About the GPS Water Vapour Meteorology project
Water vapour is one of the most important constituents of the atmosphere
as moisture and latent heat are transported through the water vapour phase.
Besides that, water vapour is the most important greenhouse gas. Accurate,
dense and frequent sampling of water vapour, is obviously of great importance
for climatological research as well as operational weather forecasting.
Currently water vapour is measured using radiosondes and ground or space
based water vapour radiometers. Radiosondes produce an accurate measurement
of the water vapour profile, but the temporal and spatial resolution is
rather poor. Ground based radiometers experience problems during periods
of rain fall and space based radiometers can be degraded in the presence
of clouds. Besides these limitations, all systems involve considerable
costs.
Over the past few years, a new technique to measure integrated water
vapour (IWV) has been developed. This technique is based on the estimation
of the tropospheric delay time of GPS signals. The delay, regarded as a
nuisance parameter by geodesists, can be directly related to the amount
of water vapour in the atmosphere, and hence is a product of considerable
value for meteorologists. Furthermore, ground based GPS water vapour estimation
is not affected by rain fall and clouds, and can therefore be called an
'all-weather' system. As it takes minor effort to obtain GPS water vapour
estimates from the existing GPS infrastructure, and since the temporal
and spatial resolution are higher than of the current techniques used,
GPS water vapour estimation is a valuable complement.
From September 1996 up to October 1998 the GPS Water Vapour Meteorology
was executed by the Royal Netherlands Meteorological Institute, Faculty
of Aerospace Engineering and Faculty of Geodetic Engineering of Delft University
of Technology, and the Survey Department of RWS.
The objectives of this project:
- set up an infrastructure for the acquisition, storage and processing of
GPS-IWV data
- assessment of the accuracy of GPS-IWV data
- investigate the usefulness of GPS-IWV data for weather forecast models
and climate research
- study the feasibility of real-time processing of GPS-IWV data
Selected Results:
-
CLARA campaigns
GPS-IWV estimates have been compared over three intervals of extensive
measurements of the atmosphere in the framework of the
CLARA project. During these campaigns a water vapour radiometer and
a GPS antenna were collocated, and radiosondes were launched three times
daily at the same location. In general a good agreement between the different systems was found.
-
Operational processing
GPS data of the AGRS.NL network complemented with a number of IGS stations
is processed daily at the Faculty of Aerospace Engineering and time series
of the tropospheric delay estimates are calculated every 6 minutes. At
the KNMI the IWV is calculated from the tropospheric using meteo data from
GPS-stations and stations from synoptical network of the KNMI. Several
different network configurations have been investigated to study the influence
of the size of the network on the IWV estimates. Additionally the influence
of the use of predicted information of the satellite orbits and polar movement
has been studied. A 15 station network has been selected with a 2-day a
priori orbit generation derived from CODE rapid orbits.
-
Orbit estimation
Operational results were not always accurate as compared to the radiosonde
data. Especially near the end of the day larger scatter was sometimes found.
Experiments with orbit estimation as a part of the processing showed promising results,
even with predicted orbits. These results have been published in Journal of Geophysical Research,
"Integrated atmospheric water vapour estimates from a regional GPS network", H. Klein Baltink, H. van der Marel,
and A.G.A. van der Hoeven, 2002, Vol. 107, No. D3, 10.1029/2000JD000094, pp. ACL 3-1 - 3-8.
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