Combining SLR and GNSS measurements
The activities at AIUB related to the analysis of SLR data can be grouped in three major topics:
1. Analysis of LAGEOS data;
2. Statistics on SLR range residuals to GNSS orbits based on microwave data;
3. Combined analysis of microwave and SLR range data to GNSS satellites.
The Bernese Software recently has been extended for the capability of analysing SLR observations to spherical geodetic satellites, e.g. LAGEOS and ETALON. Since July 2010, the contribution of BKG (one of the partners within the CODE consortium) to the International Laser Ranging Service (ILRS) is based on the SLR development version of the Bernese Software.
Seven-day solutions based on LAGEOS data are delivered on a routine basis to the ILRS. Estimated parameters are station coordinates, polar motion, LOD, range biases (for a few stations only) and satellite orbits. The ILRS has two product lines: the so-called "DAILY" product is due just one day after the data (i.e. covering the time span from "today - 8 days" until "today - 1 day"). The so-called "WEEKLY" product covers the time span from Sunday to Saturday and is due on Tuesday evening.
The comparison of SLR range data with GNSS orbits based on microwave data allows to validate the GNSS orbits. On the other hand, the SLR measurements can be calibrated, too. CODE, as an Associated Analysis Center (AAC) of the ILRS, provides daily comparisons of SLR tracking data with CODE orbits for GPS and GLONASS satellites in form of quick-look reports. These reports are distributed via e-mail to the SLR-report mail exploder every day, giving rapid feedback on the SLR data quality to the ILRS community.
Besides the pure SLR residual analysis, a combined analysis of microwave and SLR range measurements to GPS and GLONASS satellites has been performed at AIUB. In such an analysis the GNSS satellites can be used as co-location point instead (or in addition) to the co-located ground stations. Using satellite co-locations implies that one common set of orbit parameters is estimated based on microwave and SLR range observations together.
The common orbit parameters allow it to transfer the absolute scale information provided by the SLR range observations directly to the GNSS part. This is of particular interest because the GNSS-derived scale is contaminated by uncertainties in modeling the phase center of the transmitting and receiving antennas. Other studies already revealed that the satellite antenna offsets (SAO) of the GNSS microwave antenna w.r.t the center of mass (COM) of the satellite officially adopted within the IGS (given by igs05.atx) might be wrong by several centimeters. Therefore, we estimated the SAO from a combined analysis of GNSS microwave and SLR range data. Four years of data have been considered (2006-2009). The estimated corrections for the SAO in nadir direction are shown in Fig. 1. The mean correction for the GPS satellites clearly differ from the mean correction for the GLONASS satellites, i.e., 76.4 mm and -47.7 mm, respectively.
When estimating the SAO parameters, the GNSS part has a rank deficiency regarding the scale, so that the GNSS sub-network in the combined analysis will fully adapt the scale provided by the SLR data. We see a difference of 0.59 ppb for the GNSS sub-network between the solution with estimated SAO parameters and a solution with SAO fixed to the IGS05 values, whereas the scale of the SLR sub-network does not change. This behaviour clearly demonstrates that SLR provides the scale in the combined solution (although there is only a very few amount of SLR data available compared to the amount of microwave data). Furthermore, it becomes clear that the SAO values provided by IGS05 does not fit to the scale of SLR.
On the SLR side, the uncertainties in the offsets of the laser retro-reflector arrays (LRA) w.r.t. COM of the GNSS satellites and the presence of range biases have to be handled. We set-up one bias parameter per SLR station and GNSS satellite. Figure 2 shows the resulting bias parameters for two satellites.
Figure 2: Bias parameters estimated for each SLR station in order to account for discrepancies between GNSS and SLR. a) GPS PRN 06, b) GLONASS PRN 15.