COMBINED EOP SOLUTION DERIVED FROM GPS TECHNIQUE: EOP (IERS) P 01 Authors : D. Gambis and M. Sail IERS/CB, Paris Observatory, France COMBINED GPS SERIES OF POLAR MOTION AND LENGTH OF DAY Since 1995, a combined solution of the various GPS series is performed and is used in our current analyses. All series being given at one-day intervals and for the same date, the procedure of the combination is made by a weighted average of the various series. The weighting reflects the qualities of the series, long-term and short-term stability. Two different approaches are used for that purpose: a pair variance analysis based on the mutual comparisons of the series (Gray and Allan, 1974) and secondly comparisons to other reference series. Both lead approximatively to the same order of magnitude. The relative percentages of the series entering the pole and lod combination for 2004/2005 are listed in the following table. ------------------------------- X pole Y-pole lod ------------------------------ CODE 27 27 30 JPL 27 27 20 GFZ 36 36 25 ESOC 0.1 0.1 15 NOAA 0.1 0.1 0.1 SIO 0.1 0.1 9.8 EMR 9.7 9.7 0.1 --------------------------- Table. Percentage over of the various GPS series contributing to the EOP (IERS) P 01 pole and lod solutions UNIVERSAL TIME BASED ON BOTH VLBI AND GPS TECHNIQUES Due to the difficulty of determining the long-term behaviour of the non rotating system realized through the orbit orientation, Universal Time UT1 cannot be accurately derived from satellite techniques but only from inertial methods like VLBI. On the other hand, these techniques can determine the length of day variations (lod), derivative of Universal Time together with the orbital parameters. The various determinations which are made by the analysis of satellite data follow differents strategies; some of them integrate their estimates of lod to derive a "free-running" Universal Time series, some are constraining their determination using a-priori VLBI values in order to keep the consistency with the non-rotating inertial reference frame. Various studies (Gambis et al 1993; Gambis, 1996; Ray, 1996) have shown that the high-frequency signal contained in the lod estimates on time scales limited to a couple of months derived fron SLR and GPS can be used to densify the series obtained by the VLBI technique and also for near-real time earth orientation monitoring. For clarification it was felt (Ray, 1997) and accepted by the commu- nity that the acronym UT1 should be reserved to Universal universal time derived from inertiel techniques (astrometry, VLBI). Since december 1995, the Central Bureau of IERS is operationally publishing a mixed Universal Time solution based on a combined short-term GPS UT solution calibrated by the long-term VLBI UT1 series. The stategy has now evolved; since spring 1997, a combined GPS lod solution is calculated using the 7 GPS Analysis Centers estimates and integrated to give an "internal free-running" solution which is finally calibrated by VLBI and labelled EOP(IERS) 97 P 01. USE OF UT1 GPS ESTIMATES FOR NEAR REAL-TIME APPLICATIONS Another application of lod (or UT1 integrated series) derived by GPS is the estimation of Universal Time from the last available VLBI estimation. This problem is now dramatic with the availability of rapid estimation of lod estimates from CODE analysis center. These lod estimates are integrated to give a UT solution which is piped to the last VLBI value. This procedure takes into account a model to correct long-term errors in the GPS UT series (Gambis,1996). This model now consists of a linear term or an auto-regressive process. Since the beginning of 1997, CODE has implemented a rapid orbit determination including preliminary lod estimates (Rothacher, 1997) and available twice a week (once 2 days and once 5 days). These estimates are integrated into a free running UT series; this series after long-term corrections removed is piped to the last available VLBI estimate or C04 solution. The operationel accuracy of UT1 derived from this procedure is now on the range of 30 microseconds. REFERENCES Gambis, D, N. Essaifi and E. Eisop and M. Feissel, 1993, Universal time derived from VLBI, SLR and GPS, Proc. of the 1993 IGS workshop of the International Association of Geodesy, pp212-217. also published in IERS technical note 16, Dickey and Feissel (eds), ppIV15-20.. Boucher C, Z. Altamimi, D. Gambis, E.Eisop and M. Feissel, Contribution of the Central Bureau of IERS, 1995 IGS Annual report. JPL Publication, 27.. Gambis D., Essaifi N., Eisop E. and M. Feissel, 1993, Universal time derived from VLBI, SLR and GPS, IERS technical note 16, Dickey and Feissel (eds), ppIV15-20. Gambis D. , 1996, Multi-technique EOP combinations,proceedings of the 1996 IGS Analysis Center workshop, Silver spring.MD, edited by P. Van Scoy and R.E. Neilan, Pasadena, CA, JPL, JPL Publication 96-23. Gray, J.E. and Allan, D.W. 1974: A method for estimating the frequency stability of an individual oscillator, Proc 8th Ann. Symp. on Frequency Control}, 2439, 277--287. Ray J.R., 1996, Measurements of length of day using the Global Positioning System, JGR,101,20141-20149. Ray J.R., 1997, Proposal for the nomenclature and use of UT1-like measurements derived from satellite observations, personal communication.