Digital Signal Processing Reference
In-Depth Information
3.4
GNSS Precipitable Water Vapor
3.4.1
GNSS PWV Estimate
ZTD is the sum of the hydrostatic or dry delay (ZHD) and non-hydrostatic or wet
delay (ZWD), due to the effects of dry gases and water vapor, respectively. The ZHD
is related to the atmospheric pressure at the surface, i.e.
P
s
f .; H/
D
2:2779
10
3
ZHD
(3.12)
where
P
s
is the total surface pressure in millibars and
f
(,
H
) is a factor for correcting
the local gravity as
f
(,
H
)
D
1
0.00266 cos(2)
0.28
H
, where is the latitude
and
H
is the height above the ellipsoid in meters. The wet component ZWD can
be obtained by subtracting ZHD from ZTD, which can be transformed into the
precipitable water vapor (PWV) via the following function (Bevis et al.
1994
):
PWV
D
Y
:ZW D
(3.13)
where
Q
D
(10
6
R
v
[(
k
3
/
T
m
)
C
k
2
0
])
1
, is the density of liquid water,
R
v
is the
specific gas constant for water vapor,
T
m
is a weighted mean temperature of the
atmosphere,
k
2
0
D
k
2
mk
1
and
m
is
M
w
/
M
d
, the ratio of the molar masses of water
vapor and dry air (Davis et al.
1985
). Therefore, if the pressure and temperate data
are available, the water vapor can be derived from GPS-derived ZTD.
Due to the absence of local surface temperature and pressure data, the surface
synoptic observations and numerical weather models' reanalysis products are used
to calculate the
P
s
and
T
m
.The
P
s
is interpolated from global 3-h surface synoptic
observations of all land and ocean surface weather stations with more than 15,000
including in the Global Telecommunication System (GTS) and marine reports from
the Comprehensive Ocean-Atmosphere Data Set (COADS) (Dai and Wang
1999
)
6-hourly re-analysis products ERA-40 (European Centre for medium range weather
forecast 40 year reanalysis), but currently ERA-40 data are only available from
1948 to 2002, and the data after 2002 are not available. While the NCEP/NCAR
reanalysis products are available from 1948 to present, and furthermore the inter-
polated
T
m
results are almost similar with a small difference between ERA-40
and NCEP/NCAR reanalysis (Wang et al.
2007
). Therefore, in this study the 2-h
noaa.gov/Datasets/ncep.reanalysis/
)
with an interpolation technique (Wang et al.
2007
). Thus, 2-h PWV time series for 1994-2007 over all global IGS sites can
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