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The second rollover occurred on the night of April 6 to 7, 2019, when GPS Week 2,047, represented as 1,023 in the counter, advanced and rolled over to 0 within the counter. [2] The United States Department of Homeland Security, the International Civil Aviation Organization, and others issued a warning about this event.
GPS time is expressed with a resolution of 1.5 seconds as a week number and a time of week count (TOW). [13] Its zero point (week 0, TOW 0) is defined to be 1980-01-06T00:00Z. The TOW count is a value ranging from 0 to 403,199 whose meaning is the number of 1.5 second periods elapsed since the beginning of the GPS week.
GPS time was set to match UTC in 1980, but has since diverged. The lack of corrections means that GPS time remains at a constant offset with International Atomic Time (TAI) (TAI – GPS = 19 seconds). Periodic corrections are performed to the on-board clocks to keep them synchronized with ground clocks. [86]: Section 1.2.2
GPS dates are expressed as a week number and a day-of-week number, with the week number transmitted as a ten-bit value. This means that every 1,024 weeks (about 19.6 years) after Sunday 6 January 1980, (the GPS epoch ), the date resets again to that date; this happened for the first time at 23:59:47 on 21 August 1999, [ 11 ] the second time at ...
This almanac is transmitted repeatedly over 12.5 minutes. Almanac data can be received from any of the GPS satellites and is considered valid for up to 180 days. Warm or normal The receiver has estimates of the current time within 20 seconds, the current position within 100 kilometers, its velocity within 25 m/s, and it has valid almanac data.
The GPS time scale has a nominal difference from atomic time (TAI − GPS time = +19 seconds), [11] so that TT ≈ GPS time + 51.184 seconds. This realization introduces up to a microsecond of additional error, as the GPS signal is not precisely synchronized with TAI, but GPS receiving devices are widely available. [12]
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Though Selective Availability capability still exists, on 19 September 2007 the US Department of Defense announced that newer GPS satellites would not be capable of implementing Selective Availability; [41] the wave of Block IIF satellites launched in 2009, and all subsequent GPS satellites, are stated not to support selective availability.