Altimeter normal points

Each normal point is created out of 21 consecutive 1-Hz measurements, by fitting a third-order polynomial through the relative sea heights. If none of the measurements deviate from the fit by more than 3 times the RMS-of-fit, and the RMS-of-fit is less than 10 cm, the normal point is accepted. The normal point measurement is the relative sea height indicated by the polynomial at the time of the central measurement.

Before the altimeter normal points can be used in the orbit determination, the relative sea heights are converted back to corrected altimeter height measurements by subtracting them from the a priori orbital altitude. Because these measurements act as satellite altitude measurements above sea level, a reference sea level must be modelled as realistically as possible. Since the applied reference surface (the D-PAF MSS93A model) has only a limited accuracy (errors in the model and correction errors remain), the altimeter tracking data have to be down-weighted properly with respect to the SLR measurements. The RSS combination of propagation and geophysical correction errors, differences between the actual sea surface and the reference surface, data noise, and JGM-2 overall model uncertainties, was estimated to be equivalent with a 30 cm RMS altimeter measurement precision.

It was found that, because of the large (geographically correlated) errors in the reference sea level, adding altimeter data in the orbit determination was of little success.