[IGSMAIL-98] Comments on the GPS determinations of UT by the CODE Processing Centre

[Martine]

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IGS Electronic Mail       17-SEP-1992 18:35:28       Message Number 98
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>From:    Martine Feissel, Central Bureau of IERS
Subject: Comments on the GPS determinations of UT by the CODE Processing Centre
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A series of 77 values of UT1(CODE)-UTC available on Sep 17 is compared with
the EOP(IERS) 90 C 04 series (a permanently updated series, based on VLBI, LLR
and SLR). The GPS series considered is EOP(CODE) 92 P 04, recently obtained by
appending the weekly solutions in such a way that only the first value
(MJD 48792.5) is a priori fixed. The difference with the previous version,
EOP(CODE) 92 P 03, is that the cause for a spurious 14 day term which 
was present in the solution was found at CODE and corrected.

The series of UT1(CODE)-UT1(IERS) has a clean high frequency spectrum.
In the long term, the results have a daily rate of + 0.042 +- 0.001 ms/day.
The residuals after this correction suggest that a quadratic term centered
around MJD 48830 (Jul 27) might take a sizeable part of the discrepancies
left. The weighted rms residuals after these various treatments are as follows.

                Correction                       Rms residual

Bias                                               0.910 ms
Bias + drift                                       0.135 ms
Bias + drift + quadratic term                      0.088 ms

The quadratic term is indeed an ad hoc empirical model. The series is still
too short to decide whether a periodic term (for example) would be more
appropriate.

To make a comparison, the rms residuals of the VLBI daily values and the
SLR 3-day values (tied to VLBI on the long term), after taking out a bias,
are respectively 0.049 ms and 0.072 ms on the same time interval.

Although the cause of the trend still waits to be found, these results are
extremely encouraging. I would suggest that other Processing Centres consider
the possibility to estimate UT1 in their solutions.