Report of Special Commission 3 of IAG

Abstract Since the last presentation of SC-3 on numerical values of fundamental geodetic parameters at the IAU General Assembly at Kyoto in 1997 there were some conceptual as well as fundamental numerical changes. The four basic parameters of geodetic (ellipsoidal) reference systems (GRS) can no longer be considered as constant with time: J 2 , a, ω , and GM have to be replaced by clearly (±10 −8 or better) specified mean values or have to be associated with a specific epoch or, in case of GM, with specific reference frames ( a = semi-major axis of Earth ellipsoid, J 2 = second degree zonal harmonic of geopotential, ω = spin of Earth rotation). In case of ( a, J 2 ....) associated tidal reductions must be specifically defined in view of particular applications and significant differences between different tidal reduction types. Or we may replace “a” by a quantity which is independent of tides like the geopotential at the geoid, W 0 , where, however, also temporal changes are now discussed. The official geodetic reference systems such as GRS 80 and WGS 84 (revised in 97-form) are also no longer truly representing reality; a new system GRS 2000 is desired. We are, meanwhile, able to define and determine tidal and non-tidal (secular, periodic, aperiodic) variatipns of some fundamental geodetic parameters. Others are under investigation. New precession and/or nutation formulas to be adopted by IAU in 2000 or later would imply, again, changes in geodetic parameters such as H = hydrostatic flattening. Those and related other consequences are considered.