Jens Reimann

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.

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.

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.

Radiometric Performance of the TerraSAR-X Mission over More Than Ten Years of Operation

The TerraSAR-X mission, based on two satellites, has produced SAR data products of high quality for a number of scientific and commercial applications for more than ten years. To guarantee the stability and the reliability of these highly accurate SAR data products, both systems were first accurately calibrated during their respective commissioning phases and have been permanently monitored since then. Based on a short description of the methods applied, this paper focuses on the radiometric performance including the gain and phase properties of the transmit/receiver modules, the antenna pattern checked by evaluating scenes acquired over uniformly distributed targets and the radiometric stability derived from permanently deployed point targets. The outcome demonstrates the remarkable performance of both systems since their respective launch.

Geometric Accuracy of Sentinel-1A and 1B Derived from SAR Raw Data with GPS Surveyed Corner Reflector Positions

The geometric accuracy of synthetic aperture radar (SAR) data is usually derived from level-1 products using accurately surveyed corner reflector positions. This paper introduces a novel approach that derives the range delay and azimuth shift from acquired SAR raw data (level-0 products). Therefore, the propagation path is completely retrieved from SAR pulse transmission up to the reception of the point target’s backscatter. The procedure includes simple pulse compression in range and azimuth instead of full SAR data processing. By applying this method, the geometric accuracy of ESA’s Sentinel-1 SAR satellites (Sentinel-1A and Sentinel-1B) is derived for each satellite overpass by using corner reflectors with precisely surveyed GPS positions. The results show that the azimuth bias of about 2 m found in level-1 products for Stripmap acquisitions is reduced to about 15 cm. This indicates an artificial bias arising from operational Sentinel-1 SAR data processing. The remaining range bias of about 1.0 m, observed in L0-products, is interpreted as the offset between the SAR antenna phase center and the spacecraft’s center of gravity. The relative pixel localization accuracy derived with the proposed method is about 12 cm for the evaluated acquisitions. Compared to the full processed level-1 SAR data products, this accuracy is similar in the range direction, but, for the azimuth direction, it is improved by about 50% with the proposed method.

Performance Evaluation of TanDEM-X Quad-Polarization Products in Bistatic Mode

In the TanDEM-X mission, quad-polarization data (HH, HV, VH, and VV-polarization channels) can be acquired at an experimental basis by acquiring images in the dual-receive antenna (DRA) mode. This mode was activated during the so-called TanDEM-X science phase, from October 2014 up to January 2016, serving the science community with a unique dataset for the demonstration of new SAR techniques and applications. Quad-polarization data has been firstly acquired in pursuit monostatic mode and, secondly, in bistatic configuration as well. TanDEM-X is the first spaceborne mission which allows for the acquisition of quad-polarization data in bistatic formation, with across-track baselines varying up to 4 km at the Equator. The current work completes the one presented in [1] , where TanDEM-X quadpolarization data, acquired in pursuit monostatic mode only, was analyzed and recommendations were drawn, in order to optimize the acquisition parameters, aiming at improving the final data quality. Such recommendations were then taken into account for the acquisition of quad-polarization data in bistatic configuration, starting from April 2015, and the obtained results are presented in this paper. Investigations have been performed, aiming at monitoring the effective improvement in data quality. For example, we investigated the impact of different system parameters, such as noise equivalent sigma zero (NESZ) or processing bandwidth on the SAR performance, together with their influence on the interferometric SAR (InSAR) performance, assessed in terms of interferometric coherence and relative height error. Finally, we introduce and discuss an experimental acquisition mode, which allows to synthesize a quad-polarization product by combining two simultaneous dual-polarization acquisitions.

Performance Analysis of TanDEM-X Quad-Polarization Products in Pursuit Monostatic Mode

Since 2010, the two twin synthetic aperture radar (SAR) satellites TerraSAR-X and TanDEM-X have been acquiring high-resolution images to generate a global Earths digital elevation model (DEM). Both satellites have been flying in a controlled close orbit formation, acquiring data in the nominal bistatic stripmap single-polarization mode. Once the acquisition of the dataset for the generation of the DEM has been completed, the flexibility offered by both SAR instruments in terms of interferometric, imaging, and polarization modes has been further exploited to demonstrate the different capabilities of the TanDEM-X experimental modes. By activating the dual-receive antenna mode, full polarimetric data can be acquired. For the first time, it has been possible to systematically command quad-polarization acquisitions in a dedicated TanDEM-X mission science phase, started in October 2014. In this paper, we present a first performance analysis and quality assessment of such quad-polarization products. The SAR image resolution and the noise equivalent sigma zero have been evaluated to show the quality of the focused SAR products. The influence of different instrument parameters on the SAR and interferometric performance, such as chirp bandwidth, pulse repetition frequency, or block adaptive quantization, has been investigated as well. For the evaluation of the interferometric performance, key parameters such as coherence and interferometric phase error have been analyzed. In this paper, the obtained results are presented and recommendations are given for the optimization in the commanding of TanDEM-X quad-polarization acquisitions.

Tandem-L: Main results of the phase a feasibility study

Tandem-L is a highly innovative SAR satellite mission for the global observation of dynamic processes on the Earths surface with hitherto unknown quality and resolution. Thanks to its novel imaging techniques and its unprecedented acquisition capacity, Tandem-L will deliver urgently needed information for the solution of pressing scientific questions in the areas of the biosphere, geosphere, cryosphere and hydrosphere. The feasibility of Tandem-L has been analyzed and confirmed in the scope of a phase A study, which has been conducted in close cooperation between the German Aerospace Center (DLR) and the German space industry. This paper provides an overview of the Tandem-L mission concept and summarizes the actual development status.

Absolute radiometric calibration of the novel DLR “Kalibri” transponder

The technological advancement of the synthetic aperture radar (SAR) principle leads to an innovative challenge for the calibration as well. In order to provide an active reference target for an accurate absolute radiometric calibration the knowledge of the targets backscattering characteristics is essential. For the recently developed DLR C-band transponder named “Kalibri” several strategies for an accurate determination of the radar cross section (RCS) have been analyzed. Based on a comparison with respect to accuracy and feasibility, several recommendations for the best transponder calibration strategy were established. The resulting RCS of the transponders retrieved from the most suitable measurement method is presented as well as a cross-validation to prove the plausibility of these results.