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GPS users warned of potential April 6 glitch


The U.S. Department of Homeland Security has warned that those who use older GPS devices may find themselves in troubled waters come April 6.

This has to do with how GPS uses timestamps from satellite signals to calculate positions. This timing system, which began on Jan. 6, 1980, uses 10 bits to count GPS week numbers, which can have 1,024 integer values – from zero to 1,023, in this case. So every 1,024 weeks, which is about 20 years, the counter rolls over from 1,023 to zero. This rollover will happen April 6. It’s the second time this rollover will happen, the first being in 1999.

Basically, a GPS device converts GPS time to UTC (Coordinated Universal Time) using multiple parameters, including week numbers. Devices with a poorly implemented GPS time-to-UTC conversion algorithm may provide incorrect UTC following a week-number rollover, which means attempts to calculate position could fail and navigation data could be corrupted, according to DHS.

Older devices will need an upgrade or patch from the vendor in order to work properly after the rollover. According to DHS, newer devices – that is, “receivers that follow the ICD-200/IS-GPS-200 specification” – should be programmed to accommodate the rollover without adverse effects.

However, the department also notes that some of those devices are set to a parameter other than the original January 1980 date, so even though those devices may not be affected by the April 6 rollover, they may be affected by a similar rollover at a different date, for example, 1,024 weeks after the firmware’s creation date.

DHS recommends that owners and operators contact the manufacturers of their GPS devices to make sure they have the proper updates installed and are ready for the rollover.

Going forward, the new CNAV and MNAV message formats will use a 13-bit week number that will, for all practical purposes, solve the problem.

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