![]() TT = TAI+32.184s => UT1-UTC = TAI-UTC - (TT-UT1) + 32.First, Google does track your Android device’s location not solely on GPS. Table of time scales 1972-present, and some predictions Leap seconds are added (or removed, but that hasn't happened yet)Īt the end of every year, or every half-year, or every third month, TAI-UTC is always an integral number of seconds, and is varied when Used in the US together with GCT.ĭelta-T varies continuously, depending on the Earth's rotation. The mean solar time at the local meridian. ![]() When UT was adopted, GCT fell out of use. GMT starting at Greenwich mean midnight, to distinguish it from Used in the US from 1925 to mean the "new" Royal Navy Day started 12 hour before local mean solar time, thus The start of the corresponding civil date. Of the night in Europe - a new GMT date then started 12 hours after Greenwich mean noon (12h UT) to avoid a date change in the middle Prior to 1925, GMT was reckoned for astronomical purposes from ![]() Sense of UT (in astronomical navigation, GMT still means UT). Not in astronomy) in the sense of UTC in addition to the earlier It's ambiguous, and is now used (although Some older GPS receivers thenĬeased to show the correct date/time but could still be useful for giving Week number roll-overs, the first on 00:00 GPS time and the One GPS week rollover cycle is therefore 1024 Weeks are numbered fromĠ and up until 1023, then it "rolls back" to 0 and are again numberedįrom 0 and up, etc. GPS time (which back then was equal to UTC). GPS week = a numbering of weeks starting at the GPS epoch 00:00 ![]() Seconds each time a leap second is added to UTC time scale. The difference between GPS Time and UTC changes in increments of (TDT), are constant at the level of some tens of nanoseconds while The differences between GPS Time and International Atomic Time (TAI)Īnd Terrestrial Time (TT), also know as Terrestrial Dynamical Time No leap seconds are inserted into GPS time, thus Defined in 1991 along with TTĭUT = predicted value of delta-UT, rounded to 0.1s, given in some radio Replaced TDT in the ephemerides from 2001 and on. Was defined in 1991 to beĬonsistent with the SI second and the General Theory of Relativity. Originally used instead of TDT or TDB when theĭifference between them didn't matter. (higher order terms neglected g = Earth's mean anomaly) Referred to the barycentre of the solar system. ReplacedĮT (Ephemeris Time) in 1984, was replaced by TT (Terrestial Time) Was used 1984-2000 as a time-scale ofĮphemerides from the Earth's surface. Jan 1 can be regarded as a continuous time-scale. For most purposes, ET up to 1983 Dec 31 and TDT from 1984 Was used 1960-1983, and was replaced by TDT and TDB Seconds to UT1, where t is the fraction of the year (zero at 1 Jan).ĮT = Ephemeris Time. UT2 = UT1 corrected for seasonal variations in the Earth's rotational UT1 = UT0 corrected for polar wandering - usually one means UT1 when Or from more modern methods involving GPS satellites. UT0 = "raw", uncorrected UT as derived from meridian circle observations There are severalĭifferent definitions of UT, but the difference between them is always Defined by the Earth's rotation, formerly determinedīy astronomical observations but today GPS satellites are used To keep the difference between UTC and UT less than 0.9 s. When needed, leap seconds are introduced in UTC TAI-UT1 was approximately 0 on 1958 Jan 1. TAI = International Atomic Time (Temps Atomique International = TAI) isĭefined as the weighted average of the time kept by about 200Ītomic clocks in over 50 national laboratories worldwide. Added a link to Steve Allen's detailed history of the time scales. Thanks to John Stockton for a number of suggestions to this page. Major overhaul: corrected several small errors, updated tables, added new links. Replaced obsolete links with working ones Time Scales: UT1, UTC, TAI, ET, TT, GPS time T I M E S C A L E Sġ993? First version as an ASCII document.Ģ004-09-? First HTML version, for an article in the Swedish ham radio magazine QTC
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