Digital Signal Processing Reference
In-Depth Information
Chapter 6
Atmospheric Sensing Using GNSS RO
6.1
GNSS RO Atmospheric Sounding
6.1.1
Parameters Retrieval from GNSS RO
on various atmospheric properties in the GPS RO frequencies. In the neutral atmo-
sphere, which includes the stratosphere and the troposphere (i.e., below
70 km),
the refractivity primarily depends on the dry atmosphere and water vapor as firstly
described by Smith and Weintraub (
1953
). For realistic suspensions of water or ice,
contribution due to the liquid water and ice water content are generally small in
comparison with other terms and will be neglected here. Therefore, the refractivity
variation with tangent radius
r
becomes a simple function of atmospheric pressure
P
(in hPa), water vapor partial pressure
P
w
(in hPa) and temperature
T
(in Kelvin),
such as
N.r/
D
c
1
P.r/
T.r/
C
c
2
P
w
.r/
T
2
.r/
:
(6.1)
The constants are
c
1
D
77.6 (K hPa
1
) and
c
2
D
3.73
10
5
(K
2
hPa
1
). Gener-
ally, the first term on the right-hand-side (RHS) of the equation is referred as a
dry term, which dominates throughout the neutral atmosphere. However, the second
term on the RHS is referred as a wet term (i.e., water vapor contribution), which is
generally smaller than the dry term but becomes significant in the middle and lower
troposphere and can constitute to
30 % of refractivity near the Earth's surface in
lower latitudes.
Given the GNSS RO refractivity measurement, the conventional geophysical
parameters, such as air density, pressure, temperature and humidity can be further
derived. However, beside Eq. (
6.1
), other physical relations or assumptions are
necessary.
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