Virtual GPS Reference Stations in the Netherlands

H. van der Marel

Abstract:	Until now permanent GPS networks are mainly used for geodetic, geophysical and atmospheric research. They are used less frequently for surveying and remote sensing. This is mainly because of the relatively short observation time spans - several minutes rather than hours - and the on-the-fly ambiguity resolution techniques that are being used, which puts a limit on the distance to a reference station. In a network of reference stations this distance limit can be relaxed, but then the user has to download and process data from several reference stations, which is something many users don’t like. Therefore, in the Netherlands, currently the concept of virtual GPS reference stations is investigated. In combination with a web-based interface, a user can download data of a non-existent - virtual - GPS reference station at a position specified by him. The virtual reference station data is computed from five permanent GPS stations in the Netherlands (AGRS.NL), using an adjustment with data from all reference stations and interpolation of atmospheric errors. The user has only to download data for one (virtual) reference station. Because the virtual station is computed from all available reference stations, processing with user data gives results which are comparable to a network solution. Furthermore, the user will typically select a virtual station close by, therefore he can use his favorite GPS software for short baselines and ambiguity resolution. The virtual GPS reference station imitates as best as possible a real station at the same location, including atmospheric and antenna phase center delays, but except multipath. The delays cancel out when single differences are formed in the user's processing software. The virtual data is computed at a computing center. The user may select the position and antenna type for the virtual station. The network solution, atmospheric and antenna phase center corrections are handled by the computing center. At the computing center, precise orbits can be used to reduce orbit errors, whereas the user may still use broadcast ephemerides. Also, improvements in the modeling and software have only to be implemented at the computing center. Virtual stations are an attractive alternative for the use of multiple reference stations. The accuracy of a single baseline solution with a virtual reference station is comparable to a network solution with state of the art software. Therefore, surveyors may now use permanent GPS networks without having to make changes to their current setup for GPS processing.
Published in:	Proceedings of the 11th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1998) September 15 - 18, 1998 Nashville, TN
Pages:	49 - 58
Cite this article:	Marel, H. van der, "Virtual GPS Reference Stations in the Netherlands," Proceedings of the 11th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1998), Nashville, TN, September 1998, pp. 49-58.
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