On October 17, air traffic controllers at Dallas Fort Worth airport in Texas received a worrying advisory. They were warned that GPS interference was making navigation unreliable within 40 nautical miles of the airport. The incident, which closed a runway, echoed a similar GPS failure at Denver International Airport in January. According to one report, a collision in the Colorado skies was averted only at the last minute.
Aviation is just one sector that relies on GPS, or, more accurately, GNSS — the Global Navigation Satellite System, an umbrella term for all satellite navigation constellations, which include GPS and Galileo — to tell time and position. Other sectors, including energy, telecommunications and broadcast, also depend on GNSS for timing, and can be plunged into chaos if signals from space are disrupted. As a result, the National Physical Laboratory, the UK’s official timekeeper, is now designing a time-signalling system that will bypass satellites, making it more robust against accidental or deliberate interference.
While most of us rarely think about the role precision timing plays in everyday life — except, perhaps, when the clocks change by one hour to mark the shift in seasons, as will happen in much of Europe this coming weekend — efforts by the NPL to protect the integrity of time signals affirm how important this invisible utility is.
The global standard for civil timekeeping is coordinated universal time, or UTC. It is derived from data collected from atomic clocks kept in more than 70 timing laboratories worldwide. The data is averaged monthly and published in a bulletin. Leap seconds are occasionally added to ensure the time measured by atomic clocks does not drift out of alignment with the Earth’s rotation. GNSS satellites, which have onboard atomic clocks, are synchronised to UTC.
“Pretty much all digital infrastructure relies in some fashion on GPS or GNSS for time,” says Leon Lobo, who heads the NPL’s National Timing Centre and is leading its Resilient Enhanced Time Scale Infrastructure project. But that dependence exposes infrastructure to a known vulnerability: signals from space are relatively weak and can be swamped or jammed by electronic interference.
Benign interference can come from a host of sources: planned military exercises involving electronic equipment, solar storms and even some built-up urban areas. Problems can also be malign in origin. Russia is suspected of jamming GPS signals in Ukraine, hampering navigation and mapping. “Spoofing” is another risk, particularly in shipping — this happens when counterfeit signals are transmitted in order to override genuine GNSS signals, potentially luring vessels into locations where they can be hijacked.
Whether intentional or not, unreliable signals are bad news all round. They can ground flights, as in Denver and Texas, disrupt emergency services and hit power networks. Balancing electrical loads to avoid outages requires knowing currents at different nodes of the grid with accurate timing. “Different parts of the grid have to be synchronised, and that’s done through a monitoring system that gets its time signals from GPS,” Lobo explains. “It’s really important to be able to protect infrastructure like that [by providing] time signals that are independent of satellites.”
One plan is to build multiple secure sites across the UK linked by an optical fibre network, which then disperses terrestrial time signals across the country. The NPL, which broadcasts a national time signal from a radio transmitter in Cumbria (formerly broadcast from Rugby), already provides a dedicated optical fibre link into the City of London, enabling market transactions to be timestamped to the microsecond (the precision allows forensic analysis of unusual trades). Another option is to use communications satellites rather than GNSS. The key, Lobo says, is to build a terrestrial timing network with different modes of operation — and therefore different modes of failure — so there is always a back-up.
The US Federal Aviation Authority, which is yet to report on what caused the incidents in Denver and Texas, wants to move away from GNSS by 2025, a timeline that Lobo describes as “incredibly aggressive”. The UK government’s 2018 Blackett review revealed that a loss of GNSS services would cost the domestic economy about £1 billion a day — and advised this was added to the National Risk Register.
That was, of course, before the pandemic. Our digital infrastructure has since become even more critical. Times have changed — and the way we certify time must change too.
Anjana Ahuja is a contributing writer on science for the Financial Times and holds a PhD in space physics from Imperial College London.
