(changes since last update are shown in RED)
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
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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
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* 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.
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:
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TX(mm) TY(mm) TZ(mm) RX(mas) RY(mas) RZ(mas) D(ppb) EPOCH
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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
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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
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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 **
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Earth rotation parameter difference (ITRF97 - ITRF96) at epochs noted:
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PMx(mas) PMy(mas) UT(ms) EPOCH
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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
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P2 0.004 -0.014 0.003 ** RATE **
P4 0.014 0.011 ** RATE **
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