The Swiss Federal Office of Topography is currently building up the Automated GPS Network Switzerland, called AGNES. This GPS network will serve various purposes, like national surveying, engineering surveying, navigation, meteorology, and research.
For wide-area differential GPS (WADGPS), in particular precise real-time positioning using the GPS phase data of one or more reference stations of the AGNES network, one may expect that atmospheric refraction plays the crucial role, since the reference stations may be rather widely separated in Switzerland, typically by 50-100 km. This assumes that orbits and “absolute” tropospheric refraction for at least one of the sites are known with sufficient precision.
In the time period of the next solar maximum, the ionospheric refraction will probably be the most crucial error source, in particular for resolving the initial phase ambiguities, which is mandatory when aiming at a coordinate accuracy on the centimeter level. Nowadays, global and regional total electron content (TEC) maps, like those produced by the Center for Orbit Determination in Europe (CODE), are available and may be used to model the mean ionospheric refraction. However, on time scales of few minutes, ambiguity resolution usually suffers from short-term ionospheric fluctuations which are not accounted for by such TEC maps.
Significant height differences with respect to the nearest reference stations may also occur, especially in mountainous areas like Switzerland. This implies that not perfectly modeled tropospheric refraction may cause biases too.
We study the impact of tropospheric as well as ionospheric biases on WADGPS results and investigate whether it is possible to extract atmospheric information from networks of the AGNES-type for correcting data of mobile GPS receivers in (near) real time.