Global Positioning System Reference
In-Depth Information
p
wv
T
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
Z
−
1
wv
=
R
wv
ρ
wv
(6.31)
in
the integrand gives
10
−
6
k
2
+
R
wv
k
3
T
m
ZWD
=
ρ
wv
dh
(6.32)
W
e replace the integrand in (6.32) by IWV according to (6.27) and then replace
th
e specific gas constant
R
wv
by the universal gas constant
R
and the molar mass
M
wv
. The conversion factor
Q
that relates the zenith nonhydrostatic wet delay to the
pr
ecipitable water becomes
k
2
+
10
−
6
ZWD
PWV
= ρ
w
R
M
wv
k
3
T
m
Q
≡
(6.33)
[20
The constants needed in (6.33) are known with sufficient accuracy. The largest error
contribution comes from
T
m
, which varies with location, height, season, and weather.
The
Q
value varies between 5.9 and 6.5, depending on the air temperature. For
warmer conditions, when the air can hold more water vapor, the ratio is toward the
low end. Bevis et al. (1992) correlate
T
m
with the surface temperature
T
0
and offer
the model
Lin
—
-
——
No
PgE
T
m
[K]
=
70
.
2
+
0
.
72
T
0[K]
(6.34)
[20
The following models for
Q
are based on radiosonde observations (Keihm, JPL,
private communication).
Q
=
6
.
135
−
0
.
01294
(T
0
−
300
)
(6.35)
0
.
0181 PWV
2
=
−
+
Q
6
.
517
0
.
1686 PWV
(6.36)
0
.
00049 ZWD
2
Q
=
6
.
524
−
0
.
02797 ZWD
+
(6.37)
If no surface temperatures are available, one can use (6.36) and (6.37), which take
advantage of the fact that
Q
correlates with PWV (since higher PWV values are
generally associated with higher tropospheric temperatures).
6.3 TROPOSPHERIC ABSORPTION
This section deals briefly with some elements of remote sensing by microwaves. The
interested reader may consult general texts on remote sensing. We recommend the
topic by Janssen (1993) because it is dedicated to atmospheric remote sensing by mi-
crowave radiometry. The material presented below very much depends on that source.
Solheim's (1993) dissertation is also highly recommended for additional reading.
