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Reference.org
GPS week number rollover
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<p>The GPS week number rollover is a phenomenon that happens every 1,024 weeks, which is about 19.6 years. The <a href="/facts/Global_Positioning_System/DKVLnzF3">Global Positioning System</a> (GPS) broadcasts a date, including a <a href="/facts/Week_number/KJjQTKIh">week number</a> counter that is stored in only ten <a href="/facts/Bit/RwfjaYSv">binary digits</a>, whose range is therefore 0–1,023. After 1,023, an <a href="/facts/Integer_overflow/sVNnSolT">integer overflow</a> 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 a multiple of approximately 20 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. </p>