Global Positioning System Reference
In-Depth Information
p
T
Na
≈
0
d
,0
1
0
with
p
0
=
partial pressure of the dry component at standard sea level (mbar)
T
0
=
absolute temperature at standard sea level (K)
a
1
=
empirical constant (77.624 K/mbar)
e
T
e
T
Na
≈
0
+
a
0
w
,0
2
3
2
0
0
where
a
2
and
a
3
are empirical constants (
−
12.92 K/mbar and 371,900 K
2
/mbar,
respectively).
Path delay also varies with the user's height,
h
. Thus, both the dry and wet com-
ponent refractivities are dependent on the atmospheric conditions at the user's
height above the reference ellipsoid. One model that takes the height into account
and is successfully demonstrated in [20] combines parts of the works cited in [18,
19, 21, 22]. The dry component as a function of the height is determined by
µ
hh
h
−
()
Nh N
=
d
(7.25)
d
d
,0
d
and
h
d
, the upper extent of the dry component of the troposphere referenced to sea
level, is determined from
p
h
=
0011385
.
0
d
−
6
N
×
10
d
,
0
where
µ
stems from the underlying use of the ideal gas law. Hopfield [18] found that
setting
4 gives the best results for the model.
Similarly the refractivity,
N
w
(
h
), of the wet component of the troposphere is
determined from
µ =
µ
hh
h
−
()
Nh N
=
w
(7.26)
w
w
,0
w
where
h
w
is the extent of the wet component of troposphere determined by
1
1 255
,
h
=
00113851
.
+
005
.
e
w
0
N
×
10
−
6
T
0
w
,
0
The path length difference when the satellite is at zenith and the user is at sea
level is from (7.24):
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