*************************************************************************** Gazette IERS Gazette IERS Gazette IERS Gazette IERS Gazette ________________________________________ No 50, 04 January 2000 / _________________________________/ Contact: iers@obspm.fr ftp: hpiers.obspm.fr (145.238.100.28) WWW: http://hpiers.obspm.fr *************************************************************************** Subject: IERS Working Group on the ITRF Datum -- Final Report Author : Jim Ray, WG Chair (1996-1999) -------------------------------------------------------------------------- Dear colleagues, In 1995 the IERS Directing Board formed a Working Group on the ITRF datum to develop specifications for the precise datum definition to be used for future realizations of the ITRF. During the past IAG quadrennium, the WG sponsored an "Analysis Campaign to Investigate Motions of the Geocenter," the results of which were reported in IERS Technical Note 25 (April 1999). In addition, the WG prepared a "Final Report" (15 July 1999) containing recommendations for the ITRF datum specification. A copy of the full report may be obtained at the IERS web sites: ftp://hpiers.obspm.fr/iers/itrf/ITRF-WG.Report For your convenience, the final summary section of the report is reproduced below. The membership of the WG has been updated for the current IAG quadrennium and at its most recent meeting Kristine Larson (University of Colorado, USA) was chosen as the new Chair. Jim Ray Chair (1996-1999), IERS Working Group on the ITRF Datum ************************************************************************* 5. Summary of ITRF datum specifications ======================================= Definition of ITRF origin ------------------------- To account for geocenter motions, it is necessary to elaborate the expression in Chapter 5 of the IERS Conventions 1996 (McCarthy, 1996) for the basic transformation from the ITRF to an Earth-centered inertial frame, ECI, (relatable to the ICRF) as [ECI] = [P][N][R][W] {[ITRF] - [O]} where P, N, R, and W are the usual transformation matrices for precession, nutation, rotation, and wobble; all of these are time-dependent and computed explicitly with respect to the Earth's center of mass. O is a new time-dependent vector which gives the translation from the ITRF origin to the instantaneous geocenter, defined to be the center of mass of the Earth including oceans and atmosphere. The instantaneous vector position of a point on the Earth's surface can be expressed in the ITRF (see Chapter 3 of the IERS Conventions 1996) as X(t) = X_o + V_o * (t - t_o) + Sum{ delta X_i(t) } where X(t) is the vector position of the point relative to the ITRF origin as a function of time t; X_o and V_o are the vector position and velocity of the point at the reference epoch t_o; the set {X_o, V_o} is usually regarded as constituting the ITRF realization; delta X_i(t) are site-specific corrections due to various time-varying effects including solid Earth tidal displacements (the full tidal correction including the effect of the permanent tide), ocean tidal loading, etc.; The origin realized by the ITRF94 frame is adopted as defining the conventional origin of the ITRF. Subsequent ITRF realizations will maintain the ITRF94 origin thereafter by successive optimal Helmert alignments. Monitoring geocenter motions ---------------------------- A tidal model for the diurnal and semidiurnal geocenter motions should be adopted based on an ocean tide model such as CSR3.0 (M. Watkins & R. Eanes, Geophys. Res. Lett., 24(17), 2231-2234). Because the satellite techniques do not yet seem reliable for measuring variations at other frequencies, a simple seasonal model should be investigated for the principal non-tidal motions. However, results currently available do not yet justify recommending such models for general use. Realization of the ITRF scale and its time evolution ---------------------------------------------------- The ITRF scale should not be considered subject to conventional definition (which would be equivalent to redefining the timescale). Each ITRF realization should inherit the optimally weighted scale (and time evolution) of the input frames that are combined. In practice, it is usually necessary to apply a rescaling of [1 + Ue], equivalent to [1 + (0.7 ppb)], to all the contributed reference frames in order to comply with the IAU/IUGG recommendations on the use of the TCG timescale. Until it is demonstrated that the absolute scale of GPS-based frames is reliable at about the 1 ppb level, these should not be used in the realization of the ITRF scale. Conventional orientation of the ITRF and its rotational rates ------------------------------------------------------------- No change is recommended in the current procedure of aligning the axes of each new ITRF realization to remove any net rotational offsets relative to the previous realization. In this way, the IERS successively maintains its International Reference Pole (IRP) -- aligned within the measurement error with the previous Conventional International Origin (CIO) -- and International Reference Meridian (IRM), approximately aligned with the Greenwich meridian. The IUGG recommends that the ITRF have no global residual rotation with respect to horizontal motions of the Earth's surface. This is achieved by constraining the ITRF so that there are no net rotational rates relative to a global plate motion model; currently the NNR-NUVEL-1A model, with possible recent refinements, is regarded as best for this purpose. The list of actual sites included in the no-net-rotation constraint should be published. Sites located in regions of known tectonic deformation, such as near plate boundaries, and which are not used for the no-net-rotation constraint should be identified for ITRF users. Use of full variance-covariance matrices ---------------------------------------- Future ITRF combinations should use only those solutions which are submitted with full variance-covariance information accompanied by the complete a priori constraint matrices. Alignments in origin and orientation of successive ITRF realizations rely on the use of the full covariance matrices. The Analysis Centers and the technique services are encouraged to develop methods and procedures to include Earth orientation parameters together with their terrestrial frames. This would allow joint simultaneous combinations of TRF and EOP results and should permit greater consistency to be achieved between EOP and TRF results. --------------------------------------------------------------------------