Updating the International Reference Ionosphere Using a Global Network of GPS Stations: Preliminary Results (click on the title to go to paper archive from where you can download it)
A. Komjathy and R.B. Langley
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: email@example.com
Dieter Bilitza , Hughes STX Inc., Greenbelt, MD, U.S.A.
Phone: 1-301-286-0190, Fax: 1-301-286-1771, email: firstname.lastname@example.org
At the Geodetic Research Laboratory of the University of New Brunswick, Canada, we have recently made significant enhancements to our software to be able to independently produce global total electron content (TEC) maps on an hourly basis. The UNB global TEC maps can be input directly into a modified version of the International Reference Ionosphere (IRI-95) model to update its CCIR/URSI coefficient sets on an hourly basis. These updated IRI-95 coefficient sets serve as a basis for improved IRI-95 predictions by using the modified IRI-95 model as a sophisticated interpolator between two GPS-derived TEC updates.
The UNB global TEC mapping technique includes the estimation of three stochastic parameters for each IGS station in a network mode tied to a solar-geomagnetic coordinate system assuming a Gauss-Markov stochastic process. The three parameters use a spatial linear approximation of TEC above each IGS station. The L1-L2 phase-levelled geometry-free observable is used to estimate the stochastic parameters along with the satellite-receiver differential delay biases using a Kalman filter approach. Our new algorithm uses a varying ionospheric shell height concept taking into account both the temporal and spatial variation of the shell height. The same varying ionospheric shell height is an input parameter when mapping the line-of-sight TEC into the vertical using a commonly adopted geometric mapping function. Updating the IRI-95 coefficient sets consists of computing an inferred solar effective sunspot number (IG index) which is assumed to be a function of geographic latitude, longitude and Universal Time. We implemented an efficient search technique to find the IG index that results in the best match between the IRI-95 predicted TEC and the UNB global TEC maps. To find the correct IG index, we used an empirical plasmaspheric electron content model which takes into account the fact that the IRI-95 model computes TEC predictions only up to an altitude of 1000 km whereas the UNB global TEC maps provide estimates up to the altitude of the GPS satellites. We also made modifications to the IRI-95 model and to the code to increase the efficiency of the code's execution.
For the ionospheric workshop at the University of Colorado held 24-25 September 1996, we processed 3 days' worth of global GPS data (33 IGS stations for each day) at a medium solar activity time (year 1993) and 3 days' worth of global GPS data (74 IGS stations for each day) at a low solar activity time (year 1995). An additional day's worth of global GPS data was also processed to help validate the UNB global TEC maps using Faraday rotation data. We produced hourly snapshots of the global ionosphere using GPS data only (mpeg movies of these maps can be accessed via the Web at <http://gauss.gge.unb.ca/grads/attila/movie/>). We have also compared the updated IRI-95 predictions using UNB's global TEC maps, the original IRI-95 predictions, and JPL-derived TEC maps (GIM) against 6 days' worth of TOPEX-derived TEC data which has been provided by the workshop organizers for comparison purposes. The UNB results show that based on 3 days' worth of global GPS data during a medium solar activity time in 1993, there was better than a 9 TECU level (1 sigma) agreement in the total electron content on a global scale with the TOPEX-derived TEC data using UNB's technique. For the low solar activity 1995 data, the UNB's results agreed with the TOPEX data at better than the 5 TECU level (1 sigma). The UNB technique has been demonstrated to be a viable alternative to provide independently-derived ground-based ionospheric delay corrections for future single-frequency radar altimeter missions.
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