Ground GNSS Atmospheric Sensing - GNSS Remote Sensing: Theory, Methods and Applications - page 36

is the wet component ( N w ). Thus, ZTD is the sum of the hydrostatic or 'dry'
delay (ZHD) and non-hydrostatic or 'wet' delay (ZWD), due to the effects of
dry gases and water vapor, 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), which plays an important
role in energy transfer and in the formation of clouds via latent heat, directly or
indirectly influencing numerical weather prediction (NWP) model variables (Bevis
et al. 1994 ; Duan et al. 1996 ; Tregoning et al. 1998 ; Manuel et al. 2001 ). Therefore,
the Zenith Tropospheric Delay (ZTD) from GNSS is an important parameter of
the atmosphere, which reflects the weather and climate processes, variations, and
atmospheric vertical motions, etc.

3.2 Theory and Methods

3.2.1 Estimates of GNSS ZTD

3.2.1.1 Double Difference

With double difference for two-frequency GNSS observations, some common error
sources are eliminated, e.g., satellite and receiver clock errors. For a pair of
GPS observation stations and satellites (see Fig. 3.1 ), the ionosphere-free linear

j-th satellite

i-th satellite

k-th station l-th station

Fig. 3.1 GNSS DD observation geometry