Geoscience Reference
In-Depth Information
4.3 Estimating TEC Using Different Space Geodetic Techniques
Since most of the space geodetic techniques operate in at least two different fre-
quencies, they are capable of eliminating the influence of the ionosphere on the
propagation of their signals. This on the other hand provides the ability to gain infor-
mation about the ionosphere parameters. If the behavior of the ionosphere is known,
the ionospheric refraction can be computed and used for development of regional or
global models of the ionosphere. Different observation principles result in specific
features of the ionosphere parameters derived by each of the techniques. Some of
these techniques are:
4.3.1 Determining TEC from GNSS Observations
GNSS including the U.S.A. Global Positioning System (GPS), the Russian Global-
naya Navigatsionnaya Sputnikovaya Sistema (GLONASS), the upcoming European
Galileo and the Chinese Beidou system allow for the determination of the station
specific ionosphere parameters in terms of STEC values, using carrier phase or code
measurements. To extract information about the ionosphere from the GNSS obser-
vations, a linear combination is formed, which eliminates the geometric term. This
linear combination is called geometry-free linear combination
L
4
or the ionospheric
observable.
Ionospheric Observable
To form the ionospheric observable, simultaneous observations at two carriers
L
1
and
L
2
are subtracted. In this way along with the geometric term, all frequency-
independent effects such as clock offsets and tropospheric delay are eliminated.
This leads to an observable, which contains only the ionospheric refraction and the
differential inter-frequency hardware delays. The geometry-free linear combination
has the form:
L
4
=
n
1
L
1
+
n
2
L
2
=
L
1
−
L
2
,
(86)
with
n
1
=
1.
Applying the above combination to the observation equations Eqs.
61
and
68
leads
to the geometry-free LC for the code and phase measurements, respectively:
1 and
n
2
=−
c
b
R
b
S
P
4
=+
ξ
4
I
+
Δ
−
Δ
,
(87)
L
4
=−
ξ
4
I
+
B
4
,
(88)
