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7 Platform one Uncrewed Systems Technology | December/January 2023 Researchers have developed an alternative positioning system using mobile phone networks and an atomic clock that is more robust and accurate than satnav in urban settings (writes Nick Flaherty). The team, at Delft University of Technology (DUT) and Vrije Universiteit Amsterdam (VUA), achieved an accuracy of 10 cm in a prototype system. Radio signals from navigation satellites such as GPS and Galileo are weak when received on Earth, making accurate positioning impossible if the radio signals are reflected or blocked by buildings. “That can make GPS unreliable in urban settings, for instance,” said Christiaan Tiberius at DUT and coordinator of the project. “That is a problem if we ever want to use automated vehicles. So far, we had no back-up system.”  The project, called SuperGPS, developed an alternative positioning system that makes use of the mobile telecoms network instead of satellites, which could be more robust and accurate than GPS. “We realised that the telecoms network could be transformed into a very accurate alternative positioning system that is independent of GPS,” said Jeroen Koelemeij of VUA. “We have developed a system that can provide connectivity like existing mobile and wi-fi networks, as well as accurate positioning and time distribution like GPS.” The key is connecting the mobile network to a very accurate atomic clock that broadcasts precisely timed messages for positioning. This is hosted by VSL, the National Metrology Institute of the Netherlands, and distributed through radio transmitters that are connected and time-synchronised at the sub-nanosecond level through a fibre optic Ethernet network. VSL has extensive experience with a protocol called White Rabbit, developed at the CERN particle accelerator lab, that allows timing signals to be carried over long distances using fibre cables. “We had already been investigating techniques to distribute the national time produced by our atomic clocks to users elsewhere through the telecoms network,” said Erik Dierikx of VSL. “With these techniques we can turn the network into a nationwide distributed atomic clock – with many new applications such as very accurate positioning through mobile networks. “With the hybrid optical-wireless system we have demonstrated, in principle anyone can have wireless access to the national time produced at VSL. Basically it forms an extremely accurate radio clock that is good to one billionth of a second.” The system uses radio signals with a bandwidth much larger than usual. “Buildings reflect radio signals, which can confuse navigation devices,” said Gerard Janssen at DUT. “The large bandwidth of our system helps to sort out these confusing signal reflections, and enables higher positioning accuracy. “At the same time, bandwidth in the radio spectrum is scarce and therefore expensive. We circumvent that by using a number of related small-bandwidth radio signals spread over a large virtual bandwidth. That means only a small fraction of the virtual bandwidth is actually used, and the signals can be very similar to those of mobile phones.” Driverless cars ‘SuperGPS’ for urban roads The system improves on satnav positioning in urban areas by exploiting mobile phone networks