Husson gave an update on the harmonization activities, intended to give a unique and unambiguous message to the SLR stations on the quality of their observations (preferably on a pass-by-pass level, but may not be achievable to better than a few cm) (Appendix 6). Currently, 6 analysis institutes perform an operational QC analysis on at least a weekly basis, and 4 of them have shifted to ITRF2000 station coordinates (site positions is a known source of discrepancies) (action item CSR, MCC). The disagreement on pass-by-pass range biases is large (a few to several cm).
If one aggregates the QC results over a month, there can still be cm level offsets, but a time series of these offsets will generally track each other to the sub-cm level. There are seasonal trends in the long term averages from the better performing sites, which is probably due to unmodeled geocenter movement and/or atmospheric pressure loading. A change to a site's coordinates (especially height) will induce a discontinuity in the site's apparent range bias. Therefore, it is imperative than analysis centers document coordinate updates (action item QC groups). In addition, an error in a site's height rate will induce a long term drift in the apparent range bias. Husson also pointed out that a "real" range bias change at one station also affects the residuals of neighboring stations, but to a lesser extent by virtue of the orbital link between stations in close proximity. In summary when interpreting QC reports, the following items need to be taken into consideration:
Aggregate the pass-by-pass results over a month.
The station coordinates/velocities used by the analysis centers.
A bias change in a neighboring site will influence your "apparent" site bias.
Husson also explored two new complimentary analysis techniques. Recognizing the correlation between range bias trends for neighboring stations and the typically "slow" change in orbital perturbations or errors, Husson has tested a technique where the time-series of range biases for a reliable station is taken as a reference, and is subtracted from the results available for neighboring stations (Appendix 6). This has been done on a monthly basis, and has led to impressive results: as an example, steps in biases and data corrections are now fully observable. In particular, the CSR solutions are capable of following physical on-site engineering changes. Possible problems in the height rates of stations (e.g. in ITRF2000, or another station coordinates representation) are also reflected in the outcome of this so-called "short-arc collocation". In this respect, remarks were made on the ITRF2000 vertical rates being "suspect" for a number of site including Riyadh, San Fernando, and Riga. The results from the 1999 "pos+eop" test 28-day coordinate solutions were also used as another bias estimation technique. This technique is in principle the best approach for determining absolute biases, since station positions are estimated simultaneously with range bias. The weakness of this technique is that there may not be adequate LAGEOS data from most sites in 28-days, in order to separate a range bias from station height change. When 28-day site bias estimates were averaged for one year, there was excellent consistency (to a few mm) between the range bias estimates obtained from ASI, CRL and CSR.
Finally, Husson also reported on a site tie analysis, which may be used to identify errors in the SLR coordinates solution, the GPS solution, and/or the observed site tie. Details can be found in Appendix 6.
In conclusion of this agenda item, Noomen briefly reported on the transition of station coordinates in use for the QC analysis performed each week in Delft: a shift from SSC(DUT)93L05 (extrapolated over more than 10 years by now) to ITRF2000 resulted in a reduction of the rms-of-fit from about 30-35 mm to about 20 mm on average (with LAGEOS-2 being at the level of about 16 mm); suggestions for further improvements were also given (Appendix 7).