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It is widely recognised that over-reliance on GNSS (e.g GPS) for timesynchronisation represents an acute threat to modern society, and a diversityof alternatives are required to mitigate the threat of an outage. This paperproposes a GNSS alternative using time dissemination over national scaletransmission or distribution networks. The method utilises the same frequencybandwidth and coupling technology as established power line carrier technologyin conjunction with modern chirp Spread Spectrum modulation. The basis of themethod is the transmission of a time synchronised chirp from a centralsubstation. During GNSS operation, all substations can estimate the time offlight by correlating the received chirp with a time-synchronised local copy.During GNSS outage, time sychronisation to the central substation is maintainedby correcting for the precalculated time of flight. It is shown that recentadvances in chirp spread spectrum allow for a computationally efficientalgorithm with the capacity to compute hundreds of thousand of chirpcorrelations every second, improving the resolution of the system. ATP-EMTPsimulations of the method on large transmission networks demonstrate sub-$\mu$stiming accuracy even in the presence of low SNR and impulsive noise. An FPGAbased prototype is developed and experimental testing also demonstratessub-$\mu$s accuracy for time dissemination over a distance of 700 m. Averagingover time allows operation down to -20 dB, which could extend the range of thesystem to a national scale.

A GPS Alternative using Electrical Transmission Grids as Precision Timing Networks