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.
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X pole Y-pole lod
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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
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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.