Dates for the GPS constellation start at midnight on January 5th, 1980, so the first rollover occurred on August 21, 1999. The original specification for GPS had dates stored by week in a 10-bit field (2^10 or 2x2x2x2x2x2x2x2x2x2) which is 1,024 weeks. The underlying problem stems from the way GPS units store dates. Unfortunately, the bulletins also warn that not all implementations of the new specification work properly and some GPS units might be prone to trouble either at the 4/6/19 rollover or later. Both bulletins make reference to a new GPS spec (IS-GPS-200) that deals with these rollovers gracefully. The Department of Homeland Security has published a bulletin on the upcoming rollover, as has the Coast Guard. The rollover last happened in 1999, but in the last 20 years the use and dependence on GPS has increased dramatically. It turns out the way dates are stored in older GPS units has a rollover event every roughly 20 years and we are about to experience the second one since the GPS constellations came online. Those problems could include not being able to receive location data. On April 6th, 2019 older GPS units may experience a range of problems because of a limitation in how they handle dates. GPS date rollover affects older GPS units
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