Ground GNSS Atmospheric Sensing - GNSS Remote Sensing: Theory, Methods and Applications - page 51

5

4

3

2

1

0

−90 −60 −30 0 30 60 90

Latitude ( o )

Fig. 3.18 Semidiurnal ZTD amplitudes as a function of station latitude

0.1 and 4.3 mm with an uncertainty of about 0.2 mm are observed. Similar to the
diurnal results mentioned above, the largest semidiurnal amplitudes are also found in
low-latitude equatorial areas (see Fig. 3.18 ). The first peak of the semidiurnal cycle
occurs typically around local noon. These diurnal and semidiurnal cycles of ZTD
may be due to certain short time scale physical processes such as diurnal convection,
atmospheric tides, general circulation and the coupling between the lower and the
middle and upper atmosphere.
The atmospheric density changes the refractive index of the zenith column
of air under the influence of the atmospheric tides, which causes oscillations in
the ZTD at tidal frequencies. Thus, the oscillations in ZTD within periods of
a solar day (diurnal) and half a solar day (semidiurnal) may reflect the diurnal
and semidiurnal tides induced on the atmosphere by thermal and gravitational
excitation from the Sun. Under the assumption of hydrostatic equilibrium, the
change in pressure with height is related to total density at altitude h through the
approximate relationship with hydrostatic equilibrium approximation. Based on Eq.
( 3.11 ), the zenith hydrostatic delay (ZHD) can be expressed as ZHD D 2.28 p 0 .As
the hydrostatic component ZHD accounts for approximately 90 % of ZTD, ZTD
is strongly correlated with surface pressure p 0 at the site. It can be seen that if
subdiurnal surface pressure varies by 1 hPa, the scale factor predicts a subdiurnal
ZTD variation with amplitude of 2.28 mm.
The GPS-derived S 1 and S 2 signals in ZTD are compared with 3-h surface synop-
tic pressure observations from 1997 to 2007 that are archived at the National Center
for Atmospheric Research (NCAR) (DS464.0; http://rda.ucar.edu/datasets/ds464.0 ) .