Troposphere Modeling for Precise GPS Rapid Static Positioning in Mountainous Areas

P. Wielgosz, S. Cellmer, Z. Rzepecka, D.A. Grejner-Brzezinska

Abstract:	In GNSS precise positioning, double differencing of the observations is the common approach that allows for significant reduction of correlated atmospheric effects. However, with a growing distance between the receivers, tropospheric errors decorrelate causing large residual errors affecting the carrier phase ambiguity resolution and positioning quality. This is especially true in case of large height differences between the receivers. This is due to a very high correlation of the troposphere with the height component, exceeding a factor of 0.9, and therefore, affecting mostly estimated station heights. In addition, the accuracy achieved by using standard atmosphere models is usually unsatisfactory when the tropospheric conditions at the receiver locations are significantly different from the standard atmosphere. This applies to baselines of tens of kilometers, especially in mountainous areas. This paper presents an evaluation of different approaches to the tropospheric delay modelling in rapid static applications when using 10-minute long observing sessions of dual frequency pseudorange and carrier phase GPS observations. Several permanent GPS stations of EUPOS (European Positioning System) active geodetic network located in the Carpathian Mountains were selected as a test reference network. The distances between the reference stations ranged from 64 to 122 km. KRAW station served as a simulated user receiver located inside the reference network. User receiver ellipsoidal height is 267 m and the reference station heights range from 277 to 647 meters. Four 66-72 km baselines connecting rover and the reference stations were selected and processed in the rapid-static mode using the inhouse developed MPGPS™ software. A 13-h data set collected on July 3rd, 2008 (from 6:00 to 19:00 UT, LT=UT+1hour) was divided into 78, 10-minute long, sessions. Each session was processed independently. Three different approaches to the troposphere modelling were applied and tested: a) neglecting the troposphere, b) using a standard atmosphere model, c) estimating tropospheric delays at the reference station network and providing interpolated tropospheric corrections to the user. All these solutions were repeated with various constraints imposed on the tropospheric delays in the least squares adjustment. The quality of each solution was evaluated by analyzing the residual height errors calculated by comparing the estimated results to the reference coordinates. The results show that regardless of station height differences, it is recommended to model the tropospheric delays at the reference stations and interpolate them to the user receiver location. The most noticeable influence of the residual (unmodelled) tropospheric errors is observed in the station height component. In many cases, mismodelling of the troposphere disrupts ambiguity resolution and, therefore, prevents user from obtaining the accurate position.
Published in:	Proceedings of the 21st International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2008) September 16 - 19, 2008 Savannah International Convention Center Savannah, GA
Pages:	253 - 260
Cite this article:	Wielgosz, P., Cellmer, S., Rzepecka, Z., Grejner-Brzezinska, D.A., "Troposphere Modeling for Precise GPS Rapid Static Positioning in Mountainous Areas," Proceedings of the 21st International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2008), Savannah, GA, September 2008, pp. 253-260.
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