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The use of GPS receivers in wireless telephones has been proposed as a means of automatically identifying the position of wireless 911 callers. GPS simulators are an efficient means of testing the accuracy of such technology but require a channel model for GPS satellite signals. The paper presents a methodology for measuring and modelling the fading distribution of GPS satellite signals received in outdoor urban centres. GPS fading data, as collected in the downtown areas of Calgary and Vancouver, Canada, are used to generate fade histograms as a function of satellite elevation angle. These histograms are found to be sufficiently similar between the two cities and, therefore, lead to the conclusion that a generic fade distribution for urban centres is reasonable. Parameters for the urban three-state fade model (Akturan, R., and Vogel, W.J., IEEE Trans. Antennas Propag., vol.45, no.7, p.1107-16, 1997) are estimated from the empirical fading data collected in each city. Correlation between the model parameters derived for each city is significant and this also suggests that a generic model may be derived. The parameters from the two cites are averaged to produce a generic urban three-state fade model, which adequately represents the fade histograms of each city. These averaged parameters generally agree with parameters derived from data collected in Tokyo, Japan.