IGS Analysis Center Workshop - IGS - NASA

to IGS in the sp3 orbit files.In Figure 5.1 the station clock solutions for the two most precise Hydrogen Masers (HM) in theIGS network are compared, HM clocks have the highest precision and stability over periods ofseveral hours. After removing an offset and drift they in effect provide a convenient ground truthat a cm (.03ns) level. The HM clock at ALGO served as a time reference and the FAIR HM clockwas treated as unknown with a large, 1 second apriori sigma. As seen from the above figure theagreement between the two HM’s is at the 0.1 ns (3cm) level which is also consistent with formalclock sigmas. The systematic variation of about 0.2ns seen is likely due to orbit errors and changesin observed satellite constellation rather than the instability of the HM clocks. Satellite clocksolutions have formal errors comparable with the station clock solutions i.e. a few cm; but, theaccuracy and consistency tests are not possible here since there is no HM satellite yet! The situationchanges dramatically when clock estimation is based bn undifferenced phase data only (the currentsituation for AS satellites, since the problem with AS pseudorange observations has not beencorrected at most IGS stations), then formal sigmas increase to about 10ns (or 3m.).For convenience and completeness clock solutions should be also combined and included in theIGS official orbit product. Problems with the combination of different satellite clock solutions aresimilar to the problems encountered with orbit combinations, i.e. first reconciling the referencetime frame differences, then detection, rejection of outliers and weighting of the individual clocksolutions. A clock combination can be based on the orbit combination approach of Section 2.Reference time frame problems are rather specific and depend on approaches to time referencedefinition within a particular solution, There are many possible approaches which can be used,ranging from a single station (fixed) reference to weighting some apriori clocks either for somestations or satellites or both. The ideal and most desirable case would be to introduce or connectsome IGS stations to BIPM primary standards. This would not only enhance the lGS clockreference but also enhance BIPM’s time transfer and maintenance. Currently it is not possible tocombine the satellite clock information since only one Center contributes satellite clock solutions,however for the permanent service the contributing Centers should be encouraged to develop andsubmit satellite clock estimations.6. CONCLUSIONS AND RECOMMENDATIONSTwo methods for orbit combinations were presented and tested here, the first is based on aweighted average and the second is a rigorous orbit determination with orbits introduced aspseudo-observations. Both methods gave nearly equivalent combined orbits, agreeing at 5cm(RMS) in position and below O. lmas in orientation. The baseline tests in North America and inEurope also indicated that both combined orbits are practically equivalent and comparable to orbetter than the best (regional) orbits. Both types of orbits implied precision and orientation stabilityat or below the 3 ppb and 0.5mas, respectively. Satellite clock solutions and combination at the0.1 ns consistency level are also possible. Problems which require additional studies and attentioninclude: the choice of reference pole (IERS or a GPS solution), sub-daily EOP representation,mitigation or elimination of orbit discontinuities, reporting orbit accuracy, processingstandardization etc.The following recommendations are offered:1. The IGS combined orbits are based on both methods described and tested above, more55