From March to August 1990, the signals transmitted by the Block II satellites of the Global Positioning System (GPS) were dithered under a policy of ''Selective Availability'' (SA). The dithering appears as a ~10-10 deviation of the satellite oscillator frequency, which, when accumulated over several minutes, can produce an error of ~100 cycles (~20 m) in the model for carrier beat phase. Differencing between simultaneously sampling receivers minimizes the error. If, however, the receivers do not sample simultaneously, it is necessary to model the frequency deviation, which we estimate from the phase observed at a station with a stable local oscillator. We apply such a model to data collected in March 1990 by TI4100 and Minimac receivers sampling at times separated by 0.92 s. Applying the algorithm significantly improves the root mean square (RMS) scatter of the estimated relative position vectors. The RMS scatter from a data set including dithered satellites is similar for both simultaneously and non-simultaneously sampling receivers, a result which indicates that SA can be adequately modeled. ¿American Geophysical Union 1991