Ground-based observations of GPS satellites disappearing below the local horizon are analysed. Starting at +2 degree elevation angle the GPS signals are recorded in open-loop tracking mode down to −1.5 degrees. The open-loop Doppler model has negligible influence on the derived data products for strong signal-to-noise ratios; at lower signal levels, however, a notable bias is uncovered. These results may have implications for the design of future space-based GPS radio occultation missions.