Subject: [IGSMAIL-2432] ITRF96/97 IGS product changes
From: AC Coordinator
Date: Fri, 20 Aug 1999 4:17:56 PDT

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IGS Electronic Mail      Fri Aug 20  4:17:56 PDT 1999      Message Number 2432
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Author: AC Coordinator
Subject: ITRF96/97 IGS product changes

Dear IGS colleagues,

On 1 August 1999 the IGS changed its realization of the International
Terrestrial Reference Frame by switching from the ITRF96 to the ITRF97.
At the same time the set of reference stations (RF) was slightly
enhanced from 47 to 51 sites. The main change was the inclusion of a
few sites for which the accuracy was insufficient in the ITRF96 but
which now seem to be well determined in the ITRF97. A SINEX file
containing the necessary information about these 51 reference stations
may be found at the IGS Central Bureau
(ftp://igscb.jpl.nasa.gov/igscb/station/coord/ITRF97_IGS_RS51.SNX).

Although the ITRF96 and ITRF97 frames are nominally aligned globally
in all 7 Helmert components and their rates, comparison of the IGS
subset of RF sites shows significant differences between the ITRF96
and ITRF97 realizations. Several analysis centers have also observed
these differences. Some detective work by Zuheir Altamimi and Remi
Ferland has shown that this may be related to the use of the full
covariance matrices to align the two ITRF solutions. Based on the
differences observed by several of the analysis centers the expected
differences between the IGS products based on the ITRF96 and ITRF97
reference frames are given below. These values are based on the work
done by Remi Ferland at NRCan using the ITRF coordinates and velocities
of 45 IGS reference stations.

Transformation from IGS(ITRF96) to IGS(ITRF97) at epoch 01-AUG-1999:
====================================================================
         TX     TY     TZ    RX      RY      RZ       D
         mm     mm     mm    mas     mas     mas     ppb
Offset   0.3    0.5  -14.7  0.159  -0.263  -0.060   1.430
+/-      2.1    2.1    2.1   .090    .098    .088    .31
--------------------------------------------------------------------
        dTX    dTY    dTZ    dRX     dRY     dRZ     dD
       mm/y   mm/y   mm/y   mas/y   mas/y   mas/y   ppb/y
Drift  -0.7     0.1  -1.9   0.013  -0.015   0.003   0.192
+/-      .3      .3    .3    .011    .012    .011    .043
--------------------------------------------------------------------
* The IERS convention for the transformation parameters was followed.
* The equivalent changes in polar motion, in the sense
  (ITRF97 - ITRF96), may be obtained using PMx = RY and PMy = RX.

Unlike past IGS ITRF changes, the transformations should be quite exact,
rotations in particular, for the IGS Final orbits/ERPs. This is the result
of the minimum constraint solutions adopted for all AC and IGS final
products at the end of 1998.

For completeness the results of different analysis centers have been
included below. The agreement with the expected changes is very good
in most cases. I would like to thank all individuals who have taken the
time and effort to study these differences and to report them, special
thanks are due to Zuheir Altamimi and Remi Ferland who have done most
of the hard work related to the ITRF97. Many thanks also to Jim Ray and
Jan Kouba for their corrections and useful suggestions.

Hoping to have informed you sufficiently I remain,
with kind regards,

         Tim Springer
         IGS Analysis Center Coordinator

Solution description
--------------------
O1 : USNO: Rapid orbit differences for 20-26 July 1999
O2 : EMR:  EMR10217.SUM final orbit product differences for GPS week 1021

C1 : EMR:  Remi Ferland NRC SINEX vs ITRF97_GPS.SNX  (RF47)
C2 : IGN:  Zuheir Altamimi ITRF96 vs ITRF97 using diagonal matrix (not full)
C3 : EMR:  EMR10217.SUM and EMR10227.SUM final coordinate product differences
           for GPS weeks 1021 and 1022

P1a: EMR:  Remi Ferland PM differences (Oct/98-May/99)
P1b: EMR:  Remi Ferland PM differences based on MIT GNAAC (Oct/98-May/99)
P2 : USNO: Marshall Eubanks based on VLBI (1979.6-1999.3).
P3 : EMR:  EMR10217.SUM final pole product differences for GPS week 1021
P4 : CODE: ERP differences 200/1993-212/1999 (IGSMAIL #2422)

Transformations from ITRF96 to ITRF97 at epochs noted:
--------------------------------------------------------------------------
    TX(mm) TY(mm) TZ(mm)  RX(mas) RY(mas) RZ(mas)  D(ppb)  EPOCH
--------------------------------------------------------------------------
O1   1.1    0.0    8.4     0.07   -0.22    0.02     0.0    23-07-1999
O2  -0.9    1.0   -8.5     0.12   -0.22   -0.01     0.0    01-08-1999
--------------------------------------------------------------------------
C1a  1.2    0.0  -11.0     0.11   -0.21   -0.05     0.82   01-01-1997
C1b -0.0   -0.2  -16.4     0.08   -0.25   -0.11     1.25   01-01-1999
C2   0.8    0.6  -10.2     0.12   -0.15   -0.06     0.79   01-01-1997
C3   0.6    0.0  -12.0     0.12   -0.23   -0.01     1.33   01-08-1999
--------------------------------------------------------------------------
C1  -0.6   -0.1   -2.7    -0.011  -0.024  -0.028   +0.212  ** RATE **
C2  -0.3   -0.2   -1.4    -0.010   0.005   0.000   +0.22   ** RATE **
--------------------------------------------------------------------------

Earth rotation parameter difference (ITRF97 - ITRF96) at epochs noted:
--------------------------------------------------------------------------
                         PMx(mas) PMy(mas) UT(ms)          EPOCH
--------------------------------------------------------------------------
P1a                       -0.27    0.17                    01-01-1999
P1b                       -0.28    0.18                    01-01-1999
P2                        -0.18    0.19    0.013           01-01-1997
P3                        -0.23    0.13                    01-08-1999
P4                        -0.23    0.07                    19-07-1993
--------------------------------------------------------------------------
P2                         0.004  -0.014   0.003           ** RATE **
P4                         0.014   0.011                   ** RATE **
--------------------------------------------------------------------------

-- 

 -----------------------------------------------------------------------------
 Tim Springer                      Astronomical Institute, University of Berne 
 [email-masked]            Sidlerstrasse 5, CH-3012 Bern, Switzerland  
 http://www.cx.unibe.ch/aiub/      Tel: +41 31 6318592    Fax: +41 31 6313869
 http://ftp.unibe.ch/aiub
 -----------------------------------------------------------------------------

[Mailed From: Tim Springer <[email-masked]>]