One of the major error sources in GPS positioning is ionospheric refraction which causes signal propagation delays. The disturbing influences of the temporally and spatially varying ionization of the ionosphere have great impact on satellite geodesy, especially on GPS. To correct data from a single-frequency GPS receiver for the ionospheric effect, it is possible to use empirical models. In this research, we investigated the GPS single-frequency Broadcast [Klobuchar, 1986] and the International Reference Ionosphere (IRI90) [Bilitza, 1990] models. The GPS single-frequency Broadcast model is available to GPS users as part of the navigation message. The IRI90 model is a standard ionospheric model developed by the International Union of Radio Science (URSI) and the Committee on Space Research (COSPAR). After Newby [1992] investigated the IRI86 model’s performance, we decided to include the new IRI90 model in our ionospheric research. We have used Faraday rotation data as ‘ground-truth’ with which we compared the vertical ionospheric range error corrections predicted by the Broadcast and IRI90 models. Some of the results shown here have been presented earlier [Komjathy et al., 1995].