Dependancy of power grids to satellite-derived time: vulnerabilities and new protections

Power grids depend more and more on satellite-derived time and use signals from Global Navigation Satellite Systems (GNSSs) to be more efficient. GNSS signals serve to reference measurements to an absolute time scale, allowing for a better accuracy of the power grid state estimate. Phase Measurement Units (PMUs) take synchronous snapshots of current and voltage amplitudes/phases across the grid, returning a complete picture of the state of the power system at any time instant. Unfortunately, using conventional receiving hardware, GNSS signals transmitted by the satellites are well below the electromagnetic noise floor and are susceptible to interfering signals. Among all types of interference, intentional radio attacks (spoofing attacks) with the intent of disrupting GNSS services are becoming a significant concern, although no real incidents of this type have been publicly reported yet for power grids. This paper focuses on the threat posed by GNSS spoofing to PMUs. First, the paper critically discusses the vulnerability of some GNSS receivers, representative of those currently embedded in commercial PMUs. The relevance of the work consists in the methodology used to conduct the investigation. In fact, whereas spoofing effects on the computed positions have been reported in many papers, far less attention has been devoted to the computed time information and related applications. The paper intends to increase awareness, proving another fundamental aspect: the counterfeit signals leave clues in the receivers' observables, thus giving the chance of transforming a feared vulnerability into a potential resilience, if these clues are detected and smartly exploited. Finally, the paper introduces the concept of authenticated civilian GNSS signals, namely the Galileo Open Service Navigation Message Authentication (OS NMA), which will be broadcast for signal experimentation starting from 2019.