Global Positioning System Reference
In-Depth Information
temperature and correlated with the relative humidity. The mean correlation coefficient
between ZWD and surface temperature is about 0.80 and the correlation coefficient between
ZTD and ZWD is about 0.95, reflecting a good agreement between ZTD and ZWD
variations. Therefore, the seasonal cycles of the ZTD are due primarily to the wet
component (ZWD), especially in the surface temperature, even though the wet delay is only
10% of the total delay (ZTD).
The lower mean ZTD values are located at the areas of higher altitude (e.g. Tibet, Asia and
Andes Mountain, South America) and higher latitude areas (Antarctica and Arctic), and the
higher mean ZTD values are concentrated at the areas of middle-low latitudes. The mean
ZTD decreases with increasing altitude at an exponentially decreasing rate due to the
atmospheric pressure decreasing with the height increasing. The mean secular ZTD
variation trend is about 1.5±0.001 mm/yr at all GPS sites. The secular variations are
systematically increasing in most parts of the Northern Hemisphere and decreasing in most
parts of the Southern Hemisphere (excluding increasing in Antarctic). The ZTD trends are
almost symmetrically decreasing with increasing altitude, while the sum of trends at
globally distributed GPS sites is almost zero, possibly reflecting that the secular ZTD
variation is in balance at a global scale. The annual variation of ZTD ranges from 25 to 75
mm depending on the site, and the mean amplitude is about 50 mm at most sites. The
annual variation amplitudes of ZTD at the IGS sites near Oceanic coasts are generally larger
than in the continental inland. Larger amplitudes of annual ZTD variation are mostly found
at middle-low latitudes (near 20S° and 40N°), and the smaller amplitudes of annual ZTD
variation are located at higher latitudes (e.g. Antarctic and Arctic) and the equator areas.
The phase of annual ZTD variation is almost about 60° (about February, summer) in the
Southern Hemisphere and at about 240° (about August, summer) in the Northern
Hemisphere. The mean amplitude of semiannual ZTD variations is about 10 mm, much
smaller than annual variations. The significant semiannual variations with a consistent
phase of about 30° (about January) are at above 30°N in the Northern Hemisphere and the
amplitudes of semiannual variations in other parts are not significant. In addition, the
higher frequency variability (RMS of the ZTD residuals) ranges from 22 to 40 mm of delay,
once again primarily due to the wet component. The variability depends on altitude of the
station. Inland stations tend to have lower variability and stations at ocean and coasts have
higher variability. This is because these stations in particular are located in a region well
known for large abrupt changes in the weather, such as Indian, West Pacific and West
Atlantic oceans.
In addition, the responses of the key ionospheric F2-layer parameters (NmF2 and hmF2) to
the 20 November 2003 super storm have been studied using the GPS ionospheric
tomography technique over South Korea. A strong increase of NmF2 during this storm has
been found, accompanied by a significant hmF2 uplift, which is also confirmed by
independent ionosonde observations. The uplift of the F2 layer is mainly associated with a
strong eastward electric field. The increase of electron density in the F2-layer peak depends
mainly on the molecular nitrogen concentration [N2] with some contribution from
molecular oxygen concentration [O2], while the production rate depends on the atomic
oxygen concentration [O]. However, the O/N2 ratio from the GUVI instrument on board the
TIMED satellite shows no significant change during this geomagnetic storm. It suggests that
the increase in NmF2 is not caused by changes in neutral composition, but is related to other
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