Subject: [IGSMAIL-1170] Combined EOP solution derived from GPS
From: IERS / CB
Date: Fri, 22 Dec 1995 4:46:18 PST

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IGS Electronic Mail      Fri Dec 22  4:46:18 PST 1995      Message Number 1170
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Author: IERS / CB
Subject: Combined EOP solution derived from GPS

Paris Observatory
	  D. Gambis    [email-masked]
          E. Eisop      [email-masked]

  An EOP combined solution labelled EOP(IERS) 95 P 01 based on individual GPS
 series is proposed. It is available since 1994.0 and is weekly updated.
 It contains the coordinates of the pole and UT1-UTC referred to EOP(IERS) C04.
 Questions, remarks and suggestions concerning this solution are welcome.
  
DATE : 20 december 1995

ACCESS TO IERS/CB DATABASE:

ftp 145.238.2.21
login: anonymous
password: your name
directories: iers

DESCRIPTION OF THE FILES:

gpsierscb.notice   :  description of the combined GPS solution  
gpsierscb.eop94    :  combined GPS solution for 1994
gpsierscb.eop95    :  combined GPS solution weekly updated for 1995 

gpsierscbx.ps :  Post-script file relative to graphics for X polar motion 
		component of the residuals of IGS solution and individual
 		GPS solutions to gpsierscb.eop. The solution is given for a
		period of 3 months in the past.
gpsierscby.ps :  idem for Y polar motion
gpsierscbut.ps  :  idem for UT 

Graphics files concerning past periods are available and will be sent on 
request.

ANALYSIS PROCEDURE:

POLAR MOTION

	The analysis centers participating in this combination are : CODE,
EMR, ESOC, GFZ, JPL, NOAA and SIO. Each series included in this solution
has been weighted according to its formal uncertainty and scaled according 
to external comparison. 
	The combined series is given in EOP (IERS) C04 system. It is 
slightly smoothed in order to remove high-frequency white noise, the cutoff
frequency is 2-3 days. The average standard error of a dayly value is 0.2 mas. 

UNIVERSAL TIME 

	Universal Time solution combined by IERS from individual series is
mainly based on VLBI inertial techniques. The celestial reference system 
defined by satellite techniques is not stable over long periods of time; 
this prevents the satellite technique to accurately determine long term UT1
variations. However, as it has been shown (Gambis et al, 1993), it is still
possible to combine the high-frequency fluctuations contained in  GPS "UT1"
series with the long-term variations of the VLBI or IERS C04 solution to 
derive a mixed  UT1(VLBI+GPS) solution.
	The Universal Time we are computing is therefore a combined solution of 
the various available GPS Universal Time solution calibrated by EOP(IERS) C04.
At present the GPS series included in the analyses are those of CODE, EMR and JPL.
        The average uncertainty is about 0.04 ms for a daily value. 

REFERENCE

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..

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