TIME
TIME
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The development of highly accurate cesium-beam atomic clocks led to the redefinition of the second in 1967. This led to the recognition by scientists and technologists of the inadequacy of measuring time based on the erratic motion of the earth whose rate fluctuates by a few thousandths of a second a day. Attempts to couple GMT, based on the earth's motion, and the new definition of the second was highly unsatisfactory. A compromise time scale, Coordinated Universal Time (UTC), was devised and became effective on January 1, 1972.
UTC normally runs at the rate of cesium-beam atomic clocks. When the difference between this atomic time and one based on the Earth’s rotation approaches one second, a one-second adjustment (a "leap second") is made in UTC, thereby maintaining synchronization between arbitrary date-time and the Earth’s journey about its axis and the sun. Hence, the behavior of the Earth retains primacy for date-time, while actual elasped-time (laboratory time) is demarcated by the hyperfine atomic clock.
National Institute of Standards and Time (NIST) maintains cesium-beam clock systems. This data is coordinated with data from other atomic clocks located in more than 25 countries contribute data to the international UTC scale which is coordinated in Paris by the International Bureau of Weights and Measures (BIPM). The evolution in time measurement responsibility from observatories to the standards laboratories has accompanied the change from earth-time to atomic-time.
UTC normally runs at the rate of cesium-beam atomic clocks. When the difference between this atomic time and one based on the Earth’s rotation approaches one second, a one-second adjustment (a "leap second") is made in UTC, thereby maintaining synchronization between arbitrary date-time and the Earth’s journey about its axis and the sun. Hence, the behavior of the Earth retains primacy for date-time, while actual elasped-time (laboratory time) is demarcated by the hyperfine atomic clock.
National Institute of Standards and Time (NIST) maintains cesium-beam clock systems. This data is coordinated with data from other atomic clocks located in more than 25 countries contribute data to the international UTC scale which is coordinated in Paris by the International Bureau of Weights and Measures (BIPM). The evolution in time measurement responsibility from observatories to the standards laboratories has accompanied the change from earth-time to atomic-time.
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If you want to know how timekeeping has changed over time, follow this link http://physics.nist.gov/GenInt/Time/time.html
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But the main thing to remember is that GMT (or UTC) is not the same as GPS time. When the satellites were launched, there was no way of predicting how time would change with all the little fixes that have been done to GMT to adjust for the changes of earth rotation speed. The difference is in the order of 2.5 seconds (I gather).
The FMS in my jet had a frig factor to convert the time signal to GMT, and actually calls it UTC (bl**dy frogs!)
The FMS in my jet had a frig factor to convert the time signal to GMT, and actually calls it UTC (bl**dy frogs!)
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Dan,
Actually, GPS time is currently 13 seconds ahead of UTC. This time offset only changes when UTC id adjusted with a leap second, as it last was on 31 December, 1998. UTC is always within one second of GMT. That is not to say that the time displayed on your GPS receiver is 13 seconds ahead of UTC. Most, it not all, current GPS receivers will correct to and display UTC. The GPS to UTC correction is included in all the other orbit and time information in the GPS broadcast message, so your receiver "knows" how much to correct.
Actually, GPS time is currently 13 seconds ahead of UTC. This time offset only changes when UTC id adjusted with a leap second, as it last was on 31 December, 1998. UTC is always within one second of GMT. That is not to say that the time displayed on your GPS receiver is 13 seconds ahead of UTC. Most, it not all, current GPS receivers will correct to and display UTC. The GPS to UTC correction is included in all the other orbit and time information in the GPS broadcast message, so your receiver "knows" how much to correct.
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Which means UTC is the basis for all nav/atc systems we use.
And before you gentlemen start bashing the french,would you care to remember that one Mrs Thatcher of yours decided to do away with the clock at Greenwhich for -I'm sure-very important costs reasons,and as far as I know,every country in the world has approved the international system of measures which was defined with the participation of a lot of scientists
Lastly,for the few die-hard british system fanatics around,the foot,the pound...etc are defined as fractions of the appropriate metric unit.
In aviation,we keep the nautical mile as the most relevant unit,using a sexagesimal division of time and angles,and the foot as more useful for altitude,but the day they will have accurate enough alt measurement (I will be long retired by then)and they could separate aircraft by 300 ft,meter will be the better unit.
Cheers;
And before you gentlemen start bashing the french,would you care to remember that one Mrs Thatcher of yours decided to do away with the clock at Greenwhich for -I'm sure-very important costs reasons,and as far as I know,every country in the world has approved the international system of measures which was defined with the participation of a lot of scientists
Lastly,for the few die-hard british system fanatics around,the foot,the pound...etc are defined as fractions of the appropriate metric unit.
In aviation,we keep the nautical mile as the most relevant unit,using a sexagesimal division of time and angles,and the foot as more useful for altitude,but the day they will have accurate enough alt measurement (I will be long retired by then)and they could separate aircraft by 300 ft,meter will be the better unit.
Cheers;
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The difference between the three times isn't so much a matter of accuracy as of definition. The UTC clocks stop for one second every year or two to keep them roughly in sync with earth time (i.e. with what used to be called mean solar time), but I gather the GPS clocks don't stop.
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And to think Einstein spent all that time and effort proving that all time was relative!
More to the point time, according to general relativity, is dependent on the gravitational field, so those of you flying at 33,000 feet experience a different rate of time as observed by those people on the ground whether they be in Greenwich or France.
Does anyone remember that in the 70s they conducted an experiment with two atomic clocks? They were synchorised and one was kept on the ground while the other was put on a supersonnic jet (Blackbird?) and flown round the world. When it landed the the two clocks were out of synch.
Now I can't remember whether the clock in the plane gained or lost time but I don't think it is much to worry about even if you do spend large amounts of your 'time' in the air.
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JJC
More to the point time, according to general relativity, is dependent on the gravitational field, so those of you flying at 33,000 feet experience a different rate of time as observed by those people on the ground whether they be in Greenwich or France.
Does anyone remember that in the 70s they conducted an experiment with two atomic clocks? They were synchorised and one was kept on the ground while the other was put on a supersonnic jet (Blackbird?) and flown round the world. When it landed the the two clocks were out of synch.
Now I can't remember whether the clock in the plane gained or lost time but I don't think it is much to worry about even if you do spend large amounts of your 'time' in the air.
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JJC
