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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 transponder's 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 DLR's 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.

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 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 target's 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.

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.