Digital Signal Processing Reference
In-Depth Information
Fig. 6.5
Schematic diagram of radio occultation geometry with a receiver inside (e.g. aircraft
or mountain-top) and outside (LEO) the atmosphere. The
red lines
connecting the GPS with the
receiver represent three raypaths going through the atmosphere with different elevation relative
to the local horizon (zero elevation). The
blue dashed lines
represent the descending of raypath
received at LEO. Note the bending of the raypath is neglected and the relative scales of the plot are
exaggerated for illustration purposes
it is difficult to provide dense sounding measurements in a specific region within a
limited time period. With a GNSS receiver inside the atmosphere on a mountain top
or onboard an airplane, the RO technique offers much denser regional soundings
while retaining the high vertical resolution RO sounding capability which could
greatly facilitate studies of regional weather and climate.
The possibility of making RO measurements with a receiver inside the atmo-
sphere was originally considered by Zuffada et al. (
1999
). Even though fun-
damentally very similar to the spaceborne RO measurements, the mountain-top
or air-borne RO measurements must be corrected for the asymmetric sampling
geometry as the GNSS receiver is located inside the Earth's atmosphere. Zuffada
et al. (
1999
) noticed that it is possible to measure both positive and negative
elevation rays. These refer to rays that intersect the receiver from above and below
the local horizon, as shown in Fig.
6.5
. Note however that it is not possible to retrieve
information unambiguously from the positive elevation angle rays alone.
For an occultation ray with elevation angle below the local horizon, the bending
angle measured at the receiver is a sum of bending accumulated due to the
atmosphere between the tangent height and the receiver height and the bending from
the atmosphere above the receiver up to the GNSS satellite (e.g., Healy et al.
2002
;
Xie et al.
2008
). The negative elevation bending is theoretically given by
Z
r
R
Z
r
T
1
n
dn
dr
dr
p
.nr/
2
1
n
dn
dr
dr
p
.nr/
2
˛
N
.a/
D
2a
a
2
a
a
2
:
(6.14)
r
t
r
R
where
r
T
and
r
R
refer to the radius of the transmitter and receiver altitude,
respectively, and
r
t
is the radius at the tangent altitude. The constant
a
,fora
given ray, is known as the impact parameter. Assuming spherically symmetric
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