Towards a Combined Ionosphere Product

Stefan Schaer
Astronomical Institute, University of Bern,
Switzerland

New IONEX Format

For a long time, the IGS community has been aware of the fact that the worldwide IGS network offers a unique opportunity to extract information about the Earth's ionosphere. At the IGS Workshops held in Potsdam in May 1995 and in Silver Spring in March 1996, sessions were dedicated to ionospheric issues. For the latter workshop, total electron content (TEC) maps provided by Centre for Orbit Determination in Europe (CODE), Deutsches Zentrum für Luft- und Raumfahrt e.V. (DLR), European Space Operations Center (ESOC), and University of New Brunswick (UNB) were compared, considering only regional (European) maps and the corresponding portion of global maps, respectively (Feltens et al., 1996). As a consequence of these TEC comparisons, an official format for the exchange of ionosphere maps, called IONEX, has been developed (Schaer et al., 1998) and approved by the IGS community. The IONEX format allows the storage of snapshots of the electron density (including associated rms information) referring to particular epochs and to a 2- or even 3dimensional, Earth-fixed grid. IONEX is not a GPS-specific format. It is an interface to non-GPS users of IGS ionosphere products.

Monitoring for High Solar Activity

Several IGS Analysis Centers derive global ionosphere maps (GIMs) and, as a by-product of the TEC determination, differential code biases (DCBs) on a regular basis (or are close to doing so). At the 1998 IGS Analysis Center Workshop in Darmstadt, Germany, it was concluded that the IGS should monitor the ionosphere for (at least) the next period of high solar activity and study in particular the impact of the ionosphere on IGS core products (Feltens and Schaer, 1998). For that purpose, it was recommended to focus on two kinds of products: 2-hourly GIMs in 2-dimensional grid form and daily sets of DCBs for the satellites.

Figure 1 shows the evolution of the mean TEC during a period of low solar activity. The smoothed curve indicates (among other features) 27day fluctuations caused by the Sun's rotation, and reveals that we have passed the recent ionospheric minimum in summer 1996 and are approaching the next maximum! The last solar maximum occurred in 1989; the next maximum solar activity is expected in 2000?2001 (occurring on an 11-year cycle).

The development of an IGS ionosphere model may be seen as a long-term goal. In order to accomplish these goals, an IGS Ionosphere Working Group shall be established in May 1998. The start of the pilot phase, where the IONEX files as produced by the individual Analysis Centers will be sent to the IGS Global Data Centers, may be expected for mid-1998. Finally, the provision of IGS combined ionosphere maps is planned.