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MeriPlat clock/satellite time

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Interesting observation: I am currently in Michigan to do a little caching and visiting (notice which one came FIRST?) and when I power up my MeriPlat, the clock still reads Arizona time (3 hours behind). I would think that the unit would realize where in the country it was, and adjust the time automatically, but it would appear that I need to go into the menu and set it for the correct zone. Do GPS units all receive the same time signal across the country (say, for Central time) regardless of were the unit is at?

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YES. All GPSr units receive UTC (Zulu time, GMT, among other names). You have to tell it the offset from UTC by setting the local time, and from that point on, the time displayed will be calculated as an offset from the received UTC time. While it would be pretty cool if the unit knew which time zone it was in, most time zone borders aren't straight from north to south. Most newer units (MeriPlat included) can automatically adjust for DST, both in the US and Europe, but that's a user configurable setting.




... Two roads diverged in a wood, and I--

I took off through the thorns, chest high, ...

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Originally posted by Prime Suspect:

Just to clarify, UTC and GMT are _not_ interchangeable. They mean different things.


I sit corrected. For my own edumacation, what's the difference?




... Two roads diverged in a wood, and I--

I took off through the thorns, chest high, ...

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If you're not a major time nerd, or doing actual VERY precise calculations with time they have been pretty much interchangeable. But, Prime Suspect is right, they are officially NOT interchangeable and can differ from each other by as much as 0.9 seconds. icon_smile.gif


In basic terms UTC is based upon the vibration of a cesium atom in an atomic clock. It's coordinated with the time we see on the earth by adding or subtracting the occasional second - called the "leap second". GMT is tied into the rotation of the earth and its orbit about the sun.


The best explanation I've found was here:



UTC vs GMT vs vs UT1/UT1/UT2 TDF? What are these acronyms?


The results of an international compromise---though some would say an international attempt to increase confusion---UTC is refered to as "Coordinated Universal Time" (though not as CUT) in English and as "Temps Universel Coordinné" (though not as TUC) in French. (No particular information exists to explain why UTC was chosen over the equally nonsensical TCU, according to Ulysses T. Clockmeister, one of the diplomats that helped establish the international compromise.)


Universal Time UT0 is solar time, UT1 is solar time corrected for a wobble in the Earth's orbit, and UT2 is UT1 corrected for seasonal rotational variations in rotation due to the Earth's solar orbit.


GMT---Greenwich Mean Time---is UT1. GMT is the time at the classic site of the since-disbanded Royal Greenwich Observatory; at the most widely-known tourist attraction of Greenwich, England.


UTC is based on an average across multiple atomic clocks, and is kept within 0.9 seconds of GMT, through the insertion (or removal) of seconds. In other words, UTC matches GMT plus or minus up to 0.9 seconds, but UTC is not GMT.


TDF is the Timezone Differential Factor, the interval of time between the local time and UTC. Areas that celebrate daylight savings time (DST) will see periodic changes to the TDF value, when the switch-over between daylight savings time and standard time occurs. The switch-over itself is entirely left to local governmental folks, and can and has varied by political entity and politics, and the switch-over has varied over the years even at the same location.

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I find all this extremely interesting and just shake my head at the technology that utilize Atomic clocks and depend on its micro-accuracy as does the GPS. In the grand scheme of things 0.9 seconds is probably considered a very large amount of 'time' and a correcting 'leap second' even more so. I'm curious what these leap seconds will do to someone that is navigating by the GPS at the moment it occurres. No big deal for a geocacher I'm sure but what about a pilot as an example? Is it just a quick blip on the system and 1 second later he is back in business or is a course correction actually required?




"You are only young once but you can stay immature forever"

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And after all those different times, nobody mentioned GPS Time, which is the time all GPS receivers base time on.


GPS time equaled UTC time at Midnight Jan 5/6, 1980 and hasn't been changed since. The leap second difference is part of the Nav message, which allows a GPS to determine UTC time and then whatever time zone the user wants to work in. Since Dec 31, 1998 the leap second difference is 13 seconds.


"Time" and "Timing" are two entirely different things to a GPS (receiver) and all the leap seconds do is correct to UTC time and do nothing as far as position determination is concerned.


Cheers, Kerry.


I never get lost icon_smile.gif everybody keeps telling me where to go icon_wink.gif

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