Subject: [IGSMAIL-1072] Changes in USNO GPS-only Combination Procedure
From: D. McCarthy, B. Luzum
Date: Thu, 28 Sep 1995 13:16:38 PDT

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IGS Electronic Mail      Thu Sep 28 13:16:38 PDT 1995      Message Number 1072
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Author: D. McCarthy, B. Luzum
Subject: Changes in USNO GPS-only Combination Procedure

     Due to the recent results concerning the high-frequency
signal in the GPS polar motion, we have modified the procedure
used for combining the GPS series.  We have eliminated all
smoothing in the USNO combination of GPS-only results and changed
the epoch of the output series to match the input series.  These
changes will provide a better representation of the GPS polar
motion and the true Earth orientation signal.  The new USNO GPS
combination is essentially equivalent to the IGS mean pole series
but extends much farther back in time.

     The differences that will result from this change are the
smaller residuals seen in the plots and statistics beginning with
our report for week 819.  These are due to the closer tracking of
GPS at high frequencies as well as adjustments to the systematic
corrections that we apply.  Our current estimate of the GPS
accuracy is around +/- 0.1 msec. of arc.

     While no changes have been made to the basic multi-technique
smoothing algorithm, the reduced smoothing and the resulting
improved precision of the GPS data will mean that the IERS Rapid
Service will follow the GPS polar motion more closely.  If
unannounced systematic changes occur in any of the IGS Analysis
Center procedures, the Rapid Service results may be adversely
impacted since we have reduced the ameliorating effects of the
smoothing.  This could effect the quality of EOP predictions and
a priori estimates for future data analysis.
     
     Please be careful about any small changes in operations
which might have a systematic effect on your results.  As in the
past, we would appreciate any advance announcement of all such
changes and estimates of the resulting differences.  We can make
adjustments for known modifications, but for unannounced system
changes, there will be little recourse.
--
Dennis D. McCarthy                       [email-masked]
Head, Earth Orientation Department       phone: (202) 653-0066
U.S. Naval Observatory                   fax: (202) 653-0587
Washington, DC 20392-5420

[Mailed From: Dennis McCarthy <[email-masked]>]