Global ionosphere maps (GIM) are generated on a daily basis at CODE using
data from about 200 GPS/GLONASS sites of the IGS and other institutions.
The vertical total electron content (VTEC) is modeled in a solar-geomagnetic reference frame using a spherical harmonics expansion up to degree and order 15. Piece-wise linear functions are used for representation in the time domain. The time spacing of their vertices is 2 hours, conforming with the epochs of the VTEC maps.
Instrumental biases, so-called differential P1-P2 code biases (DCB), for all GPS satellites and ground stations are estimated as constant values for each day, simultaneously with the 13 times 256, or 3328 parameters used to represent the global VTEC distribution. The DCB datum is defined by a zero-mean condition imposed on the satellite bias estimates. P1-C1 bias corrections are taken into account if needed.
To convert line-of-sight TEC into vertical TEC, a modified single-layer model (MSLM) mapping function approximating the JPL extended slab model mapping function is adopted.
The global coverage of the GPS tracking ground stations considered at CODE is shown here (PS/PDF), or here including 4-figure abbreviations for station identification (PS/PDF).
CODE GIM results correspond to the results for the middle day of a 3-day combination analysis solving for 37 times 256, or 9472 VTEC parameters and one common set of satellite and receiver DCB constants. In this way, discontinuities at day boundaries can be minimized. Furthermore, a time-invariant quality level is achieved.
This model improvement concerns the final GIM results as of day 076, 2002 (GPS week 1158). The change was announced in IGSMAIL-3823 (and BSW Mail 144).
Starting with day 307, 2002 (GPS week 1191), all provided CODE IONEX files include 13 VTEC maps. The first map refers to 00:00 UT, the last map to 24:00 UT (instead of 01:00 and 23:00 UT, respectively). The time spacing of the maps (snapshots) is still 2 hours.
Due to the fact that each new daily file contains ionospheric information covering not only 22 but 24 hours, data interpolation becomes more user-friendly.
This model change was announced in IGSMAIL-4162 (and BSW Mail 153).
Starting with day 117, 2003 (GPS week 1215), GLONASS tracking data from GPS/GLONASS-combined receivers is considered. Data of 43 such GNSS receivers is regularly analyzed (simultaneously with that of GPS-only receivers).