Does the Inclusion of Regional Navigation Satellite Systems Help or Harm Global Solutions?

Within the last years, the number of Global Navigation Satellite Systems (GNSS) was completed by adding the full global constellations of Galileo and BeiDou to the established GPS and GLONASS satellite configurations. These satellites are typically flying on circular orbits in medium heights between 19000 and 24000km above the Earth surface. The resulting orbit revolution period and the ground track repeatability varies for the individual constellations (two revolutions per one sidereal day for GPS, 17 revolutions taking seven, eight, and ten sidereal days for BeiDou, GLONASS, and Galileo respectively). It has been shown that the combination of measurements from several of these constellations with different orbit characteristics reduces the influence of the constellation-specific frequencies into GNSS-derived results, like station coordinate time series or Earth rotation parameters. There are a number of satellites in so called inclined geosynchronous orbits for densifying the global GNSS constellations in certain regions (e.g., Japan with QZSS or BeiDou for China). These satellites have a completely different orbit characteristics than the other GNSS satellites (36000km above the Earth surface with one revolution per sidereal day). According to the experience with the combination of different GNSS constellations the inclusion of these satellites should be beneficial for reducing constellation-specific effects. On the other hand, these satellites are only available in the Asian region covering only about a quarter of the equator. Such an asymmetric satellite distribution may reduce the robustness of global datum parameters for station coordinates or Earth rotation parameters. With dedicated experiments we will answer the question whether global GNSS solutions targeting geodynamical parameters do benefit from the inclusion of such satellites or whether they degrade the solution.