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    Re: What time is it, really?
    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:
     

    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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