Radiometric Accuracy and One-Year-Stability of Sentinel-1A Determined using Point Targets

Abstract

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.