GNSS Atmospheric and Ionospheric Sounding – Methods and Results - Global Navigation Satellite Systems: Signal,Theory and Applications - page 360

where c is the speed of light in a vacuum, τ is the delay measured in units of time and N is
the neutral atmospheric refractivity. The N is empirically related to standard meteorological
variables as (Davis et al. 1985)

P P

w w

Nk =+ k + k (2)

1 2 3

2

ZT

ZT

w

w

where

i ki = is constant, ρ is the total mass density of the atmosphere, P is the
partial pressure of water vapor, Z is a compressibility factor near unity accounting for
the small departures of moist air from an ideal gas, and T is the temperature in degrees
Kelvin. The integral of the first term of equation (2) is the hydrostatic component ( )

( 1,2,3)

N

h

and the integral of the remaining two terms is the wet component ( )

w N . Thus, ZTD is the
sum of the hydrostatic or dry delay (ZHD) and non-hydrostatic or wet delay (ZWD),
respectively. The dry component ZHD is related to the atmospheric pressure at the
surface, while the wet component ZWD can be transformed into the precipitable water
vapor (PWV) and plays an important role in energy transfer and in the formation of
clouds via latent heat, thereby directly or indirectly influencing numerical weather
prediction (NWP) model variables (Bevis et al, 1994; Tregoning et al. 1998; Manuel et al.,
2001). Therefore, the Zenith Tropopospheric Delay (ZTD) is an important parameter of the
atmosphere, which reflects the weather and climate processes, variations, and
atmospheric vertical motions, etc.

In the last decade, ground-based GPS receivers have been developed as all-weather, high
spatial-temporal resolution and low-cost remote sensing systems of the atmosphere (Bevis
et al., 1994; Manuel et al., 2001), as compared to conventional techniques such as satellite
radiometer sounding, ground-based microwave radiometer, and radiosondes (Westwater,
1993). With independent data from other instruments, in particular water vapor
radiometers, it has been demonstrated that the total zenith tropospheric delay or
integrated water vapor can be retrieved using ground based GPS observations at the same
level of accuracy as radiosondes and microwave radiometers (Elgered et al. 1997;
Tregoning et al. 1998). Currently, the International GPS Service (IGS) has operated a
global dual frequency GPS receiver observation network with more than 350 permanent
GPS sites since 1994 (Beutler et al., 1999). It provides a high and wide range of scales in
space and time to study seasonal and secular variations of ZTD as well as its possible
climate processes.

2.1 GPS data and analysis

The IGS (International GPS Service) was formally established in 1993 by the International
Association of Geodesy (IAG), and began routine operations on January 1, 1994 (Beutler et
al. 1999). The IGS has developed a worldwide network of permanent tracking stations with
more than 350 GPS sites, and each equipped with a GPS receiver, providing raw GPS orbit
and tracking data as a data format called Receiver Independent Exchange (RINEX). All
available near-real-time global IGS observation data archived in the Global IGS Data Center,
which contributes to geodesy and atmosphere research activities in a global scale. Here the
globally distributed 150 IGS sites with better continuous observations are selected with
spanning at least four years of measurements (Figure 1), and most sites observations are
from 1994 to 2006.