Multi-Base RTK Positioning Using Virtual Reference Stations

In the recent past, multiple approaches have been proposed to implement the benefits of using multiple reference stations for centimeter-level accurate GNSS positioning. A very promising and practically approved variant is the use of the Virtual Reference Station concept. Here, the modeling of error sources and the information fusion of all reference data are done centrally in a network computation center. The user is provided with Virtual Reference Station data acting like a normal local reference station. No modification of the user GPS receiver is required to use this approach opposing to other multi-base algorithms. This includes no additional requirements on the processing power of the user equipment. Another key advantage is the use of externally provided information like predicted ephemeris. Virtual Reference Stations lead to substantial improvements for real-time positioning by reducing atmospheric, orbital and multipath errors resulting in a performance for long baseline as experienced from short baselines using only one reference station. The paper presents the techniques necessary to process the data from the different reference stations in the network, to resolve the ambiguities inside the network and to generate the error models needed for Virtual Reference Station generation. This covers the main error sources in satellite navigation: ionosphere, troposphere, satellite orbit errors and multipath. Fundamental limitations of ambiguity resolution in realtime, even on a network with known reference station coordinates, are presented. They impact the design and layout of a reference station network for high-precision applications. Another paper in this publication “Long-Range RTK Positioning Using Virtual Reference Stations” presents practical user experiences from several operating Virtual Reference Station networks, providing fast and reliable initializations even in times of high solar activity with typically less that 1 minute initialization time on 30 km baselines, while standard reference station applications can be limited to 10 km.