Global Ionospheric total electron content mapping using the Global Positioning System
Space-based radio navigation systems such as the Navstar Global Positioning System (GPS) can provide us with a unique opportunity to study the effect of the ionosphere as the signals propagate from the satellites to the GPS receivers. Based on a modified version of UNB’s DIPOP software package, I developed an algorithm to produce regional or global total electron content (TEC) maps on an hourly basis using dual frequency GPS observations from stations of the International GPS Service for Geodynamics (IGS). The algorithm uses a spatial linear approximation of the vertical TEC above each IGS station using stochastic parameters in a Kalman filter estimation to describe the local time and geomagnetic latitude dependence of the TEC. I used a new concept to take into account the temporally and spatially varying ionospheric shell height as opposed to a commonly adopted fixed shell height. I demonstrated that the UNB algorithm was capable of modelling the diurnal variation of TEC even during a geomagnetic storm period. I also have modified the International Reference Ionosphere 1995 (IRI-95) model to update its coefficient sets using the UNB GPS-derived regional ionospheric maps, based on a 5 week long GPS campaign, in order to provide more precise IRI-95-derived ionospheric delay predictions for e.g., single frequency GPS receivers. I enhanced the UNB algorithm to estimate ionospheric model parameters from a global set of GPS stations to independently produce hourly snapshots of the global ionosphere. The previously modified IRI-95 model as a sophisticated interpolator was used between two GPS-derived TEC updates to provide improved IRI-95 ionospheric delay predictions. During the update procedure, I enhanced the IRI-95 model with an empirical plasmaspheric electron content model. Based on 3 days’ worth of global GPS data during a medium solar activity period in 1993 (33 IGS stations for each day) there was better than a 9 TECU level (1 sigma) agreement in the TEC on a global scale with TOPEX/Poseidon-derived (T/P) TEC data. For a low solar activity 1995 data set (74 IGS stations for each day), the UNB results showed an agreement with the T/P data at better than the 5 TECU level (1 sigma). The UNB global ionospheric TEC modelling technique in conjunction with the IRI-95 update procedure has been demonstrated to be a viable alternative to provide independently-derived ground-based ionospheric delay corrections for single frequency applications such as single frequency radar altimeter missions.