Ionospheric Delay Correction for Single-Frequency Radar Altimetry
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
At the University of New Brunswick, Canada, we have developed the capability to independently produce hourly global total electron content (TEC) maps. UNB's hourly GPS-derived global TEC maps can be ingested directly into a modified version of the International Reference Ionosphere 1995 (IRI-95) model to update its CCIR/URSI coefficient sets. These updated IRI-95 coefficients provide improved IRI-95 predictions by using the modified IRI-95 model as a sophisticated interpolator between two hourly GPS-derived TEC updates. The updated IRI-95 coefficient sets will make it possible to provide ionospheric delay corrections for single frequency radar altimeter missions such as the upcoming GFO mission. Since the spacecraft will be orbiting 'inside' the ionosphere, our GPS-updated IRI-95 electron density profile will allow us to integrate the electron densities up to the spacecraft altitude to remove the bias imposed by the ionosphere. This would not be possible using GPS-derived TEC alone since it provides integrated electron content up to the altitude of the GPS satellites (20,200 km).
During the presentation, we will describe the algorithm we use to generate the UNB global TEC maps, the modifications we made to the IRI-95 model, and the IRI-95 update procedure. We will also give a description of the varying ionospheric shell height concept we developed taking into account both the temporal and spatial variation of the shell height. We will show the method we use to update the IRI-95 model which we have enhanced with an empirical plasmaspheric electron content model.
To demonstrate our capabilities, we will present results based on 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). We will also compare our updated IRI-95 predictions using UNB's global TEC maps, the original IRI-95 predictions, and JPL-derived global ionospheric maps (GIMs) against 6 days' worth of TOPEX-derived TEC data. Based on 3 days' worth of global GPS data during the medium solar activity time in 1993, the UNB results show that 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 1995 medium solar activity time, our results agreed with the TOPEX data at better than the 5 TECU level (1 sigma). These results suggest that our method may be viable for providing ionospheric delay corrections for future single-frequency altimeter missions.
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