Calibration and early results of the ASAR on ENVISAT

This paper presents the approach for the in-flight calibration of the ENVISAT-1 ASAR and the verification of the ground processing facility PF-ASAR during the commissioning phase. The philosophy presented is a logical progression from the experience gained during calibration of the ERS SARs. The ASAR has a comprehensive internal calibration loop, which is described distinctly from the external calibration and characterisation. The antenna patterns of the various beams have been fully measured during on-ground flight-model testing and have been used for initial performance predictions. In-flight characterisation of the main beams is performed over the South American rainforest. As for ERS, absolute gain calibration is achieved using three fixed and one transportable precision calibration transponders situated in the Netherlands. These transponders are also capable of recording the azimuth beam patterns and supporting the external characterisation mode of ASAR.