A Global Analysis of the Mean Atmospheric Temperature for GPS Water Vapor Estimation

Torben Schueler, Andrea Posfay, Guenter W. Hein and Robert Biberger

Abstract:	It is well-known that GPS as a stand-alone system is unable to measure integrated water vapor. Two meteorological quantities are needed to do so: surface total pressure and the mean tropospheric temperature. Using these data, ground-based GPS water vapor estimation basically works as follows: First, the surface pressure is used to separate the hydrostatic from the total delay. The remaining part is the wet delay and can be converted into integrated water vapor with help of the mean temperature of the atmosphere. This, however, requires the complete temperature and humidity profile above the antenna site and thus, either radiosondes need to be launched, or the data must be taken from numerical weather models. The use of radiosondes is economically inefficient in the long run and, in any way, it would render GPS useless or at least questionable since water vapor samples can already be derived from the radiosonde profile measurements, so there would be no need for an additional water vapor sounder. The second method is feasible, but since the primary aim is to improve weather models with help of GPS meteorology, it is desirable to introduce GPS results into the weather models that are as uncorrelated with the numerical weather model data as somehow possible. So, the best solution would be a fully-featured, independent GPS integrated water vapor sensor, i.e. a GPS receiver and antenna plus a pressure and temperature sensor. Such a system works as water vapor sensor due to the fact that a relation can be established between time, surface and weighted mean tropospheric temperature. Based on a global-scale analysis of T170L42 GDAS numerical weather fields of the National Center for Environmental Prediction (NCEP, NOAA), the following issues are addressed in this paper: Several models deriving the mean temperature as a function of the dry surface temperature or the day of year or both are presented and assessed on a global 1° x 1° grid. Regions with good and poor fit are identified, shortcomings are analyzed and compensated if possible. Site-specific conversion coefficients to compute the mean temperature were also processed for a large number of IGS, EUREF and GREF stations and are presented in this paper. Furthermore, the height reduction problem is examined. Important questions on how the mean temperature decreases with increasing height and their impact on the accuracy of GPS water vapor estimation are answered. Concluding remarks indicate that GPS water vapor estimation without any direct use of numerical weather models or radiosonde data is possible up to a satisfactory degree of accuracy.
Published in:	Proceedings of the 14th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2001) September 11 - 14, 2001 Salt Palace Convention Center Salt Lake City, UT
Pages:	2476 - 2489
Cite this article:	Schueler, Torben, Posfay, Andrea, Hein, Guenter W., Biberger, Robert, "A Global Analysis of the Mean Atmospheric Temperature for GPS Water Vapor Estimation," Proceedings of the 14th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2001), Salt Lake City, UT, September 2001, pp. 2476-2489.
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