Empirical Characterization and Modeling of GPS Positioning Errors Due to Ionospheric Scintillation

Ionospheric scintillation degrades GPS positioning accuracy in a number of ways. Along each scintillating satellite-receiver link, errors are introduced in the measurements of pseudorange and carrier phase. When the scintillation on a given link is sufficiently intense, the link becomes intermittently unavailable for use in the position solution. The loss of each link leads to an increased dilution of precision as the geometry of the overall satellite constellation is degraded. These effects reduce the accuracy of the computed position and result in temporary losses of positioning service when fewer than four links remain available at any given time. The duration of these positioning service “outages” depends on the duration and severity of the scintillation event, the geometry of the satellites in view, and the recovery time of the equipment. In this work we characterize and model the effects of scintillation on GPS positioning accuracy using data acquired with an Ashtech Z-12 receiver at Ascension Island. This campaign was conducted by the Air Force Research Laboratory (AFRL) during solar maximum conditions in March of 2002. Positioning errors exceeding 75 meters in the horizontal and 150 meters in the vertical were routinely encountered, and positioning outages occurred nearly every evening. We demonstrate the effects of scintillation on the pseudoranges, and model the propagation of these errors into the position solution in terms of an empirical parametrization of the scintillation environment. A key feature of the model is its ability to simulate link-by-link measurement errors and the effective degradation in the constellation geometry, both individually and in concert. Results generated by the model are presented and shown to compare favorably with actual measurements of the positioning error. The parameters of the model may be tuned for different GPS receiver designs, either through field testing as it was done in this study or through hardware-in-the-loop simulation using tools such as the AFRL Antenna WaveFront Simulator (AWFS). Coupled with a regional specification of the scintillation environment, based either on data or climatology, the model may be used to provide regional GPS position error maps suitable for integration into space weather forecast products.