REVIEW OF THE IMPACT OF ERS-2 PILOTING MODES ON THE SAR DOPPLER STABILITY

ERS-2 was piloted in yaw-steering mode using three gyroscopes since the beginning of the mission until February 2000, when a new yaw-steering mode using only one gyroscope was implemented. The ERS-2 gyroscopes have experienced several problems during the mission and the new mono-gyro mode (1GP) was intended to ensure the mission continuity even in case of additional failures. In January 2001 a new test piloting mode using no gyroscopes, the Extra- Backup Mode (EBM), was implemented as a first stage of a gyro-less piloting mode. The aim of this challenging mode was to maintain the remaining gyroscopes performance only for those activities absolutely requiring them, such as some orbit manoeuvres. A more accurate version of this yaw- steering zero-gyro mode (ZGM) was operationally used since June 2001 and the performance was further improved with the implementation of the Yaw Control Monitoring mode (YCM) at the beginning of 2002. The evolution from the nominal and extremely stable three-gyro piloting mode (3GP) to the YCM has allowed to successfully continuing the ERS-2 operations despite of the gyroscopes failures. Nevertheless, this evolution has significantly affected the stability of the satellite attitude and the SAR Doppler Centroid frequency. The scope of this paper is to provide an overview of the different ERS-2 piloting modes and to assess their impact on the platform attitude, the SAR Doppler stability, the product quality and the applications performance, particularly for interferometry.