Global Positioning System Reference
In-Depth Information
trends at globally distributed GPS sites is almost zero, which possibly indicates that the
secular variation is in balance at a global scale, but subjecting to unevenly distributed GPS
stations, etc. It need further be confirmed with much denser GPS network in the future.
These secular ZTD variation characteristics reflect the total variations of surface atmospheric
pressure, temperature and relative humidity, atmospheric vertical motions, etc.
90
60
30
0
-30
-60
2 mm/yr
Scale
-90
0
60
120
180
240
300
360
Longitude (deg)
Fig. 4. Secular trend of ZTD variations at global IGS sites. The red upwards arrows represent
the increase of secular ZTD variations and the green downwards arrows stand for the decrease
of secular ZTD variations
2.5 Seasonal cycles
Meanwhile, the seasonal components are also obtained using least square at annual and semi-
annual scales, which can be used to study the seasonal cycle, including amplitude and phase
shift. The fitted phase shift is used to determine in which month the seasonal maximum takes
place. The annual variation of ZTD ranges from 25 to 75 mm depending on the site, and the
average amplitude is about 50 mm at most sites (Fig. 5). The annual variation amplitudes of
ZTD at the IGS sites near Oceanic coasts are generally larger than in the continental inland. In
addition, larger amplitudes of annual ZTD variation are mostly found at middle-low latitudes
(near 20S° and 40N°), and the amplitudes of annual ZTD variation are especially smaller at
higher latitudes (e.g. Antarctic and Arctic) and the equator areas (see Fig. 6). Sites on the
eastern Atlantic and northeast Pacific coasts have lower annual variations, probably because of
the moderating effect of the ocean on climate. Sites on the lee side of the Alps have higher
annual variation, possibly due to the combined effects of a rain shadow in the winter and high
moisture from the Mediterranean in the summer (Haase et al., 2003; Deblonde et al., 2005).
Figure 7 shows the annual phase distribution with the latitude, where phase values are
counted as clockwise from the north. It can be seen that the phase of annual ZTD variation is
almost found at about 60° (about February, summer) in the Southern Hemisphere and at about
240° (about August, summer) in the Northern Hemisphere, which is just a half-year difference.
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