Global Positioning System Reference
In-Depth Information
h
(5)
log
P
≈
5-
15.5
10
where
P
is the pressure in Pascals and
h
is the height in millimeters. Based on the Eq. (4),
ZHD can be expressed as
(5
−
h
/15.5)
2.28 * 10
. Therefore, ZTD at all GPS sites can be
approximately expressed as:
(5
−
h
/15.5)
ZTD
=
2.28 * 10
(6)
where the units of
ZTD
and
h
are in millimeters, respectively. Comparing GPS-derived ZTD
with the empirical formula estimations, it has shown a good consistency.
mm
90
2600
60
2380
30
2160
0
1940
-30
1720
-60
-90
1500
0
60
120
180
240
300
360
Longitude (deg)
Fig. 3. Distribution of mean ZTD at global IGS sites
2.4 Trend analysis
The GPS ZTD time series have been analyzed for 4-12 years at globally distributed 150 GPS
sites. Using the least square the fitting parameters of all GPS site are obtained, including
trend and seasonal variation terms (Jin et al., 2007). The mean secular ZTD variation trend is
about 1.5±0.001 mm/yr. Figure 4 shows the distribution of the secular ZTD variation trends
at all GPS sites as the yearly increase or decrease. It can be seen that the trends are positive
in most parts of the Northern Hemisphere and negative in most parts of the Southern
Hemisphere (excluding positive in Antarctic), corresponding to a systematic increase or
decrease of ZTD. It is interesting to note that the downtrend in Australia is larger than other
regions. This downtrend of ZTD is probably due to the highly deserted in Australia. In
addition, the ZTD variation trend decreases with increasing altitude, and furthermore, the
ZTD trends are almost symmetrical with altitude. This indicates that the secular ZTD
variations are larger at the lower altitude and at the higher altitude the secular ZTD
variations hardly increase or decrease. In addition, the sum of downward and upward
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