The Global Positioning System Week Number Rollover is a phenomenon that happens every 1024 weeks, which is 19.7 years. The Global Positioning system broadcasts a date, including a weekly counter which is stored in only ten binary digits. The range is therefore 0–1023. After 1023 the internal value "rolls over", changing to zero again. Software which 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 August 21 to August 22, 1999, when GPS Week 1023 advanced and rolled over to 0 within the counter. The second rollover occurred on the night of April 6 to April 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.
Effects
Due to relatively limited commercial and consumer use of GPS during the 1999 rollover, disruption was minor. 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 CityWireless Network crashed. 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. iPhone 4s, iPad mini with cellular, iPad 2 with CDMA cellular or iPad with cellular require software updates with at least iOS 9.3.6. iPhone 5 and iPad with cellular require software updates with at least iOS 10.3.4. If these devices do not have these software updates before November 3, 2019, the devices will lose over-the-air software updates and iCloud Backup.
