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An outline of the astrometric series of the ERP of their accuracy and of the results obtained from analyses of these data are given in the paper. Presently known systematic differences between the astrometric ERP series and those determined by VLBI and Lageos ranging techniques are discussed. Some geophysical applications of latitude and longitude variations determined by astrometric techniques are mentioned.

30 Observational series which include the ERP component, latitude or time results of 15 main networks using six techniques (Ast, Doppler, SLR, LLR, VLBI and CEI) have been reduced to give a combined determination of the ERP for the period 1980 Jan 1 – 1984 October 31. The systematic error of each series is modeled by a constant plus a drift, and these are adjusted together with the ERP unknowns. The accuracies and systematic errors of the 15 networks so found are also given.

Before we introduce the intercomparison of celestial reference frames, let us first examine the structure of a reference frame, what are the causes of difference between them and how they can be compared.