CoAuthors

Alexandre Couhert (CNES, France)

Currently, the GPS orbits are determined using GPS pseudo-range and phase measurements with floating ambiguities (the phase ambiguities are determined with fractional parts). On ground stations, the phase ambiguity fixing to integers improves significantly the stability of the solutions, for example the solutions produced by the IGS analysis centers.
The ambiguity fixing on LEO satellites has been demonstrated on Grace, HY2A and Jason 1, but is not currently used for the delivered products. Unfortunately, the Jason 2 GPS receiver was not producing correct ambiguity references (lot of half cycles are present in the phase measurements), and a reliable ambiguity fixing is very difficult to achieve on Jason 2, although the receiver is similar to the Jason 1 and Grace receivers.
For Jason 3, the first tests performed at CNES show that the initial step of the procedure works correctly (widelane ambiguity fixing). Some passes have important pseudo-range biases, usually just after complete interruptions, this must be taken into account. The second step (narrowlane ambiguity fixing, 10.7 cm wavelength) seems possible, but further investigations are needed. The results of this on going work will be shown.