Scatterometer ground processing review for gyro-less operations

The scatterometer on-board of the ERS spacecraft is range-gated, in opposition to Seasat/Nscat scatterometers that are Doppler filtered. It can thus admit some Doppler frequency shift in the returned echo. However, the bandwidth of the filter on-board of the ERS satellite limits the Doppler shift that can be tolerated on the returned echo data. For these reasons, the ERS spacecraft needs to be yaw-steered in order to minimize the residual Doppler shift. Due to a malfunction of several of the on-board gyroscopes used to govern the attitude of the ERS-2 spacecraft, precise yaw steering of the spacecraft cannot be guaranteed anymore. This appear reviews modification to the existing processing chain that are needed to be able to process the data acquired by the ERS-2 spacecraft in degraded attitude mode, so called zero-gyro mode (ZGM). The main modification is the introduction of the on the fly estimation of the yaw angle as input to a model of the acquisition geometry. This paper also details how the yaw angle is estimated from the received raw data by measuring the residual Doppler frequency shift. Finally, several improvements such as the impact of an increased spatial resolution of the backscattering coefficients are also discussed.