Multi-Base RTK Positioning Using Virtual Reference Stations
Multi-Base RTK Positioning Using Virtual Reference Stations
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发表时间：
2000-09
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通讯作者：
U. Vollath;Alois Buecherl;H. Landau;Christian Pagels;B. Wagner
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作者：
U. Vollath;Alois Buecherl;H. Landau;Christian Pagels;B. Wagner
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. INTRODUCTION Differential GPS is a well-established technique to improve the positioning precision since many years. This allows even centimeter-level accurate positioning using the so-called integer ambiguity resolution technique. The basic concept is to mitigate the main error sources, ionospheric and tropospheric delay, orbit errors and satellite clock errors (where we had S/A effects, too until recently) by receiving satellite data at a well-known location. All common errors between this reference receiver and the user receiver cancel out. Though this already works quite well for many applications, the special de-correlation (i.e. the change of the errors when moving away from the reference station) of the errors leaves large error contributions in the corrected signals. A new concept addresses this problem. The idea is to generate Virtual Reference Stations (VRS) that simulate a local reference station near by the user receiver. Thus, the errors cancel out better than by using a more distant reference station. Integrating the data of a whole network of reference receivers, precise models of the error sources are derived. They are used to interpolate the expected errors at the user location, thus simulating a near reference receiver. REFERENCE STATION CONFIGURATIONS Besides Virtual Reference Station operation, operating a reference station network has multiple tasks. Following the different requirements for GPS reference station networks the operation of a network can be classified into 4 levels: • Level 0: Station Data Integrity. This level performs analyses using the data of single stations to perform quality control / quality assurance procedures. • Level 1: Differential Integrity. This extends level 0 to the differential observables using baselines, i.e. data from two reference stations to increase integrity. • Level 2: Modeling of Error sources. Networkwide models for the error contributions on the positioning signals are derived in real-time to improve meter and decimeter-level positioning. • Level 3: Virtual Reference Stations. Using the error models of level 2, the carrier phase ambiguities within the network are resolved. This is used to derive centimeter accurate measurements of the error contributions and generate a local Virtual Reference Station for the user. The following table gives an overview of quality checks performed and stations used: Level Error Sources Stations Used 0 Code chips errors (wrong code ambiguity) Coarse code outliers (tens of m) Coarse carrier phase fluctuations
