Daniel Rudolf

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.

SUMIRAD: a low-cost fast millimeter-wave radiometric imaging system

For many military or peace-keeping operations it is necessary to provide better situational awareness to the commander of a vehicle with respect to possible threats in his local environment (predominantly ahead), at a distance of a few ten to a few hundred meters. Such a challenging task can only be addressed adequately by a suitable multi-sensor system. As a beneficial part of that, an imaging radiometer system with a sufficiently high frame rate and field of view is considered. The radiometer, working 24 hours in all weather and sight conditions, generates quasi-optical images simplifying the microwave image interpretation. Furthermore it offers the advantage to detect and localise objects and persons under nearly all atmospheric obstacles and also extends the surveillance capabilities behind non-metallic materials like clothing or thin walls and thin vegetation. Based on constraints of low costs and the observation of a large field of view, the constructed radiometer still offers a moderate resolution at a moderate scan speed. This paper describes the challenges for the design of a vehicle-based imaging radiometer system at W band, providing high-quality images of sufficient resolution for a large field of view at a moderate frame rate. The construction is briefly outlined and imaging results for several situations are presented and discussed. Those comprise measurements on target detection and a visual comparison of different SUM (Surveillance in an Urban environment using Mobile sensors) data products.

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.

SUMIRAD – a near real-time MMW radiometer imaging system for threat detection in an urban environment

The armed forces are nowadays confronted with a wide variety of types of operations. During peace keeping missions in an urban environment, where small units patrol the streets with armored vehicles, the team leader is confronted with a very complex threat situation. The asymmetric imminence arises in most cases from so called IEDs (Improvised explosive devices) which are found in a multitude of versions. In order to avoid risky situations the early detection of possible threats due to advanced reconnaissance and surveillance sensors will provide an important advantage. A European consortium consisting of GMV S.A. (Spain, “Grupo Tecnològico e Industrial”), RMA (Belgium, “Royal Military Academy”), TUM (“Technische Universität München”) and DLR (Germany, “Deutsches Zentrum für Luft- und Raumfahrt”) developed in the SUM project (Surveillance in an urban environment using mobile sensors) a low-cost multi-sensor vehicle based surveillance system in order to enhance situational awareness for moving security and military patrols as well as for static checkpoints. The project was funded by the European Defense Agency (EDA) in the Joint Investment Program on Force Protection (JIP-FP). The SUMIRAD (SUM imaging radiometer) system, developed by DLR, is a fast radiometric imager and part of the SUM sensor suite. This paper will present the principle of the SUMIRAD system and its key components. Furthermore the image processing will be described. Imaging results from several measurement campaigns will be presented. The overall SUM system and the individual subsystems are presented in more detail in separate papers during this conference.

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.

SUMIRAD - A close to real time MMW radiometer imaging system

The armed forces are nowadays confronted with a wide variety of types of operations. During peace keeping missions in an urban environment, where small units patrol the streets with armored vehicles, the team leader is confronted with a very complex threat situation. The asymmetric imminence arises in most cases from so called IEDs (Improvised explosive devices) which are found in a multitude of versions. In order to avoid risky situations the early detection of possible threats due to advanced reconnaissance and surveillance sensors will provide an important advantage. The aim of the SUM project (Surveillance in an urban environment using mobile sensors) is to develop a low-cost multi-sensor vehicle based surveillance system in order to enhance situational awareness for moving security and military patrols as well as for static checkpoints. The SUMIRAD (SUM imaging radiometer) system is a fast radiometric imager and part of the SUM sensor suite. This paper will present the principle of the SUMIRAD system and its key components. Furthermore the image processing of the final image product reconstructed from the data stream of each radiometer receiver will be described. Imaging results from several measurement campaigns will be presented.

Investigation of fully-polarimetric signatures from targets with some relevance to security applications

The actual and continuous threat by international terrorism and the increasing number of terroristic attacks raise the danger to the public and create a new and more complex dimension of threat. This evolution must and can only be combatted by the application of new counter-measures like advanced imaging technologies for wide-area surveillance and the detection of concealed dangerous objects. Passive microwave remote sensing allows a daytime independent non-destructive observation and examination of the objects of interest under nearly all weather conditions without artificial exposure of persons and observation areas, hence fully avoiding health risks. Furthermore the acquisition of polarimetric object characteristics can increase the detection capability by gathering complementary object information. The recent development and construction of a fully-polarimetric receiver at W band allows the acquisition of a new dimension of information compared to former imaging capabilities. The new receiver can be part of various imaging systems used at DLR over the years. This paper will show some imaging results recorded recently from different sceneries.

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.