Calibration of the ERS-2 scatterometer in gyro-less mode

The finite bandwidth of the filter on-board of the ERS-2 satellite limits the Doppler shift that can be tolerated on the received echo signal. For this reason, the spacecraft need to be yaw steered. However, due to malfunctions of several gyroscopes, precise yaw steering cannot be guaranteed any more. This prompted for a review of the scatterometer processing chain [6] in order to estimate the actual yaw angle and to take it into account in the derivation of the normalized backscattering coefficients. The calibration of the instrument is performed using transponders and distributed targets. Transponders at known locations (on ground) can be used to perform an absolute calibration of the instrument. The distributed targets can be used to perform a relative calibration of the instrument, assuming an isotropic behaviour of the distributed targets. This paper will present the reviewed absolute calibration chain, the validation of the results with the previous calibration chain and the results in degraded modes. We will also present the calibration pulse I/Q characterization and the results of the natural targets processor (relative calibration over the rain forest).