Improvement of a Global Ionospheric Model to Prvovide Ionospheric Range Error Corrections for Single-frequency GPS Users
Both at: Geodetic Research Laboratory, Department of Geodesy and Geomatics Engineering
University of New Brunswick, P.O. Box 4400, Fredericton, N.B. E3B 5A3 Canada
Phone: 1-506-453-4698, Fax: 1-506-453-4943, email: lang@unb.ca
The electromagnetic waves propagating from the satellites of the Navstar Global Positioning System (GPS) to a GPS receiver on or near the earth's surface must travel through the earth's ionosphere. GPS receiver users must correct for the carrier phase advance and pseudorange group delay imposed on the signals by the ionosphere to achieve the highest possible positioning accuracies. Since a new solar cycle has just begun, this effect will become increasingly important.
It is possible to use global empirical and/or physics-based ionospheric models to account for the ionospheric effect using single-frequency GPS receivers. Moreover, dual-frequency GPS observations can also allow us to take advantage of the dispersive nature of the ionosphere and compute the total electron content (TEC). The permanent network of GPS receivers administered by the International GPS Service for Geodynamics (IGS) may be used to determine the spatial and temporal variation in TEC. Several analysis and processing centers of the IGS are currently developing the capability to make TEC maps available to users as an official IGS product. In this paper, we report on the use of five weeks' worth of dual-frequency GPS pseudorange and carrier phase observations from 6 European IGS stations to derive regional TEC values.
Furthermore, we have investigated the use of a modified version of the latest International Reference Ionosphere model enhancement of IRI-90, also designated as IRI-95, to provide ionospheric range error corrections for single-frequency GPS users. We used our GPS-derived TEC maps to provide updates to the IRI-95 model on an hourly basis. After updating IRI-95 for each hour, we used the updated coefficient set of IRI-95 to compute TEC predictions between two updates. The updated IRI-95 model was then used to compare the model performance against the GPS-derived TEC. The predictions provided by the original version of IRI-95 were also compared with our GPS-derived TEC maps. After updating IRI-95, we found that the original model performance was improved overall by 32.5 percent. We also made modifications to the model and to the code to increase the efficiency of the code's execution. We tested the practicability of the model and found that it takes about 0.03 seconds (using an 85 MHz MicroSparc II processor) to execute the IRI-95 model for computing TEC or ionospheric range error corrections for one epoch at any geographic location. We believe that such a short execution time will make the updated IRI-95 model suitable for both post-processing and real-time applications for providing TEC estimates which can be used for ionospheric range error corrections for single-frequency GPS users.
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