MetOp GRAS: Signal tracking performance results

The GRAS radio occultation instrument on board MetOp has continuously provided measurement data during almost four years. The measurement data are here used as a means to analyze the signal tracking performance of the GRAS instrument. The results demonstrate that the atmospheric Doppler shift is measured to an accuracy of 1.1 mm/s and that the on-board Doppler model is accurate within 4 m/s (or 20 Hz). It is also found that rapid signal amplitude fluctuation, exceeding 10 dB, frequently occur at higher altitudes than expected and that it is possible to improve the signal tracking during such events.