Geoscience Reference
In-Depth Information
290
285
280
275
270
265
100
200
300
400
500
600
700
800
900
1000
1100
Frequency (THz)
Fig. 3
The total optical refractivity as function of frequency. The total pressure is 1013 hPa, the
Temperature 300 K, and the relative humidity is 100%
Owens (
1967
), Ciddor (
1996
), and Ciddor and Hill (
1999
) can be used. For the
accurate calculation of the optical refractivity, IAGhas recommended (Rüeger
2002b
,
Chap. 3) the formulae proposed by Ciddor (
1996
) and Ciddor and Hill (
1999
) that are
expressed as
1
ρ
ds
5792105
238
167917
k
d
(ν)
=
(
1
+
0
.
534
×
10
−
6
(
x
c
−
450
))
×
10
−
2
+
,
(24)
.
0185
−
Υ
2
57
.
362
−
Υ
2
295
6
1
.
022
ρ
ws
k
w
(ν)
=
2
4
10
−
2
.
235
+
2
.
6422
Υ
−
0
.
032380
Υ
+
0
.
004028
Υ
×
,
(25)
ρ
ds
is the density of dry air at 15
◦
C, 101325 Pa,
x
w
=
where
0 (where
x
w
=
p
w
/
p
is
themolar fraction of water vapor inmoist air).
ρ
ws
is the density of pure water vapor at
20
◦
C, 1333 Pa,
x
w
=
λ
=
c
0
)
and
x
c
is the fractional carbon dioxide content. A plot of the optical refractivity as
function of frequency is shown in Fig.
3
.
is the wave number (reciprocal of the wavelength,
1
1.
Υ
3 Definition of Path Delay in the Neutral Atmosphere
In space geodesy normally the travel time (or difference in travel time) between a
source in space (a satellite or a quasar) to a receiver on the surface of the Earth is
