Ionosphere sounding by ground and space based GNSS measurements

The world-wide use of Global Navigation Satellite Systems (GNSS) such as GPS and GLONASS offer the unique chance for a permanent monitoring of the total ionization (Total Electron Content - TEC) of the global ionosphere/plasmasphere up to about 20000 km height. Whereas ground based measurements provide a good horizontal resolution of the total ionization, space based GNSS measurements provide global information on the vertical structure of the ionosphere and on the 3D- electron distribution of the topside ionosphere/plasmasphere. The talk addresses the capabilities of global GPS networks, such as generating global and regional maps of the total electron content (TEC) for research and studying the ionospheric impact on GNSS applications from the International GPS Service (IGS) network. During ionospheric storms TEC is highly variable and indicates a close correlation with geomagnetic activity indices and even with geo-stationary satellite particle flux data. The Ionospheric Radio Occultation (IRO) technique has the big advantage to unify profiling through the entire ionosphere with global coverage. Additionally, the GPS navigation measurements onboard LEO's may be used to reconstruct the electron density distribution of the topside ionosphere and plasmasphere with high resolution. Both techniques are considered when presenting results obtained so far from GPS measurements onboard CHAMP. Since 11 April 2001 more than 20000 vertical electron density profiles have been retrieved from the IRO data. Due to the rather low orbit of the CHAMP satellite (h < 450 km) the classical Abel transform algorithm is problematic to apply. To overcome this problem, a model assisted retrieval technique has been developed. Validation studies reveal RMS deviations of the F2 layer parameters f0F2 and hmF2 of about 1 MHz and 45 km, respectively. Entire IRO derived electron density profiles are compared with corresponding profiles derived from vertical sounding measurements obtained at some European ionosonde stations and incoherent scatter facilities during selected campaigns. The topside electron density reconstruction has been made by assimilating calibrated TEC data into a global ionospheric model (PIM) providing a 3D- electron density reconstruction near the CHAMP orbit. The results indicate that ground and space based GNSS measurements have a very high potential for a permanent and high resolution monitoring. In the future, this potential will be fully utilized considering the enhanced GNSS signal availability by Galileo from 2008 onward and the international activities in preparing multi-satellite missions