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    Re: Time and cel nav, a stupid question
    From: Lu Abel
    Date: 2008 Apr 22, 15:12 -0700

    Fred:
    
    I'm not an expert on cell phone technologies, but it may be that AT&T is
    the one provider that may not need accurate timekeeping to make its
    particular technology work.
    
    When you say you're on a network time server, are you on a Windows, Mac
    or Unix machine?   Windows only resynchronizes the computer's
    time-of-day clock with an NTP server once a week (or even less
    frequently);  I've seen at least one of my computers drift considerably
    in between.   I just checked my new Vista PC computer's clock against
    time.gov, it's three seconds slow with a resynchronization scheduled for
    Friday.   But it could just as easily be fast, as yours might be.   I
    personally have a lot of faith in time.gov (especially with a broadband
    connection) because it attempts to measure and compensate for any
    network delay.
    
    Wait, I forgot that I also have one of those digital clocks that
    synchronizes itself to the low-frequency time signal broadcast by WWVB.
    It appears to be between 0.3 and 0.5 seconds slow as compared to the
    www.time.gov display on my computer.   On the other hand, considering
    that it cost me all of $6, I have to wonder whether its time display is
    more or less accurate.
    
    Lu
    
    Fred Hebard wrote:
    > I followed up on this when it was mentioned previously on this list,
    > and I believe the accuracy of cell phones may vary with the network.
    > I'm on AT&T, and my impression is that the time is not accurate.  For
    > instance, my phone now says it's 9:51:55 and my computer says it's
    > 12:52:42.  The computer is getting its time from a network time
    > server.  These may be more accurate than the USNO site, but, I don't
    > know this one way or another.
    >
    > Fred Hebard
    >
    > On Apr 22, 2008, at 11:56 AM, Lu Abel wrote:
    >
    >
    >> Bill:
    >>
    >> The time difference between your Garmin and the USNO web site is
    >> likely
    >> due to network delays (ie, the USNO sends you a time pulse at the
    >> correct time, but it takes almost a second to arrive at your
    >> computer).   The other web site run by USNO, www.time.gov, says it
    >> attempts to measure network delays and compensate for them.   Another
    >> source for accurate time is your cell phone.  Mine only keeps time to
    >> the minute, but it very accurately rolls over at the 60-second mark.
    >>
    >> Lu Abel
    >>
    >> Bill wrote:
    >>
    >>> The current discussion on time, relativity and leap seconds is, in
    >>> many
    >>> areas, well above my head.  Noting the various flavors of UT
    >>> (currently), an
    >>> observation and a pragmatic question for cel nav.
    >>>
    >>> The seconds readout between my GPS (Garmin GPS 76) and
    >>> http://tycho.usno.navy.mil/cgi-bin/anim
    >>> differ by about one (1) second.
    >>>
    >>> The web site (dial up modem at the moment) is nominally one (1)
    >>> second
    >>> behind the GPS readout.
    >>>
    >>> Bringing it into the realm of traditional navigation, which time
    >>> do I use to
    >>> set my hack watch? ;-)  Potentially a quarter mile intercept
    >>> difference
    >>> under ideal circumstances.
    >>>
    >>> I do not have access to a time-signal radio now.
    >>>
    >>> Bill
    >>>
    >>>
    >>>
    >
    >
    >
    > >
    >
    
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