Processing Description
Product Overview
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).
Note 1:
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).
Note 2:
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).
Note 3:
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).