GPS week number rollover
The GPS week number rollover is a phenomenon that happens every 1024 weeks, which is about 19.6 years. The Global Positioning System (GPS) broadcasts a date, including a week number counter that is stored in only ten binary digits, whose range is therefore 0–1023. After 1023 an integer overflow causes the internal value to roll over, changing to zero again. Software that is not coded to anticipate the rollover to zero may stop working or could be moved back in time by 20 or 40 years. GPS is not only used for positioning, but also for accurate time. Time is used to accurately synchronize payment operations, broadcasters, and mobile operators.
The first rollover took place midnight (UTC) August 21 to 22, 1999.
NavCen issued an advisory prior to the rollover stating that some devices would not tolerate the rollover. Because of the relatively limited use of GPS during the 1999 rollover, disruption was minor.
The second rollover occurred on the night of April 6 to 7, 2019, when GPS Week 2047, represented as 1023 in the counter, advanced and rolled over to 0 within the counter. The United States Department of Homeland Security, the International Civil Aviation Organization, and others issued a warning about this event.
Products known to have been affected by the 2019 rollover include Honeywell's flight management and navigation software that caused delays for a KLM flight and cancellations for numerous flights in China because the technicians failed to patch the software.
Furthermore, the New York City Wireless Network (NYCWiN) crashed. (This is NYC's private municipal services wifi network not for public use; the public free wifi service is known as LinkNYC and apparently not affected.)
Other products that were affected by the rollover include cellphones that were sold in 2013 or earlier, Australian Bureau of Meteorology's weather balloons, NOAA's weather buoys many scientific instruments, and consumer GPS navigation devices. Prior to return to normal standard time from daylight saving time during the morning of November 3, 2019, Apple issued a warning to owners of iPhone and iPad, which were sold before 2012, that these Apple products could lose internet.
The third rollover will occur between November 20 and 21, 2038.
The above rollovers are due to a ten-bit week number; the more recent CNAV protocol, successor to the original NAV protocol, uses thirteen-bit week numbers, which amounts to a 157-year cycle; therefore, using the same epoch of 1980, the first rollover won't be until 2137.
- Time formatting and storage bugs
- Structure of the time-encoding components of GPS signals, NAV and CNAV versions
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