GPS satellite

GPS ‘rollover’ could confuse location-tracking devices

Scientists based at the UK’s National Physical Laboratory have warned that the ‘rollover’ of the GPS method for counting time could affect devices with GPS receivers.

As well as allowing for location tracking and navigation across the Earth’s surface, GPS is also used as an extremely reliable clock. Being able to measure time with great precision is necessary for GPS satellites and receivers, as errors of just nanoseconds can result in metres of error in location estimation.

GPS satellites count time with atomic clock. GPS time is reset every 19.6 years when the system reaches the maximum number of values it can use to count ‘week number’ and is reset to zero. Week number is recorded using 10 binary digits, of which there are only 1,024 different combinations, requiring a reset every 1,024 weeks (not quite 20 years).

The first GPS ‘rollover’ took place in August 1999. The second rollover is due at 01.00 BST on 7 April 2019.

The National Physical Laboratory (NPL) has warned that devices which incorporate GPS receivers which were manufactured before the 1999 rollover, or which are not designed to acknowledge the GPS time reset, will experience some problems. The number of devices that could be affected is unknown.

According to NPL, UK businesses and users should check their GPS-enabled devices after the rollover to ensure that they continue working correctly over the weekend. Devices that use Russia’s own satellite navigation system, Glonass, will not be affected.

The GPS receivers affected may jump to an incorrect date and time. While this will not affect location tracking, other services may be affected. These vary from timing information used to load ships at ports, the synchronisation of energy networks on the power grid, or the recording of financial transactions.

Device manufacturers hope to minimise the impact of the rollover on unprepared devices by providing updates to improve handling of the rollover in firmware.

“The effect of the rollover is truly unpredictable. When GPS was first created, the processing power of computers was not what it is today and limiting the week number to just 10 bits helped to keep down the amount of data that had to be transmitted,” said Peter Whibberley, a senior research scientist at NPL. “Now we rely on precise timing in more and more applications and, naturally, we need reliability to match.”

“GPS can be susceptible to jamming, spoofing, and even solar storms, and many organisations have turned to alternative or supplementary methods […] to provide the resilience as well as the accuracy that they need for timekeeping in critical applications.”

In the future, GPS is expected to use 13 bits rather than 10 bits. This would significantly extend the amount of time between rollovers, such that the next rollover would not occur for another 157 years.

Meanwhile, a UK man is four days into a project which involves drawing a giant outline of his body by walking more than 300km within the M25 orbital motorway around London and mapping his route using GPS. So far, the artist has completed half of his head, his right arm and half of his right leg.

In February, E&T spoke to the creators of GPS as they were awarded the 2019 Queen Elizabeth Prize for Engineering.

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