Altimeter height residuals are a means to monitor the radial orbit error, although part of the residuals are caused by the fact that the reference surface will not be a perfect representation of the actual instantaneous mean sea level. The radial orbit error can, however, be distinguished from reference model errors in the spectrum of the height residuals. Orbit errors will be concentrated around the lower frequencies (particularly around 10-cpr), whereas reference model errors are expected to occupy the high-frequency part of the spectrum (above 10-cpr).
From analyses of the altimeter residuals it appeared that the origin of the MSS93A model is offset from the actual orbital reference frame imposed by the SLR station coordinates. From a harmonic analysis of the altimeter height residuals it was found that, in order to match both origins, the MSS93A model has to be translated along a vector cm, yielding a reduction of the standard deviation of the altimeter height residuals from 23.6 to 19.6 cm. A similar result was found with altimeter height residuals based on the D-PAF orbit.
Figure 5: (top left) Residuals of the altimeter height normal points with respect to the MSS93A mean sea surface model with application of the DUT JGM-2 orbits and origin correction. (top right) Periodogram of the altimeter residuals. (left centre) Amplitudes of the sine and cosine part of the 1-cpr signal in the residuals, obtained through complex demodulation. (bottom left) Altimeter residuals after demodulation. (bottom right) Histogram of the demodulated signal.
Figure 5 gives a detailed analysis of the remaining altimeter height residuals with respect to the MSS93A mean sea surface model after implementation of the estimated offset: