NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: What time is it, really?
From: Geoffrey Kolbe
Date: 2008 Jul 17, 15:03 +0100
From: Geoffrey Kolbe
Date: 2008 Jul 17, 15:03 +0100
Radio clocks, which pick up very long wave signals from terrestrial
transmitters containing an encoded time signal, are never usually rated
at much better than one second of time - if you read the fine
print.
If you really want accurate time, by which I mean better than a millisecond, a specific GPS clock device (rather than the clock on a GPS device) is the way to go. Such GPS clocks automatically account for the signal delay time from the satellites, including a model for refraction of the signal through the earth's atmosphere and the finite refractive index (which slows the microwave signal) in the atmosphere. Such clocks can be accurate to about 50 nanoseconds, which is the length of time it takes light to travel 50 feet....! Cost will be about $150
Geoffrey Kolbe
At 09:59 17/07/2008, you wrote:
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If you really want accurate time, by which I mean better than a millisecond, a specific GPS clock device (rather than the clock on a GPS device) is the way to go. Such GPS clocks automatically account for the signal delay time from the satellites, including a model for refraction of the signal through the earth's atmosphere and the finite refractive index (which slows the microwave signal) in the atmosphere. Such clocks can be accurate to about 50 nanoseconds, which is the length of time it takes light to travel 50 feet....! Cost will be about $150
Geoffrey Kolbe
At 09:59 17/07/2008, you wrote:
A while ago there was a thread on time and the affect of dropping leap
seconds on cel nav.
I asked a question regarding US Navy computer time lagging behind my GPS
time (a Garmin 76). The answer passed my common-sense test--server/router
lag.
Recently I obtained a clock that polls the Colorado, USA broadcast time
signal. (Radio Shack, so perhaps not the optimal atomic-based timepiece.) My
understanding, the clock should give me UTC (factoring in the time lag of a
broadcast traveling at 186,000 miles per second). It seems to to run
nominally a half second faster than my GPS (within minutes after the clock
has polled the Colorado signal and reset).
If I understand correctly, the Nautical Almanac is based on "UT", which
Dutton's claims is UT1. This may differ from UTC.
Searching the web, the articles on GPS time versus UTC are confusing to me
(perhaps the date of the articles are part of the problem.)
I am informed:
* GPS time will always lag behind UTC.
* GPS time will always be ahead of UTC.
* GPS satellite clocks do factor in leap seconds so can run
19 seconds or more behind UTC.
* GPS time is "steered" to approximate UTC. (Meaning?)
Dutton's also provided two URL's for UTC/UT1 corrections (DUT1) which proved
to be dead ends. I finally found a March posting with a correction factor of
-0.4 seconds, which seems to sync up with my observations.
My questions:
Does anyone know of a URL for up-to-date UTC/UT1 corrections (DUT1)?
How does GPS time relate to UTC or UT1? Is the relationship somewhat
dependent on the make/model/age/firmware of the GPS unit?
Can a person with two watches (clocks) ever know what time it is? <g>
Thanks
Bill B.
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