A Comparison of GPS Performance in a Scintillation Environment at Ascension Island

Post-sunset disturbances in the equatorial ionosphere routinely cause rapid phase and amplitude fluctuations (i.e., scintillation) of radio waves propagating through the disturbed regions. The intensity of scintillations is positively correlated with the solar cycle and the associated signal fades will often exceed 20 dB at L-band frequencies during solar maximum. The effect of such an environment on the performance of GPS navigation systems is poorly understood. In March 2000 AFRL conducted a campaign at Ascension Island to test the performance of several GPS receivers under potentially severe scintillation conditions. Ascension Island is located at approximately 16 S magnetic latitude, a region of intense ionospheric disturbances. The systems tested included a Plessey GPS Builder, a Novatel-based prototype GPS Silicon Valley (GSV) Ionospheric Scintillation Monitor (ISM) modified specifically for scintillation applications, a custom High Gain Advanced GPS Receiver (HAGR) developed for AFRL by NAVSYS Corporation and an Ashtech Z-12. Overall, the Ashtech proved to be very robust at tracking the carrier signal amplitude and phase, but it experienced scintillation-induced navigation outages on four of the eight nights of observations. The responses of the different receivers during severe scintillation varied significantly, suggesting that models to simulate ionospheric effects on GPS performance must be receiver-specific.