Combining the Observations from Different GNSS (Invited)

For a very long time GPS has clearly dominated the use of GNSS measurements for scientific purposes. This picture is changing: we are moving from a GPS-only to a multi-GNSS world. This is, e.g., reflected by changing the meaning of the abbreviation IGS in March 2005 from International GPS to GNSS Service. The current situation can be described as follows: GPS has the leading role in the GNSS because it has provided a very stable satellite constellation over many years. Some of the currently active GPS satellites are nearly 15 years old. These old satellites are expected to be decommissioned within the next years. On the other hand, due to the increasing number of active GLONASS satellites and the improved density of multi-GNSS tracking stations in the IGS network, the quality of the GLONASS orbits has drastically improved during the last years. The European Galileo system is under development: currently two test satellites (GIOVE-A and GIOVE-B) are in orbit. The IOV (in-orbit-validation phase) will start soon. Also the first test satellites for the Chinese Compass system are in space. For the maximum benefit the observations of these GNSS will be processed in a combined multi-GNSS analysis in future. CODE (Center for Orbit Determination in Europe) is a joint venture between the Astronomical Institute of the University Bern (AIUB, Bern, Switzerland), the Federal Office of Topography (swisstopo, Wabern, Switzerland), the Federal Agency for Cartography and Geodesy (BKG, Frankfurt am Main, Germany), and the Institut für Astronomische und Physikalische Geodäsie of the Technische Universität München (IAPG/TUM, Munich, Germany). It acts as one of the global analysis centers of the IGS and has started in May 2003 with a rigorous combined processing of GPS and GLONASS measurements for the final, rapid, and even ultra-rapid product lines. All contributions from CODE to the IGS are in fact multi-GNSS products -- the only exception is the satellite and receiver clock corrections. The procedure to derive the satellite and receiver clock corrections is under the transition from the currently operational GPS-only to the multi-GNSS mode including GPS and GLONASS. When CODE started with its multi-GNSS processing more than 6 years ago the network density and the number of active GLONASS satellites was very limited. Nowadays this situation has changed, which brings us into the position to review the strategy to combine the measurements from different GNSS in the data analysis. The presentation will discuss the advantages and disadvantages of the highest (only one constant inter-system bias) and lowest (a minimum number of common parameters) possible correlation between the observations of the individual GNSS.