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Sentinel-1A is a space-borne SAR system developed in the frame of the Copernicus Program. DLR supported the radiometric and polarimetric calibration of Sentinel-1A by the analysis of point target responses of several acquisitions considering different modes (all except wave mode), beams and polarization channels. An elevation dependent bias, which had not been properly predicted by the used antenna model, was found for all investigated modes. Offsets up to 2 dB were determined during the SAR instrument calibration phase, in particular for low and high elevation angles. Therefore, in order to correct these elevation biases, a radiometric refinement campaign was carried out by ESA in November 2015. After that, Sentinel-1A radiometric accuracy and radiometric long-term stability (over a period of one year) were analyzed. For this period, the radiometric accuracy and the channel imbalance in amplitude were determined for the main mode IW and V-polarization. Moreover, a slight drift of the derived calibration factor has been observed since July 2016, at the same time as an anomaly affecting several transmit and receive modules in one tile was detected. The radiometric behavior of Sentinel-1A should therefore be monitored for a longer period of time, especially to detect potential aging effects of the SAR instrument.

Polarimetric synthetic aperture radar (SAR) systems require verification and calibration of their polarimetric performance after launch. Dual-antenna calibration transponders with freely rotatable, linearly-polarized antennas are one of the most common calibration targets for this task. Because the transponders are used as a measurement standard, any misorientation of the polarization direction of the transponder antennas leads to characterization errors for the SAR instrument. In this paper, we present a novel calibration approach with which the polarization direction of linearly polarized antennas can be accurately determined; no additional reference antenna with a previously known polarization is required. The novel approach will reduce polarimetric transponder calibration errors in the future and will lead to a higher accuracy in calibrated, fully polarimetric SAR systems.

For the radiometric calibration of spaceborne synthetic aperture radar systems, active targets with known backscatter, so called transponders, are used. These serve as an external absolute reference, and the quality of the derived calibration parameters depends on the quality of the deployed transponders. Due to the temperature dependent behavior of the implemented active radio frequency components the transponder radar cross section is sensitive to the temperature of the components. This effect requires the implementation of a reliable and precise temperature management System to stabilize the temperature of the radio frequency components to a constant value under all relevant environmental conditions. In order to fulfill the increasing requirements for future transponders towards radiometric accuracy new temperature control concepts must be developed. For this purpose, existing temperature management systems are analyzed and a promising concept is investigated. In the final version of this paper the results of measurements in a climatic chamber are presented in order to analyze the performance of the new control concept.

The radiometric calibration of synthetic aperture radar (SAR) systems is typically based on the known backscatter of calibration point targets such as transponders. Before a SAR calibration campaign can begin it is therefore necessary to determine the backscatter (radar cross section or RCS) of the transponder itself. Known methods suffer from unnecessarily high uncertainty contributions and therefore also affect the radiometric uncertainty of the calibrated SAR system. In this paper we present a novel calibration approach which neither requires to disassemble the amplifier path nor necessitates a separate RCS calibration target such as a corner reflector or a circular disk. Significant sources of measurement uncertainties can therefore be outright avoided. The method is not applicable to all transponders though, and a certain type of transponder design is assumed. Nevertheless, direct metrological traceability to a realization of the SI base unit "meter" can be achieved for the first time. The reduced measurement uncertainties of the novel approach will subsequently allow to reduce the radiometric measurement uncertainties of air- and spaceborne SAR sensors in the future.

Transponders are, besides trihedral corner reflectors, the most commonly used measurement standards in radiometric SAR calibration. They allow signal recording for the reconstruction of the azimuth pattern of the SAR system, adjustments of the backscattering matrix for polarimetric applications, and radar cross sections (RCSs) which are potentially much larger than those of passive point targets. These advantages led DLR to develop, manufacture, and install three new, accurate C-band “Kalibri” transponders in South Germany, which are now being used for the calibration and monitoring of the Copernicus Sentinel-1A satellite. Before the transponders could be used as radiometric measurement standards, they needed to be calibrated themselves. In an effort to find the most accurate RCS calibration approach for the given transponder design, several existing methods were compared [1], and a new, potentially highly accurate method, devised which exploits the specific design of the Kalibri transponders [2]. The new “three-transponder method” is similar in principle to the known “three-antenna method”, but is based on the radar equation instead of the Friis transmission formula. The approach exploits the fact that modern transponders like the “Kalibri” device can also be operated as radars because of the integrated digital sub-system (which is needed to implement a digital delay line and incorporates an AD and DA converter). To conduct a complete measurement, three transponders and three measurements (with one transponder pair each) are required; refined measurement schemas are also possible. In comparison to existing methods, no additional radiometric measurement standard is needed, which so far has been one of the limiting factors in accomplishing lower calibration uncertainties. Measurement traceability is achieved by tracing a comparatively simple length measurement back to a national realization of the meter. Such a length measurements can be performed with high accuracy. The presentation will include the setup and the measurement results of a first demonstration measurement campaign. Despite remaining challenges in the practical implementation, the uncertainty analysis shows that the method is a good candidate for highly accurate transponder RCS calibrations in the future.

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.

Spaceborne synthetic aperture radars (SAR) are systems capable of acquiring high-resolution earth-observation data independent on time of day and weather. These SAR systems have to be calibrated before first use and at regular intervals. For these radiometric calibrations active targets with known backscatter, so called transponders, serve as an external reference. For determining the frequency-dependent radar cross section (RCS) of these transponders, different measurement methods are available. This paper gives an overview of four different strategies with a following uncertainty analysis according to the Guide to the expression of uncertainty in measurement (GUM). After comparison with respect to accuracy and feasibility, a recommendation for the best absolute radiometric transponder calibration method is given.