www.gpsworld.com February 2013 | GPS World 61 Road | TRANSPORTATION R eal-time vehicle localization is one of three key enabling technologies for the concepts of vehicle-to- vehicle and vehicle-to-infrastructure (V2V and V2I, collectively termed V2X, see OPENING GRAPHIC), a classification of intelligent transport systems (ITS). The further enabling technologies are ad-hoc dynamic networking of agents, and accurate dynamic local traffic maps. Jointly, these require that positioning be accurate, reliable, available, and continuous. A natural evolution in road transport, V2X promises to deliver the next major safety breakthrough. The concept moves away from vehicles making individual decisions about road safety, as in advanced driver assistance systems, and towards a cooperative driving approach that shifts the emphasis from collision protection to collision prevention. The U.S. National Highway Traffic Safety Administration estimates that V2X technology can avoid or minimize up to 80 percent of collisions of unimpaired drivers, and that even a small number of deployed vehicles will provide tangible safety benefits. Network RTK GNSS positioning, like V2X applications, requires a communication system; and by its nature V2X has a positioning solution requirement. Thus it is envisioned that network RTK will play an essential role in the implementation of V2X systems. The consensus between car manufacturers and research organizations is that the future of V2X communication lies with Dedicated Short Range Communication (DSRC) devices, and a large pilot study is currently underway. However, in the short term many V2X applications could be achieved using existing technology, such as cellular communication, offer ing a legacy solution, and initiating early uptake of V2X applications. Previous research by the Nottingham Geospatial Institute (NGI) at the University of Nottingham showed that network RTK positioning can provide a high-accuracy positioning solution during real-world trials, but also revealed two areas of concern: the loss of the fixed-integer ambiguity during satellite line-of-sight outages; and the fragility of the data communications service that delivers the real-time correction information. During road tests, a fixed-ambiguity network RTK solution was available for less than 50 percent of the time on UK roads. Network RTK Vehicle Positioning Networks of continuously operating reference stations (CORS) extend across Europe, North America, Australia, and East Asia. Networks vary in size from five or six reference stations for agriculture to systems of hundreds of CORSs providing national or regional service. FIGURE 1 shows the location of the OS Net CORS run by Ordnance Survey in Great Britain. FIGURE 2 shows the main advantage of network RTK as compared to traditional RTK. The individual reference stations on the left suffer from the spatial decorrelation of errors as distance between reference and rover receivers increases. Adequate vehicle positioning would require individually operating reference stations to be placed approximately 20–30 kilometers apart. However, a CORS network can be used to develop a model of differential corrections, as shown at right, from which a rover receiver can interpret RTK correction information and use this during the computation of its position. The geometry of a CORS network allows two adjacent reference stations to be Network RTK for Intelligent Vehicles Accurate, Reliable, Available, Continuous Positioning for Cooperative Driving Adoption of network real-time kinematic GNSS positioning can lead to major improvements in vehicle localization, although implementation must overcome some real-world challenges. This article assesses the extent of GNSS signal outage in a motorway environment. The average total GNSS outage period and the average time to resolve ambiguity for the network RTK solution can help assess complimentary sensors for a ubiquitous positioning system. Scott Stephenson, Xiaolin Meng, Terry Moore, Anthony Baxendale, and Tim Edwards