Virtual Reference Station

Abstract: We will focus on single-frequency single-baseline real-time kinematic (RTK) combining four Code Division Multiple Access (CDMA) satellite systems. We will combine observations from the Chinese BeiDou Navigation Satellite System (BDS), European Galileo, American Global Positioning System (GPS) and the Japanese Quasi-Zenith Satellite System (QZSS). To further strengthen the underlying model, attention will be given to overlapping frequencies between the systems. If one can calibrate the inter-system biases, a common pivot satellite between the respective systems can be used to parameterize double-differenced ambiguities. The LAMBDA method is used for ambiguity resolution. The instantaneous (single-epoch) single-frequency RTK performance is evaluated by a formal as well as an empirical analysis, consisting of ambiguity dilution of precision (ADOP), bootstrapped and integer least-squares success rates and positioning precisions. The time-to-correct-fix in some particular cases when instantaneous RTK is not possible will also be analyzed. To simulate conditions with obstructed satellite visibility or when low-elevation multipath is present, various elevation cut-off angles between 10 and 40° will be used. Four days of real data are collected in Perth, Western Australia. It will be shown that the four-system RTK model allows for improved integer ambiguity resolution and positioning performance over the single-, dual- or triple-systems, particularly for higher cut-off angles.

Abstract: The past few years have seen substantial growth in multiple-reference-station networks which are used to overcome the limitations of standard real-time kinematic (RTK) systems. The use of a multiple-reference-station network, as opposed to a single reference station, results in a larger service area coverage, increased robustness, and a higher positioning accuracy. However, real-time application is still a difficult task to implement in practice. The virtual reference station (VRS) concept is an efficient method of transmitting corrections through a data link to the network users for RTK positioning. A novel method of creating VRS for high-precision RTK positioning has been developed and tested at the Nanyang Technological University (NTU). The emphasis has been on real-time implementation. A number of tests were conducted using the Singapore Integrated Multiple Reference Station Network (SIMRSN). The tests were done at different locations in Singapore to assess the achievable accuracy and initialization times for VRS RTK positioning using the NTU method. The results confirmed that VRS RTK positioning can be achieved to within 3-cm accuracy in horizontal position. Height accuracy is in the range of 1 to 5 cm. The average initialization time is within 2 min.

Abstract: Methods and apparatus are described and illustrated for producing GPS corrections, comprising: collecting measurements from a plurality of network reference stations; determining network corrections from the measurements; determining residual errors at one or more vernier-cell reference stations; and preparing vernier-cell corrections to compensate the residual errors within a vernier-cell region. Network correction streams are described and illustrated which contain network corrections derived from a plurality of network reference stations and residual error corrections derived from one or more vernier-cell reference stations. Methods and apparatus are described for employing such network correction streams in a virtual reference station to produce corrections and/or virtual measurements for use in a GPS receiver.

Abstract: The navigation signal processing theory is described within this text for generic navigation signals to allow a broad range of applications, beyond that of Global Navigation Satellite System (GNSS). Chapter 1 introduces requirements for navigation signals and are illustrated with one Global Positioning System (GPS), one Galileo, and two pulsed signals. Chapter 2 covers software-defined radio technology, together with the architecture and the data flow of a permanent GNSS reference station in Chapter 3. Theoretical signal-processing aspects are the focus of Chapters 4 and 5, and the focus is shifted to implementation in Chapters 6 through 9. Chapter 10 presents an innovative high-precision software radio concept using double-difference correlators, in addition to double-difference pseudorange and carrier-phase observations to increase carrier-phase tracking stability for real-time kinematic applications. Some MATLAB (a high-level technical computing language) and assembler programs that illustrate the core signal-processing concepts of a navigation receiver may be found for this book at the Artech House website, www.artechhouse.com, and this software is described in Chapter 11. This book should help in building advanced navigation software receivers, although it is not for beginners.