Geoscience Reference
In-Depth Information
4.2.2 Eliminating Higher-Order Ionospheric Effects in GNSS Measurements
The elimination of the ionospheric refraction is the huge advantage of the two
ionospheric-free linear combinations Eqs.
66
and
71
. Although the term“ionospheric-
free” is not completely correct as in this combination the higher-order terms as well
as the curvature effects which are less than 0.1% of the total value in L-band, are
neglected.
Based on the geometrical optic approximation Brunner and Gu (
1991
) proposed
an improved model for the ionospheric-free linear combination that considers the
significant higher-order terms, the curvature effect of the ray paths, and the effect of
the magnetic field. The improved model is written as:
1
L
3
=
(
L
1
−
Γ
L
2
)
−[
κ
1
−
κ
2
]
,
(72)
1
−
Γ
where
κ
1
is the geometric bending effect,
Γ
1
Γ
2
,
Γ
=
(73)
Γ
κ
2
=
ν,
(74)
1
−
Γ
with the electron collision frequency
ν
and
1
C
X
2
f
L
1
C
Y
f
L
1
C
X
4
f
L
1
Γ
1
=
±
N
e
B
0
cos
θ
−
N
max
η
,
(75)
1
C
X
2
f
L
2
C
Y
f
L
2
C
X
4
f
L
2
Γ
2
=
±
N
e
B
0
cos
θ
−
N
max
η
.
(76)
A comparison of Eq.
71
with Eq.
72
shows that the improved model replaces
γ
by
the more complete
Γ
and includes two curvature correction terms
κ
1
and
κ
2
.
4.2.3 Using Multi-Frequency Observations
For this topic we refer to the IERS Conventions 2010 (Petit and Luzum
2010
) and
references therein.
4.2.4 Very Long Baseline Interferometry and the Ionosphere
Like other space geodetic techniques that operate in the microwave frequency band,
Very Long Baseline Interferometry (VLBI) is affected by dispersive delays caused by
the ionosphere. Two or more radio telescopes are pointed towards a common radio
