Receiver/Payload Hardware Biases Stability Requirements for Undifferenced Widelane Ambiguity Blocking
Mercier, F.; Laurichesse, D.
CNES

Integer ambiguity blocking on the GPS phase observables is a key contributor to precise applications in geodesy, orbit determination and time transfer. Usually this blocking is performed on double differences (for geodesy for example) or more recently on single differences measurements. Ambiguity blocking on undifferenced measurements is a step forward to improve the performance of these applications.

Within the CNES Orbit Determination Group we have developed a new method for blocking undifferenced Widelane ambiguities directly at receiver level, using only receiver measurements and a few GPS system specific parameters. The undifferenced Widelane ambiguities are blocked on a worldwide network, without any external data such as receiver trajectory, constellation orbits and clocks, ionospheric and tropospheric effects. Currently the method has been applied on Ashtech Z12 and Rogue ACT receivers, giving a reliable Widelane ambiguity estimation for almost all passes (close to 100 %). The method has also been recently extended to Trimble NETRS receivers. Results of our GPS Widelane blocking at receiver level are validated on short baselines between different receivers, where single difference ambiguities can be found directly on each frequency.

Once Widelane ambiguities are known, it is very easy to directly block the remaining ambiguity of each pass, even for long baselines (up to 2000-2500 km). In addition, it is possible to solve for the remaining undifferenced ambiguities on a global receiver network, giving very precise solutions for positioning and time.

The difficulty of undifferenced measurements processing is that the biases due to receivers and GPS satellite payloads have to be taken into account to obtain the integer property of the ambiguities. We have computed these biases and studied their stability over long periods. Their long term stability is a key feature of the GPS system, both at satellite payload and receiver level, even though it does not appear to have been specified as such. This same level of stability needs to be obtained with the GALILEO system if we want GALILEO to provide at least the same level of precision as GPS for science applications. Our results on the stability of GPS biases could provide a first level of stability requirements for GALILEO.

In our presentation we will cover the basic elements of the approach, and present biases observed on different receiver networks and at different epochs. Then we will present different applications. First a global time synchronisation on a European receiver network, which is an application where undifferenced integer ambiguities are necessary to compute receiver and satellite clocks. Second, results for in orbit dual frequency receivers will be shown (for Jason). The possible improvements on precise orbit determination will be detailed. The impact of the new GPS signal L2C on our Widelane ambiguity blocking will also be discussed.