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    Re: What time is it, really?
    From: Greg R_
    Date: 2008 Jul 17, 12:09 -0700

    --- Bill  wrote:
    
    > What time is it, really?
    
    I believe the musical group Chicago answered that question back in the
    late '60s... ;-)
    
    > A while ago there was a thread on time and the affect of dropping
    > leap seconds on cel nav.
    
    Don't think I was on the list for that thread, but as I understand it
    leap seconds are added to UTC as needed to keep it within 0.9 seconds
    of astronomical time.
    
    The rule that I remember from back when I was first learning celnav was
    that your observation time had to be accurate within 4 seconds,
    otherwise your LOP could be off by up to 1 NM just from that error
    alone (I interpret that to mean +/- 2 seconds). So I would say that
    unless you need exceptional accuracy with your celnav sights you're
    probably OK just ignoring the leap seconds.
    
    --
    GregR
    
    
    
    --- Bill  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? 
    >
    > Thanks
    >
    > Bill B.
    >
    >
    >
    >
    > >
    >
    
    
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