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GPS week rollover

NPL scientists encourage UK businesses and users to check devices ahead of GPS timing change.

  • GPS method for counting time due to reach maximum limit and reset, potentially affecting performance
  • Scientists was to check to ensure devices are working correctly

LONDON 3 April 2019 – At 1am BST on 7 April 2019 (midnight 6 April UTC) the Global Positioning System (GPS) method for counting time will reach its maximum limit and will reset, or rollover, potentially affecting the performance of some GPS receivers. Scientists from the National Physical Laboratory (NPL) encourage UK businesses and users to check their GPS devices to ensure they continue to work correctly over the weekend.

GPS is a reference for time, as well as for positioning and navigation. Any GPS receivers not programmed to reset correctly on Sunday may jump to an incorrect date and time, potentially affecting service. While navigation will not be affected, timing will be. Those affected could include financial traders who use GPS to record trades, ports combining GPS location and timing information to load ships, or even the power grid, as GPS is used to synchronise energy networks.

Concern comes as the true number of devices to be affected is unknown, making it hard to predict the impact the change will have at the weekend. If GPS devices have not been programmed to deal with the rollover correctly, it could potentially cause the shutdown of key equipment.

The rollover will happen as GPS has reached the maximum allocation of values it can use to count weeks. An associated number for each week is encoded into the navigation message from GPS satellites, however, this system only allows for 10 bits of information for the week number, which is equivalent to 1024 different values (from 0 – 1023). This means that every 1024 weeks (or 19.6 years) GPS ‘week counting’ has to reset to zero.

The first rollover occurred in August 1999, but devices produced prior to this, or those not designed appropriately for the change since, will experience problems. Manufacturers aim to solve this by providing updates to improve handling of the rollover in firmware (the system that helps a device to function operationally, by controlling elements like the time of day for instance).

Peter Whibberley, senior research scientist, National Physical Laboratory (NPL) said: “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. 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, such as the NPLTime® service delivered over optical fibres, to provide the resilience as well as the accuracy that they need for timekeeping in critical applications.”

Businesses at risk of being affected should contact their equipment supplier to ensure firmware is updated to the latest version. It may be that they need an upgrade from a manufacturer to avoid any problems. Also, where possible, organisations should monitor for any anomalous behaviour from devices on 7 April. You can find the contact details for many GPS manufacturers here.

Other forms of Global Navigation Satellite Systems (GNSS) will not be affected, as they use alternative systems for coding the data for tracking weeks. A modernised form of GPS is also being deployed, which uses 13 bits to encode the week number. With more values available under this newer system, we will not see a third rollover in the foreseeable future. In fact, with 13 bits, a rollover will not occur for 157 years.

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