Geographically correlated errors – problem solved?
It is known that gravity field errors map differently to ascending and descending passes of a satellite – either by the sum or by the difference of a ‘mean’ (geographically fixed) error and a ‘variable’ error component. In satellite altimetry, mean or geographically correlated errors are of particular concern, because they directly map into the sea surface heights. They are, however, not visible in single satellite crossover differences, because they cancel each other. Thus, with a single altimeter satellite there is no way at all to assess this type of errors – except by a retrospective comparison between old and new orbits, the latter reprocessed with improved gravity field models.
14 years ago – with the launch of TOPEX/Poseidon – a period began with two or even more altimeter satellites operating simultaneously. Subsequently dual-satellite crossovers could be used to estimate the mean orbit error – above all for those satellites whose orbits were considered to be less precise. This way ERS- and GFO-orbits have been adjusted to TOPEX, taken as reference. Long-term means of dual satellite crossover have also been used to invert the crossover and to estimate errors for the gravity field harmonics which in turn implies knowledge about the geographically correlated error. Dual satellite crossovers have been used to evaluate the spectral accuracy of the gravity field models which required to harmonize as far as possible the altimeter data and to use orbits based on the same gravity field. Dual satellite crossovers have been even used for a successful tuning of the Earth gravity field model (e.g. DGM-04) which in turn was taken to reprocess satellite orbits. Later on the quasi continuous tracking with GPS allowed to follow a “reduced dynamic” orbit determination procedure – minimizing gravity field induced orbit errors. With new generation gravity field models derived from the CHAMP and GRACE missions, the gravity field has now dramatically improved (and will further improve with the GOCE mission). Again, new (GPS-based) orbits reveal significant geographical error pattern – even for TOPEX which has been considered as a reference for other altimeter missions. Gravity field induced orbit errors are now minimized.
If geographically correlated error pattern are still discovered they are governed by other error sources. Multi-mission satellite crossovers, computed – in all combinations – from two or more contemporaneous altimeter systems, perform a dense network with high redundancy. A common analysis allows to estimate errors of the radial component of all altimeters and to verify if there are still geographically correlated error pattern.