Geoscience Reference
In-Depth Information
Fig. 3
Bi-variate interpola-
tion using the nearest four
TEC values (modified from
Schaer et al.
1998
)
VTEC
1,1
VTEC
0,1
q
VTEC
0,0
VTEC
1,0
p
RMS maps. Additionally, daily and monthly values of the satellite and receiver DCB
are provided as well.
The routine generation of ionosphere VTEC maps is currently done at four
IGS Associate Analysis Centers (IAAC) for ionosphere products. These IAAC are
namely:
•
Center for Orbit Determination in Europe (CODE), University of Berne, Switzer-
land,
•
European Space Operations Center of ESA (ESA/ESOC), Darmstadt, Germany,
•
Jet Propulsion Laboratory (JPL), Pasadena, U.S.A.,
•
Technical University of Catalonia (gAGE/UPC), Barcelona, Spain.
These centers provide results computed with different approaches, which are
transmitted to the IGS Ionosphere Product Coordinator, who calculates a weighted
combined product. Presently the weights are defined by the IAAC global TEC maps
evaluation carried out at the Geodynamics Research Laboratory of the University
of the Warmia and Mazury (GRL/UWM) in Olsztyn, Poland (Krankowski et al.
2010
). IGS releases a final ionosphere map in IONEX format with resolution of 5
◦
in longitude and 2
5
◦
in latitude with a latency of 10days and a rapid solution with
a latency of 1day. The IGS GIM and the corresponding RMS maps are available
through the IGS server in IONEX format (CDDIS-IONEX
2011
).
From long term analysis, it is believed that the IGS VTEC maps have an accuracy
of few TECU in areas well covered with GNSS receivers; conversely, in areas with
poor coverage, the accuracy can be degraded by a factor of up to five (Feltens et al.
2010
).
.
4.3.2 Obtaining TEC from Satellite Altimetry Measurements
Satellite altimetry is a particular way of ranging inwhich the vertical distance between
a satellite and the surface of the Earth is measured (Seeber
1993
). The range between
the satellite and the Earth's surface is derived from the traveling time of the radar
impulse transmitted by the radar-altimeter and reflected from the ground. Therefore
the method is best applicable over the oceans, due to the good reflective properties
