Improved GPS Ionosphere Correction Modeling

The objective of this project is to improve the GPS ionospheric correction algorithm. The proposed improvements include extending the model to higher solar activity levels, improving the low latitude model, incorporating the UT (or longitude) dependence, and adding the capability of ingesting additional ionospheric data (e.g., a magnetic activity index, equatorial vertical drift) in near real time to improve the accuracy of the algorithm. The revolution in microprocessor and microchip technology that has occurred since the present algorithm was designed allows much more sophistication to be incorporated in an improved version. For Phase I, we propose to produce a preliminary design of a algorithm based on the Global Theoretical Ionospheric Model (GTIM) developed at Phillips Laboratory (Geophysics Directorate). It will be designed to fit in a single Programmable Read-Only Memory (PROM) chip. It will also be designed to be backward compatible with the present algorithm and to require little or no additional telemetry so that the impact on GPS operations will be minimal while providing the maximum benefit to the user. We anticipate that the RMS error in the ionospheric range correction can be reduced from 50% for the present algorithm to 25% for the proposed improved algorithm.