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    Re: Time and cel nav, a stupid question
    From: Fred Hebard
    Date: 2008 Apr 23, 00:18 -0400

    Lu,
    
    Wife's AT&T phone also is slow on time.gov and time.apple.com.
    Interestingly, both phones show the exact same time, best as I can
    determine.  So it's the AT&T network clocks.  I'm out in the
    boondocks (SW Virginia), and they're not competing for service here
    due to old partnerships.  Might explain the greater inattention to
    clocks in SW Virginia (~50 seconds slow) than in San Francisco (3-5
    seconds).
    
    Fired up WWV just for yucks and the computer was correct, as
    indicated already.
    
    Fred
    
    On Apr 22, 2008, at 9:45 PM, Lu Abel wrote:
    
    >
    > Wow!
    >
    > I guess I stand corrected on cellphones telling time.   I live in the
    > San Francisco area and work in high tech.  Just polled a bunch of my
    > friends who have AT&T service; they report that their phones are 3
    > to 5
    > seconds slow as compared to either time.gov or Unix NTP.
    >
    > AT&T now runs GSM (the cell phone scheme used in most of the world
    > except the US), so I wonder about T-Mobil (the other major GSM
    > vendor in
    > the US).
    >
    > At the same time, I wish I knew more about GSM so I could
    > understand how
    > it sends time to phones.  50 seconds slow sounds like a LOT!
    >
    > Lu
    >
    > Fred Hebard wrote:
    >> 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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