 mission is to ensure the continuity of C-band SAR data for global earth monitoring. Sentinel-1A is the first of two C-band satellites launched in April 2014. In addition to the commissioning of Sentinel-1A executed by the European Space Agency (ESA), an independent verification of the system calibration has been performed by DLR under an ESA contract. For this purpose, the complete calibration chain was developed and established, starting with a calibration concept, a detailed in-orbit calibration plan and the software tools for analyzing and evaluating all the measurements up to the calibration targets serving as accurate reference. Based on an efficient calibration strategy, this paper describes the different activities performed by DLR and presents the results obtained during the commissioning phase (CP) of Sentinel-1A.

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.

Reference Target Correction Based on Point-Target SAR Simulation

The backscattering from man-made point targets like passive corner reflectors and active transponders is often used as a radiometric calibration standard for synthetic aperture radar (SAR) calibration. As new systems emerge and the demand for more accurate systems increases, it becomes necessary to better understand the effects of real or imperfect targets on the radiometric calibration results. Therefore, a point-target SAR simulator is presented which models the complete external radiometric calibration process. It incorporates a number of target properties like frequency response, transponder internal calibration strategies, noise, and interference signals, and it takes the instrument SAR mode settings into consideration. Thereby, the relevant target backscatter variation as observed in the processed SAR image with respect to an ideal or any other target can be determined. The simulation results are relevant during the design process of a new target as well as during the actual calibration of a SAR system. Based on these point-target simulations, correction coefficients can be stated for each target and SAR mode, therefore decreasing the remaining radiometric point-target uncertainties. The quantitative examples in this paper show that these corrections can influence the absolute radiometric calibration by more than 1 dB.

TerraSAR-X calibration - first results

As TerraSAR-X, due for launch in 2007, will be an operational scientific mission with commercial potential, product quality is of crucial importance. The success or failure of the mission essentially depends on the calibration of the TerraSAR- X system ensuring the product quality and the correct in-orbit operation of the entire SAR system. The paper describes the in- orbit calibration procedure for TerraSAR-X and dedicated activities performed during the commissioning phase. First results could not be described because TerraSAR-X was not launched up to the time of uploading the full paper.

The Three-Transponder Method: a Novel Method for Accurate Transponder RCS Calibration

Transponders (also known as polarimetric active radar calibrators or PARCs) are commonly used for radiometric calibration of synthetic aperture radars (SARs). Currently three methods for the determination of a transponders frequency-dependent radar cross section (RCS) are used in practice. These require either to measure disassembled transponder components, or a separate radiometric measurement standard (like a flat, metallic plate or a corner reflector), leading to additional uncertainty contributions for the calibration result. In this paper, a novel method is introduced which neither requires disassembly nor an additional radiometric reference. Instead, the measurement results can be directly traced back to a realization of the meter, lowering total measurement uncertainties. The method is similar in approach to the well known three-antenna method, but is based on the radar equation instead of Friis transmission formula. The suitability of the method is demonstrated by a measurement campaign for DLRs three new Kalibri C-band transponders, completed by an uncertainty analysis. The method is not universally applicable for all transponder calibrations because (a) three devices are necessary (instead of only one for the known methods), and (b) the transponders must provide certain additional features. Nevertheless, these features have become standard in modern SAR calibration transponder designs. The novel, potentially more accurate three-transponder method is thus a viable alternative for transponder RCS calibration, ultimately contributing to synthetic aperture radars with a reduced radiometric measurement uncertainty.

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.