Do We Need a New Reference System?

Abstract

Since IAU has adopted at its last General Assembly a fully relativistic new reference system the relativistic corrections and reductions to terrestrial fundamental parameters as evaluated by Kopejkin and others became more relevant to geodesy. IAU has decoupled TT from W⁰ so that basically IAG would be fi ee to introduce fully conventional new reference systems as advocated, e.g., by Rummel at the IAG-meeting at Banff. However, it should be carefully considered whether IAG should decouple itself from new developments where highprecision reference systems now are badly needed in various types of global geophysics. Purely conventional systems, based on „defining constants“, cannot replace high-precision frames based on well determined (± 10^-99 and better) parameters and/or constants. Moreover, the update of WGS 84 by NIMA in ’97 reveals the interest of user communities in ellipsoidal systems. Even though WGS 84 is not a consistent system in the Somigliana-Pizetti sense it was recently adopted by FAA. M. Kumar and others have recently pointed out the discrepancy between IAG-resolution 16 and ITRF. This assumption is not quite correct. At the COSPARmeeting at Warsaw the incorporation of dynamic parameters in ITRF has been considered. By introducing W⁰ instead of the semi-major axis of the earth’s ellipsoid the problem of a triple-system of tidal regimes could be avoided which is never fully understood by non-experts. The transition from W⁰ to the semi-major axis of the earth ellipsoid is a well-known problem which was recently discussed within SC-3 of IAG.

SC-3 of IAG has carefully considered tidal and non-tidal variations of fundamental parameters but before temporal changes have been carefully determined by GRACE etc. new systems should not be introduced. With GRACE, GOCE, JASON and ENVISAT together with new results from WOCE and GLOSS (incl. repeat GPS etc.) the evaluation of new high-precision reference systems becomes feasible. Pioneering work by Bursa et al. as well as by Grafarend et al. has clearly shown that new and efficient global vertical datums and SomiglianaPizzetti reference systems, respectively, can become available soon. If IAG wants to play its role in determnining and defining global terrestrial reference systems it will soon be able to introduce new systems corresponding to the new IAU systems. If not,

others will do it as IAPSO and other user groups obviously need up-to-date high precision reference frames and systems. Consistent ellipsoidal systems of Somigliana-Pizzetti type are needed even though the evaluation of really independent four-parametersets is now difficult and problematic.