Verification of the Sentinel-1A SAR Instrument Calibration using active and passive Point Targets

Abstract

In the frame of the GMES program of European Space Agency (ESA), the Sentinel-1A satellite was launched in April 2014 to ensure the continuity of SAR data acquisitions in C-band for global earth monitoring. The calibration of the SAR instrument was performed by ESA and their partners during the commissioning phase. In order to execute an independent verification of the instrument calibration on behalf of ESA the complete calibration chain was developed and established by German Aerospace Center (DLR), starting with a calibration concept, the calibration algorithms, a detailed in-orbit calibration plan, the software tools for analyzing and evaluating all the measurements up to the calibration targets serving as accurate reference. For this purpose passive and active point targets (three corner reflectors and three transponders) are permanently installed in Southern Germany covering an east-west range of about 80 kilometers. Each target is remotely manageable to consider the individual setup for each satellite overpass, e.g. the azimuth and elevation angles to the corresponding Zero-Doppler plane of the SAR acquisition. The paper gives a detailed view on the system calibration of Sentinel-1A. Transmitted SAR pulses have been detected by transponders in order to analyze the pulse characteristics after propagation through the atmosphere and ionosphere and to verify the antenna pattern. Furthermore, the L0 and L1 products of the SAR instrument are evaluated to verify the geometric calibration by exploiting the accurate target positions and to characterize the radiometric performance of Sentinel-1A for different modes (e.g. stripmap and TOPS).