Global Positioning System Reference
In-Depth Information
600
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
IS R
GPS/MET
500
400
300
200
[19
100
Lin
—
1
——
Lon
PgE
0
5 10
10
110
11
1.5 10
11
210
11
0
[m
-3
]
electron density
Fi
gure 6.6 GPS/MET electron density profile.
Source:
Hajj and Romans, 1998. Permis-
sio
n by American Geophysical Union.
[19
Th
e medium is called dispersive if
n
is a function of the frequency. When applying
(6
.3) to the troposphere the real parts
n
and
n
(f )
determine refraction that causes
th
e delays in pseudoranges and carrier phases. The nondispersive part of the index of
re
fraction is
n
. For frequencies in the microwave range the frequency-dependent real
te
rm
n
(f )
is negligible. The latter term causes delays around the millimeter level at
60
GHz and centimeter level at 300 GHz (Janssen, 1993, p. 218). In general,
n
(f )
an
d
n
(f )
are due to interactions with line resonances of molecules in the vicinity of
th
e carrier frequency. The GPS frequencies are far from atmospheric resonance lines.
Th
e imaginary part
n
(f )
, however, quantifies absorption (emission) and is important
to
the WVR observable. When applying (6.3) to the ionosphere the term
n
(f )
is very
im
portant.
¯
6.2 TROPOSPHERIC REFRACTION AND DELAY
The index of refraction is a function of the actual tropospheric path through which
the ray passes, starting at the receiver antenna and continuing up to the end of the
effective troposphere. Let
s
denote the distance; the delay due to refraction is
n(s) ds
ds
(n(s)
ν =
−
=
−
1
)ds
(6.4)
