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    Re: Time and cel nav, a stupid question
    From: Lu Abel
    Date: 2008 Apr 22, 18:45 -0700

    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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