Subject: [IGSMAIL-0824] ITRF92/ITRF93 IGS product change
From: AC Coordinator
Date: Tue, 3 Jan 1995 13:28:20 PST

******************************************************************************
IGS Electronic Mail      Tue Jan  3 13:28:20 PST 1995      Message Number 0824
******************************************************************************

Author: AC Coordinator
Subject: ITRF92/ITRF93 IGS product change

   
    Dear colleagues,

    As agreed by all the IGS analysis centres, starting on January 1, 1995, all
    the individual centre and IGS combined products will be based on ITRF93. 

    This reference frame change is necessary as the new ITRF93 represents a 
    significant consistency improvement with respect to ITRF92 which was used 
    for all the IGS products in 1994. The reference frames in the IGS products 
    are realized by fixing or constraining a set of 13 stations at the ITRF 
    coordinates & velocities, see the IGSMAIL #430 for ITRF92 used in 1994 and 
    the IGSMAIL #819 for ITRF93, to be used in 1995. 
    
    Unfortunately, on Jan 1/95, this reference frame change will introduce
    small discontinuities in all the IGS products. Since in 1995  we are still
    using the same 13 stations to realize ITRF93 we can estimate  the
    transformation between the 1994 and 1995 IGS products more precisely  than
    the nominal transformation and time evolution published in the  1993 IERS
    Annual Report and the IERS Technical Note #18. Below are the  estimated and
    expected changes between the 1994 and 1995 IGS products: 

    
     EXPECTED/ESTIMATED DISCONTINUITIES OF IGS PRODUCTS (ORBITS, EOP, SSC,
    CLOCKS) AT 1995.00 (IGS(1995)-IGS(1994)
    
    PRODUCTS      T1(cm) T2(cm) T3(cm) D(ppb) R1(mas) R2(mas) R3(mas) Remarks
                                               Y-pole  X-pole  -UT1     (1)
    IGS AC's(orb,                                  
    EOP,SSC,clocks)-.9    -1.2     .3   -.6      1.4     1.1     .4     (2),(6)
    sigma           .5      .5     .5    .8       .2      .2     .2     (3),(6)
                                                    
    Rates (./year) -.02    -.11    .15  -.13      .16     .30   -.13    (4),(6)
    
    IGS Combined
    (orbits,EOP)   -.9    -1.2     .3   -.6      1.66    .61     .4     (2),(5)
    sigma           .5      .5     .5    .8       .5      .5     .2     (3),(5)
                                                    
    Rates (./year) -.02    -.11    .15  -.13      .12     .15    .13    (4),(5)
       --------------------------------------------------------------------
    Remarks:
     (1) The actual changes in EOP have the opposite signs,i.e. the 1995 EOP x-
    pole and y-pole will be decreased by about 1.1 and 1.4mas w.r.t. 1994.
    The transformation parameters (T1-3,D,R1-3) are consistent with the 
    1993 IERS Annual Rep.,( eqn 3, p. II-52); i.e ITRF92 => ITRF93.
    (2) Applicable only to the SSC of the daily constrained IGS  solutions.
    Clock solutions are insensitive to the rotation parameters.
     (3) Sigmas represent the consistency of both ITRF(IGS) realizations, the
    precision of the 1994/95 product discontinuity should be much higher
     (4) can be used to precisely transform the 1995 IGS products to be 
    consistent with the 1994 and vice versa. (e.g. for plate tectonics, time 
    evolution, etc.) The associated epoch for the parameters and rates above is 
    1995.0. 
    (5) For the IGS combined products the ITRF-IERS(EOP) misalignments in R1,R2
    only as it is used in the IGS combination, i.e. the difference (at 1995.0)
    computed from the Tables II-3 of the 1993 and 1992 IERS Annual Reports
    (p.II-19 and II-17, resp.)
    (6) The transformations & rates based on the following 13 ITRF constraining
    stations (identity weighting): ALGO, FAIR, GOLD  HART, KOKB, KOSG, MADR,
    SANT, TIDB, TROM, WETB, YAR1, YELL (see IGSMAIL #819, #430)

    The first set was obtained from the 7 parameter transformation for the 13
    station coordinates/velocities in ITRF92 and ITRF93 (see IGSMAIL#430,#819)
    and should be a good approximation of the expected change for all the IGS
    AC's. Since individual AC may be constraining more stations, using some
    different stations, data weighting, etc., the actual changes may vary
    slightly from Centre to Centre and from day to day. (For more details see
    the individual AC summary report for GPS Week 782 or later, which may
    include better estimates of the centre product changes). The transformation
    above is also quite consistent with the published transformation between
    ITRF92 and ITRF93, which is based on all ITRF stations, nevertheless, on
    the average, the  above values should be more representative of the actual
    IGS product changes. The second set of transformation parameters,
    applicable to the IGS  combination was obtained the same way as above,
    except for the R1,R2 orientation parameters which were computed from the
    IERS-ITRF92 misalignment(see the IERS 1992 Annual Rep., p.II-17) applied to
    the IGS combinations in 1994 and the IERS-ITRF93 misalignment (see 1993
    IERS Annual Rep, p.II-19) which  will be applied to the IGS Combined
    products in 1995. 

    Also note the ITRF93 velocities are slightly biased w.r.t. NNR NUVEL1A,
    however the ITRF93 velocity field greatly reduces the drift between
    IERS(EOP) and ITRF93 frames. The non NNR ITRF93 velocities causes only
    small orientation changes with practically the same relative precision. The
    rates for R1, R2, and R3 above, which again are consistent with the
    differences between NNR NUVELl1 and ITRF93 in the IERS Technical Note #13,
    p.17) can be used if the past time evolution of IGS products is to be
    maintained, or when transforming the 1995 IGS products to a NNR reference
    system. Again, the above values should be more consistent and
    representative of the IGS products.

    For more information/comments and suggestions regarding the above, please
    contact:

		       J. Kouba ([email-masked])

    ACKNOWLEDGEMENTS: Cooperation of all the IGS Analysis Centres (AC), Drs Z.
    Altamimi and C. Boucher of the IERS ITRF Section are greatfully
    acknowledged. Also received able and rapid  assistance of the 'emr' AC team
    members, Remi Ferland and Pierre Tetreault.

[Mailed From: Jan Kouba <[email-masked]>]