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Guivre46
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Joined: Apr 14, 2010
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Location: West London
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 Posted: Tue Apr 05, 2011 12:13 am Post subject: Optical Lattice Clocks - Improved GPS Accuracy |
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Super Accurate Clock
Not sure that this would have a positional impact on satnavs for road navigation, as they are 'blunt instruments', but the possibility for improved height recognition might be useful for 'double-decker' roads.
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Mike R [aka Wyvern46]
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Last edited by Guivre46 on Sun May 15, 2011 11:45 am; edited 1 time in total |
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Skippy
Pocket GPS Verifier
Joined: 24/06/2003 00:22:12
Posts: 2946
Location: Escaped to the Antipodies! 36.83°S 174.75°E
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| Posted: Tue Apr 05, 2011 8:52 am Post subject: Re: Optical Lattice Clocks - Improved GPS Accuracy |
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| Guivre46 wrote: | | the possibility for improved height recognition might be useful for 'double-decker' roads. |
Not to mention aircraft landing. 
Though International Atomic Time is 24 seconds adrift from the time we use, the article is not correct in saying that UTC and GMT differ.
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Andy_P
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Joined: Jun 04, 2005
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Location: West and Southwest London
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| Posted: Thu Apr 07, 2011 2:04 pm Post subject: Re: Optical Lattice Clocks - Improved GPS Accuracy |
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| Skippy wrote: | | [the article is not correct in saying that UTC and GMT differ. |
Yes they do....(but only a bit  )
| Quote: | | An atomic clock is highly accurate, however atomic time is slightly faster than earth time. As a result, atomic time will gradually drift out of synchronisation with GMT. UTC thus uses atomic clocks to measure the rate of time and adjusts the absolute time when necessary to keep UTC within 0.9 seconds of GMT. This adjustment is known as a "leap second". |
(http://www.horology-stuff.com/time/standards.html |
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Skippy
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Joined: 24/06/2003 00:22:12
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Location: Escaped to the Antipodies! 36.83°S 174.75°E
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| Posted: Fri Apr 08, 2011 10:23 pm Post subject: Re: Optical Lattice Clocks - Improved GPS Accuracy |
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| Andy_P wrote: | | Skippy wrote: | | [the article is not correct in saying that UTC and GMT differ. |
Yes they do....(but only a bit )
| Quote: | | An atomic clock is highly accurate, however atomic time is slightly faster than earth time. As a result, atomic time will gradually drift out of synchronisation with GMT. UTC thus uses atomic clocks to measure the rate of time and adjusts the absolute time when necessary to keep UTC within 0.9 seconds of GMT. This adjustment is known as a "leap second". |
(http://www.horology-stuff.com/time/standards.html |
OK, this is geeky stuff but:
UTC is based on the time kept by an atomic clock.
UT1 is based on the actual rotation of the earth (which varies from one year to the next) and leap seconds are added to UTC to keep it within 0.9 seconds of UT1.
That's all well and good, but here's the question:
Does GMT = UTC or does GMT = UT1?
I would argue that for all intents and purposes, when people speak of GMT they are talking about UTC, therefore GMT = UTC. A lot of sat navs re-enforce this confusion by reporting the time as GMT (perhaps with an offset for your current timezone) but they are actually reporting UTC. GMT has become ambiguous but effectively GMT is now exactly = to UTC.
To argue that GMT differs from UTC (even by less than a second), you would have to show me a timesource that reports GMT as UT1 and I don't think there are any... Certainly in the scientific community (where they would care are about the UT1 time) they wouldn't use the abbreviation GMT because it would cause confusion. They would say UT1 or UTC.
So is the < 0.9 seconds really important between friends? Absolutely! I have worked with a number of systems over the years where times needed to be synchronised within a few milliseconds of UTC though we still talked about the time in GMT (being right across the river from Greenwich where time was invented, old habits die hard!).
I guess the confusion arises because historically, GMT was indeed based on solar observations (as UT1 is) but today everyone who refers to GMT name is talking about UTC, not UT1. Indeed, I would be surprised if anyone beyond the scientific and astronomical communities even knows or cares what the current UT1 time is. Anyone here know what the current UT1 to UTC offset is, roughly?
Or is all that just too geeky. I think I need a beer. 
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Guivre46
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M8TJT
The Other Tired Old Man
Joined: Apr 04, 2006
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Location: Bexhill, South Sussex, UK
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| Posted: Fri Apr 08, 2011 11:22 pm Post subject: |
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We Brits invented GMT so there. All you other UTC's UT1's Whatevers, go shove it.  You'll be trying to take the Grenwich Meridian off us next. Oops, you already have  |
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