Mapping and predicting the ionosphere

The Center for Orbit Determination in Europe (CODE) produces daily maps of the Earth’s ionosphere on a regular basis since January 1, 1996. These global ionosphere maps (GIMs) are derived from exactly the same GPS tracking data — doubly differenced carrier phase measurements — as those used for the determination of CODE core products delivered to the IGS like precise GPS orbits, earth orientation parameters (EOPs), station coordinates and velocities. For the ionospheric product we have to analyze the so-called geometry-free linear combination (LC), which primarily contains ionospheric information, as opposed to the ionosphere-free LC, which contains the “geometrical” information and completely eliminates the influence of the ionospheric refraction (ignoring higher-order terms). At present (March 1998), the GPS tracking network processed at CODE consists of more than 110 globally distributed stations of the International GPS Service for Geodynamics (IGS). After reprocessing all 1995 IGS data using the “Bernese Processing Engine” [Rothacher et al., 1996a], a long-time series of daily GIM parameters covering a time span of about 3.2 years is at our disposal. On the one hand this ionosphere time series reveals the evolution of the total electron content (TEC) on a global scale, on the other hand it indicates that short-term as well as long-term predictions for CODE GIM parameters are possible. We discuss the time series for a few selected TEC parameters and develop a method to predict the TEC parameters. Futhermore, we describe how the temporal resolution can be increased when using spherical harmonic (SH) expansions to model the global TEC. First attempts estimating 2-hour maps are encouraging. CODE’S IONOSPHERE PRODUCTS — AN OVERVIEW The principles of the TEC mapping technique used at CODE were described in [Schaer et at. , 1995] and [Schaer et al., 1996a]. At present the following ionosphere products are generated on a routine basis: Proceedings of the IGS AC Workshop, Darmstadt, Germany, February 9–11, 1998 • 24-hour global ionosphere maps (GIMs) are produced using double-difference phase or phase-smoothed code observations. The phase-derived TEC maps proved their usefulness for ambiguity resolution (AR) on long baselines [Rothacher et al., 1996b]. • Rapid global maps are available with a delay of about 12 hours, the final ones after 3 days (in the IONEX format [Schaer et al., 1998]). • Regional (European) maps are produced as well and are also used to support AR. On the average 90% of the initial carrier phase ambiguities can be resolved reliably — without making use of code measurements. Daily IONEX files containing hourly snapshots of the ionosphere are made available via anonymous ftp. • Daily sets of differential code biases (DCBs) for all GPS satellites (and the contributing receivers) are estimated at CODE since October 1997. Figure 1 shows the daily DCB estimates (dots) for 27 GPS satellites from day 022, 1998, to day 071, 1998, and the combined DCBs (circles) aligning all satellite-specific DCBs in the sense that the overall mean becomes zero (to obtain a virtual, but very stable reference). However, there are a couple of PRNs with drifting DCBs with respect to the remaining PRNs. PRN 08, which was launched few months ago, shows a significant drift of almost −0.5 ns over 50 days. We observe an increased root-mean-square error (RMS) for this satellite when assuming and modeling the DCBs as constant quantities (see Figure 1 and Table 1). Figure 1. Daily PRN-specific DCB estimates (dots) for 27 GPS satellites from day 022, 1998, to day 071, 1998, and combined DCBs (circles)