The time "0" which first GPS timing started is midnight on January 6th, 1980 (UTC time): in that year, the GPS time coincided exactly with UTC time, as a result of corrective measures to UTC time, today it differs by 18 (it was 17 until December 31st 2016 and 16 seconds until June 30th 2015), seconds compared to GPS time, which as written above, is not subject to any astronomical adjustment. The GPS time is based on atomic clocks and has no correction with respect to proper time variations of the Earth's rotation: the only corrections made on atomic clocks located on satellites are very small adjustments to ensure that they remain perfectly synchronized with atomic clocks installed on the Earth (usually to correct drifts due to relativistic effects). Sometimes the UTC time is also called "Zulu time", simply for the fact that time is briefly referred to as "Z time" in naval-air navigation and the letter Z is articulated as "Zulu" in the NATO phonetic alphabet. This second is called "leap second" and its calculation starts from 1972. It is typically taken off one second every 36 months, although of course this is not a certain and scientific rule. With this change in duration of minutes, you do so in order to keep the delay between the UTC and the "earth time" within 0.9 seconds: when needed (typically on June 30th or December 31st), on the recommendation of the International Earth Rotation and Reference Systems Service (IERS, which deals, among other things, the measure of Earth's rotation), is considered a minute with duration of 59 or 61 seconds. On last June 30th 2015 a minute with 61 seconds has been considered: this variation depends on the time of rotation of the earth around the sun, which, due to fluctuations in its velocity, is not constant. Simplifying, the UTC time divides time into days, hours, minutes and seconds, but while one day is always of 24 hours and one hour always of 60 minutes, a minute, while almost always consists of 60 seconds, sometimes it can be a period of 59 or 61 seconds. It is similar to Greenwich Mean Time (GMT), but unlike this time, it is not based on the time of rotation of the Earth, but on measurements made by atomic clocks that are most accurate and above all more stable over time. UTC time - Coordinate Universal TimeĬoordinated Universal Time is the time zone "0" from which all other time zones around the world are then calculated. There are many standard definitions of the time, but the main four are: UTC, GPS, LORAN and TAI. Typically this time is maintained by specific atomic clocks and has a satellite distribution. See the " Current number of leap seconds" section for the number of leap seconds inserted to date.A lot of the current technology to work properly requires a very precise definition of the time. Leap seconds are inserted as necessary to keep UTC within 0.9 seconds of the UT1 variant of universal time. The current version of UTC is defined by International Telecommunication Union Recommendation (ITU-R TF.460-6), Standard-frequency and time-signal emissions, and is based on International Atomic Time (TAI) with leap seconds added at irregular intervals to compensate for the accumulated difference between TAI and time measured by Earth's rotation. A decision whether to remove them altogether has been deferred until 2023. This CCIR Recommendation 460 "stated that (a) carrier frequencies and time intervals should be maintained constant and should correspond to the definition of the SI second (b) step adjustments, when necessary, should be exactly 1 s to maintain approximate agreement with Universal Time (UT) and (c) standard signals should contain information on the difference between UTC and UT." Ī number of proposals have been made to replace UTC with a new system that would eliminate leap seconds. This change also adopted leap seconds to simplify future adjustments. The system has been adjusted several times, including a brief period during which the time-coordination radio signals broadcast both UTC and "Stepped Atomic Time (SAT)" before a new UTC was adopted in 1970 and implemented in 1972. UTC was first officially adopted as CCIR Recommendation 374, Standard-Frequency and Time-Signal Emissions, in 1963, but the official abbreviation of UTC and the official English name of Coordinated Universal Time (along with the French equivalent) were not adopted until 1967. The coordination of time and frequency transmissions around the world began on 1 January 1960.
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