Ground GNSS Atmospheric Sensing - GNSS Remote Sensing: Theory, Methods and Applications - page 45

90

60

30

0

−30

−60

2 mm/yr
Scale

−90

0 60 120 180 240 300 360

Longitude (deg)

Fig. 3.9 Secular variation trend of ZTD 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

fitting lines at TOW2 station, Australia. The solid line is the fitting results, consisting
of a linear decrease and seasonal components (annual and semiannual terms). It has
shown a clear trend and seasonal variations of ZTD time series at the TOW2 in
Australia with lower values in the winter and higher values in the summer. Using the
model from Eq. ( 3.9 ), the fitting parameters of all GPS site are obtained, including
trend and seasonal variation terms. The mean secular ZTD variation trend is about
1.5 ˙ 0.001 mm/year. Figure 3.9 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, Fig. 3.10 shows the relationship
of secular ZTD variation trend with the altitude. It has been seen that 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 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