*************************************************************************** Gazette IERS Gazette IERS Gazette IERS Gazette IERS Gazette _______________________________________ No 26, 12 January 1998 / __________________________________/ Contact: iers@obspm.fr ftp: hpiers.obspm.fr (145.238.100.28) WWW: http://hpiers.obspm.fr *************************************************************************** Title: Recommendations on monitoring geocenter motions Author: J. Ray The IERS Working Group on the ITRF Datum met during the Fall AGU Meeting last month in San Francisco and considered results presented during the special geodesy sessions on "Investigations of Geocenter Motions" as well as other results from the Geocenter Campaign. The overall impression of the work presented thus far is that the net motion of the terrestrial reference frame relative to the Earth's center of mass is detectable but small, probably no more than about 1 cm in any component. The diurnal and semidiurnal tidal variations appear to be well determined and in good agreement with modern ocean tidal models. There seems to be some general agreement among the techniques in detecting seasonal variations, although more work remains to be done in this area. Geophysical computations of the expected motions based on global fluid motions are only roughly consistent with the observations. Based on these observations, the Working Group is presenting the following recommendations to the IERS. It is expected that appropriate changes will be made in the next edition of the "IERS Conventions". Recommendations to the IERS --------------------------- 1) 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, GRL, 24(17), 2231-2234]. For most users this can be accomplished with no additional effort by including the geocenter effect into the standard ocean loading model, as suggested by H-G. Scherneck. Most likely, the formalism for ocean loading should be changed from the use of tabular coefficients for partial tides to an admittance formulation to increase accuracy and reduce the number of coefficients needed. Using an SLR solution, Eanes will test new ocean loading parameters to be computed by Scherneck. 2) Since the satellite techniques do not yet seem reliable for measuring higher frequency variations, an empirical seasonal model is recommended for non-tidal motions. A weighted combination of the existing analysis results (and others presented in the near future), similar to the procedure described by H. Montag, will be used for this empirical model. The combination will be performed by the IERS TRF section at IGN. Validation of the results by other groups is encouraged. 3) Future geocenter monitoring can be continued in two ways: the new IERS coordinating center for Monitoring of Global Geodynamical Fluids (MGGF) will have a subcenter devoted to computing offsets of the ITRF origin from the geocenter due to large-scale fluid motions; and the ITRF section will soon begin to compute monthly realizations of the terrestrial frame which can be compared with the fluid motion effects. When detection of higher frequency geocenter motions appears feasible the situation can be reassessed. Publication of Results ---------------------- The Working Group requests brief reports and data contributions from all participants in the Geocenter Campaign for inclusion in an IERS Technical Note. The goal is to complete the TN before the 1998 IERS Workshop in September. The deadline for submission of reports is 01 June 1998. Details on the preparation of the TN are available from the Geocenter Campaign Web site at http://maia.usno.navy.mil/geoc.html .