Pre-combined GNSS-SLR Solutions: What Could be the Benefit for the ITRF?

Kurzfassung

In standard combination approaches (e.g., for the International Terrestrial Reference Frame), the space-geodetic techniques are connected by the Earth rotation parameters and by the station coordinates at co-located sites, using the so-called local ties. These local ties are usually derived from terrestrial measurements together with GNSS measurements for linking to a global reference system. The local ties often differ from the coordinate differences derived from space-geodetic observations. However, the sources for the discrepancies are not always clear, and a validation within the combination is difficult as long as the local ties are needed for the combination. We provide an alternative combination method by using the co-location of GNSS and SLR observations onboard GNSS satellites. As the local ties do not need to be applied in this approach, the resulting station coordinates are consistently estimated, but independent of the local ties. This allows us to evaluate the agreement of the terrestrial local ties and the space-geodetic coordinate differences derived from a 12-years solution. The 61 colocations investigated in a multi-year solution show an agreement better than 1 cm in the horizontal and height components for 41 and 27 co-locations, respectively. Co-locations showing big discrepancies can be explained by the shortness of the data set included, or the rare distribution over the time span. Only the co-location site San Fernando shows unexplained differences of several centimeters. When using satellite co-locations, the counterpart of local ties at stations are space ties at satellites. The offsets of the microwave satellite antenna form one component of the space tie, with the offset of the laser retro-reflector array (LRA) forming the second part. We show that corrections to the space ties can be estimated from combined GNSS-SLR solutions. The corrections to the LRA offsets are only 5–6 mm, whereas the corrections estimated for the microwave antennas can exceed 1 dm, with a mean correction of -86.1 mm and -110.4 mm for GPS and GLONASS, respectively.