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    Re: Time and cel nav, a stupid question
    From: Fred Hebard
    Date: 2008 Apr 22, 18:26 -0400

    On a mac, using time.apple.com.  Just synced, and still about 50
    seconds slow.
    
    On Apr 22, 2008, at 6:12 PM, Lu Abel wrote:
    
    >
    > 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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