ABSOLUTE CALIBRATION OF THE ERS-2 SCATTEROMETER IN GYRO-LESS MODE USING TRANSPONDERS

The scatterometer on-board ERS-2, earth-observation satellite developed by the European Space Agency, is an active real aperture radar instrument designed to measure the backscatter coefficient 0 from the earth. Its main utility is to measure physical parameters such as wind speeds and directions over the sea surface. Due to malfunctions of the on-board gyroscopes and even though a new attitude and orbit control system is used, the yaw-orientation of the spacecraft still exhibits random variations from the nominal attitude. This unpredictable attitude requires re-developing the calibration chain of the scatterometer instrument. Distributed targets, such as the rain forest, are used to perform a relative calibration while transponders situated at known locations in Spain permit an absolute calibration of the instrument. The scope is to expose the obtained results and the improvements brought by the new chain regarding the absolute calibration. An analysis of the received echo with a sub-pixel resolution (better than the nominal resolution) allows getting accurate transponder localization. The localization error is used to determine, among other quantities, the antenna mounting angles errors. Since the signal emitted by the transponders is calibrated, radiometric calibration is also possible, including determination of a large set of parameters such as the absolute gain of each antenna. The raw data quality (such as DC offsets, gain imbalance, phase imbalance) and the spatial resolution are also computed