Troposphere: Signal or Noise?

U. Vollath, E. Brockmann, X. Chen

Abstract:	The motivation for studying the non-dispersive part of the atmosphere, also known as troposphere, is manifold. Meteorologists study the atmosphere to learn about its physical properties and to do weather predictions. For GPS positioning the modeling of the troposphere is essential for unbiased coordinate results. One of the common interests for both parties is the zenith total delay (ZTD) of troposphere. GPS networks can serve as an array of sensors to provide continuous tropospheric zenith total delay for numerical weather prediction applications. Several research institutes work actively in this area to derive more accurate estimates for ZTD in near real time (normally with one or two hours delay). In this paper, we will show that ZTD estimation in real-time can give comparable results as in postprocessing. Since 1999 the Swiss Federal Office of Topography has been active in the European project COST-716 (exploitation of ground-based GPS for climate and numerical weather prediction application). After a successful benchmarking (van der Marel et al., 2001), swisstopo has been contributing zenith total delay (ZTD) estimates in near real-time (NRT-ZTD) since December 2001. In addition to the 29 Swiss AGNES sites, 20 EUREF sites are processed on an hourly basis. Furthermore, about 12 sites from other networks, mainly in France, are being used in order to improve the station distribution in the western part of Europe. This area is important because the dominating weather conditions from the Atlantic Ocean usually pass over France before they reach Switzerland. Real-time availability of the zenith total delays is an interesting alternative to the post-processing values if the same quality of the estimates is guaranteed. As reference also post-processing ZTD values using the final IGS orbits are generated with a time delay of 21 days. Networked reference station software for GPS processing must estimate tropospheric delays to provide a reliable network solution of the error models. The aim is to provide the field users with information about their local errors to improve productivity and accuracy of the positioning. In addition to the tropospheric delay, ionospheric delays and satellite orbit improvements are applied. Since January 2003 ZTD values can be extracted for the Swiss AGNES network using the real-time network RTK software GPSNet 2.0. This is possible with accumulation intervals of 1 minute and a negligible time delay. A comparison of the postprocessing and real-time results proves to be interesting for meteorology as well as for networked RTK. Therefore the GPS networking software benefits from the additional know-how provided by the meteorology data, and the meteorologists gain access to real-time data. This paper assesses real time ZTD estimation with postprocessing results. For the AGNES network, the absolute ZTD as well as the differential values between the reference stations were compared. The analyses show that 1 cm accuracy for absolute ZTD and 7 mm for relative ZTD can be achieved in real time. Finally it can be said that coordinating both the needs and the efforts of meteorology and GPS networking leads to better stochastic models for the networked GPS and real-time availability of absolute tropospheric zenith delays for meteorology.
Published in:	Proceedings of the 16th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS/GNSS 2003) September 9 - 12, 2003 Oregon Convention Center Portland, OR
Pages:	1709 - 1717
Cite this article:	Vollath, U., Brockmann, E., Chen, X., "Troposphere: Signal or Noise?," Proceedings of the 16th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS/GNSS 2003), Portland, OR, September 2003, pp. 1709-1717.
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