Implementation of Wide-Area Broadcast NRTK on a Communication Satellite Platform

L. Yang, C. Hill, T. Moore

Abstract:	Real-Time Kinematic positioning technology plays an important role in the area of precise positioning. Differencing the observations from a reference station (RS) with its own, a roving receiver can achieve centimetre-level positioning accuracy in real-time. In Network-based RTK (NRTK), the observations from a cell of RSs are used to create precise corrections for the rover location, effectively mitigating the distancedependent errors, which mostly consist of atmospheric biases. As a benefit, the rover mobility constraint in the traditional single reference RTK can be improved, and the rover-to-RS separation can increase from 20 km to 50-70 km. In the area of Intelligent Transportation Systems and Services (ITSS), such as road pricing, lane detection and assisted driving, the requirement for accuracy is not as stringent as the centimetre level. Instead, ITSS applications have a greater emphasis on the positioning mobility, convenience, efficiency and reliability. The current implementation of NRTK could not fulfil these requirements adequately. In the current NRTK implementation, if the rover is not restricted in a small area, a two-way communication link is required. The rover needs to report its approximate location regularly to the NRTK Central Processing Facility (CPF), in order to be assigned an appropriate RS cell and retrieve local precise corrections while the rover moves. A hand-over issue occurs when the rover moves from one cell to another, because the rover corrections will be computed from a different set of observations. The correction data stream may be interrupted or even disconnected during the switching. NRTK service quality and reliability cannot be guaranteed in this period, and the positioning solution may drop to stand-alone. Another drawback of the two-way communication requirement is that the number of simultaneous users a CPF can support is limited by its computing capacity and I/O bandwidth. As a result the whole service will have difficulty addressing a mass market. A one-way communication, or broadcast, NRTK implementation for wide area is proposed in this paper. It is developed from the Master-Auxiliary-Concept (MAC) NRTK concept. In this proposed broadcast implementation, the MAC data from different cells will be integrated into a large cell covering a wide area. A set of cell merging and data verification algorithms will be applied to ensure the consistency of the data integration. The rover will receive all the RS data for the whole area, which enables it to perform NRTK positioning without interruption. Thus a greater mobility and convenience can be achieved. Unlimited simultaneous users can be supported via this broadcast approach, and service cost for the end user could be greatly reduced. Another problem of current NRTK is the wireless communication media. As well as the good quality (correctness and completeness) of the NRTK data transmission, a wide and full coverage of the NRTK service is also required by ITSS applications, which cannot always be satisfied by the current communication media. Currently, most NRTK services are implemented on the commercial cell phone network services. Local tests on UK roads show that even on the highway, the data service of the cell phone network cannot be guaranteed, and the commercial cell phone networks are not available at all in some remote rural areas. The communication satellite is proposed in this study as a complementary NRTK communication medium to the cell phone network. Its signal coverage could be country-wide or even continent-wide, and the NRTK data could be readily transmitted to remote areas where the cell phone network is not available. The one-way nature of the satellite-based transmission determines that developing broadcast mode NRTK is necessary in this implementation. Bandwidth resource in satellite communication is much more valuable than in the cell phone network communication. Various efforts have been carried out to optimize the efficiency of the bandwidth usage. Instead of transmitting the raw MAC correction data, a parameter-based model is investigated. In this approach, firstly an EGNOS troposphere model is applied to the raw NRTK correction data, removing the estimated correction value, then a mathematical model is applied to the residue, to generate a set of parameters which can represent the major characteristics of the residue distribution over a wide area. This set of parameters will be broadcast via the communication satellite. At the rover end, a backward conversion will be carried out to rebuild the NRTK raw correction data, in the industry standard message formats. Therefore this approach acts as an extra layer between the CPF and the rover, and is transparent to both ends. It has been demonstrated that through this approach the required transmission bandwidth is reduced significantly. This proposed broadcast NRTK implementation has been tested on three different communication satellites, Thuraya, Worldspace and Solaris. The former two are L-band data service satellites, both supporting IP-based media content. Solaris is the first S-band data service satellite in Europe, available from 2009. It has a wider bandwidth, and has an advantage in integrating with other mobile services. Via a supporting infrastructure of terrestrial repeaters, it also provides a stronger signal penetration, more reliable signal reception during movement, and better coverage in areas where there is no line of sight to the satellite. All three of the above satellite communication channels are compared and analyzed in terms of transmission latency, data loss rate, and signal reception ability in both static and kinematic mode. Tests are carried out in two independent test sites, one in the East Midlands area of the UK, and the other in Paris, France. This work is supported by the European Commission under the 6th Framework Programme (FP6) for Research and Development. It is a part of The Satcoms in Support of Transport on European Roads (SISTER) project, which promotes the integration of satellite communications with GALILEO to enable mass market take-up by road transport applications. In this project, through a number of proofs of concept, the role of satellites (both telecommunication services and navigation technology) was investigated for a series of comprehensive road transport applications, including road pricing, traffic information, emergency position reporting, driver navigation assistance, map updating, tracking of dangerous goods and enhanced NRTK implementation.
Published in:	Proceedings of the 23rd International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2010) September 21 - 24, 2010 Oregon Convention Center, Portland, Oregon Portland, OR
Pages:	2522 - 2533
Cite this article:	Yang, L., Hill, C., Moore, T., "Implementation of Wide-Area Broadcast NRTK on a Communication Satellite Platform," Proceedings of the 23rd International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2010), Portland, OR, September 2010, pp. 2522-2533.
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