In-orbit calibration strategy for Sentinel-1

Paul Snoeij, Ignacio Navas-Traver, Dirk Geudtner, Allan Østergaard, Bjorn Rommen, Michael Brown, Ramon Torres, Marco Schwerdt, Björn Döring, Manfred Zink, Roland Meynart, Steven P. Neeck (+1 others)
2012 Sensors, Systems, and Next-Generation Satellites XVI  
The ESA Sentinels constitute the first series of operational satellites responding to the Earth Observation needs of the EU-ESA Global Monitoring for Environment and Security (GMES) programme. The GMES space component relies on existing and planned space assets as well as on new complementary developments by ESA. ESA is developing the Sentinel-1 European Radar Observatory, a constellation of two polar orbiting satellites for operational SAR applications. The two C-band radar satellites will
more » ... satellites will provide continuous all-weather day/night imagery for user services, especially those identified in ESA's GMES service elements programme and on projects funded by the European Union (EU) Framework Programmes. Radiometric calibration must be performed as part of the normal operation of the SAR. The calibration process is divided into two components: internal and external calibration. Internal calibration provides an assessment of radar performance using internally generated calibrated signals, especially in the context of pre-flight testing and in-orbit verification and during the nominal operational phase. The Sentinel-1 SAR instrument has demanding requirements for measurement stability (0.5dB, 3σ). Considering in orbit conditions and ageing effects, the hardware alone cannot provide sufficient stability to fulfill these requirements. Therefore an internal calibration system is implemented to measure the actual instrument gain and phase changes in order to apply them later, in the ground processing, for correction of the image data. The parameter to be derived by internal calibration is the so-called PG product, a quantity proportional to the product of transmit power of a polarization channel and gain of both the co-and cross-polarization receiver. It is to be noted that the PG product is complex and allows correcting both amplitudes and phases of the image data. External calibration makes use of ground targets of known RCS to render an end-to-end calibration of the SAR system, thereby assessing the impact of those elements that are difficult, if not impossible, to assess using internal methods. External calibration methods can involve the use of: passive and precisely constructed targets, such as corner reflectors and spheres; natural terrain with known backscattering properties, such as the Amazon rainforest; or active transponders. The most important point with respect to the in-orbit calibration and performance verification of this flexible SAR system is the tight performance with an absolute radiometric accuracy of only 1 dB (3 σ) for all operational
doi:10.1117/12.964992 fatcat:nq5xtwsyvzhvvdc5w3syu3ipzy