We are dangerously dependent on GPS to tell time

The writer is a scientific commentator

On October 17, air traffic controllers at Dallas-Fort Worth Airport in Texas received an alarming message. They were warned that GPS interference made navigation unreliable within 40 nautical miles of the airport. The incident, which closed the runway, echoed a similar GPS failure at Denver International Airport in January. According to one report, the collision in the skies of Colorado was only averted at the last minute.

Aviation is just one of the sectors that relies on GPS, or more precisely GNSS – the Global Navigation Satellite System, an umbrella term for all satellite navigation constellations, including GPS and Galileo – to determine time and location. Other sectors, including energy, telecommunications and broadcasting, also depend on GNSS for timekeeping and could be thrown into chaos if signals from space are disrupted. As a result, the National Physical Laboratory, the UK’s official timekeeper, is now developing a time signaling system that will bypass satellites, making it more robust against accidental or deliberate interference.

While most of us rarely think about the role that precise time plays in our daily lives – except perhaps when clocks are set forward one hour to mark the change of seasons, as will happen in much of Europe this coming weekend – the efforts of the NPL by protecting the integrity of the time signals proves how important this invisible utility is.

The world standard for civil time is Coordinated Universal Time, or UTC. It is derived from data collected from atomic clocks stored in more than 70 time measurement laboratories around the world. The data is averaged monthly and published in the newsletter. Leap seconds are added from time to time to ensure that the time measured by atomic clocks does not deviate from the Earth’s rotation. GNSS satellites carrying atomic clocks are synchronized to UTC.

“Virtually all digital infrastructure relies on GPS or GNSS in some way in terms of timing,” says Leon Lobo, who heads NPL’s National Time Scale Center and directs the Resilient Enhanced Time Scale Infrastructure project. But this reliance exposes the infrastructure to a known vulnerability: signals from space are relatively weak and can be blocked or jammed by electronic interference.

Benign interference can come from many sources: planned military exercises involving electronic equipment, solar storms, and even some built-up urban areas. Problems can also have a malignant origin. Russia is suspected of jamming GPS signals in Ukraine, which interferes with navigation and mapping. “Spoofing” is another risk, particularly in shipping – this occurs when fake signals are transmitted to cancel out genuine GNSS signals, potentially luring vessels into locations where they can be intercepted.

Whether it’s intentional or not, unreliable signals are bad news. They can ground flights, as in Denver and Texas, disrupt emergency services and knock out power grids. Balancing electrical loads to avoid outages requires knowing the currents at different network nodes with precise timing. “Different parts of the network must be synchronized, and this is done by a monitoring system that receives time signals from GPS,” Lobo explains. “It is very important to be able to protect such infrastructure [by providing] satellite-independent time signals’.

One plan is to set up multiple secure sites across the UK linked by a fiber optic network, which would then disperse terrestrial time signals across the country. NPL, which transmits the national time signal from a radio transmitter in Cumbria (previously broadcast from Rugby), already provides a dedicated fiber optic link to the City of London, allowing market transactions to be timestamped to the microsecond (accuracy enabling forensic analysis of unusual transactions) . Another option is to use communications satellites instead of GNSS. The key, Lobo says, is to create a ground-based synchronization network with different operating modes – and therefore different failure modes – so that there is always a backup.

The US Federal Aviation Administration, which has yet to say what caused the incidents in Denver and Texas, wants to phase out GNSS by 2025, a timeline that Lobo describes as “incredibly aggressive.” The UK Government’s 2018 Blackett Review found that the loss of GNSS services would cost the domestic economy around £1bn a day – and recommended that it be added to the National Risk Register.

This was, of course, before the pandemic. Since then, our digital infrastructure has become even more important. Times have changed—and the way time is certified must change, too.