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    Re: GPS time display unreliable
    From: Jared Sherman
    Date: 2003 Sep 11, 18:19 -0400

    Paul, those threads appear to deal with a bug or idiosynchracy in one model 
    from one source (the Magellan Sportrak) and evn then there is some question 
    about whether the problem just revolves around one firmware level or upgrade.
    The only solid information that I have found is that a properly functioning 
    GPS does not get "all" information instantly. In my literally fevered mind I 
    recall that there is a master string which is only broadcast something like 
    once every 14 minutes that contains data the GPSes need in order to ensure 
    full accuracy. Therefore, any GPS which has just been turned on is 
    suspect--simply because it has not necessarily had time to read the full 
    string unless it has been running for at least 15 minutes. Let's say 1/2 hour 
    in order to ensure the string was read fully and clearly at least once out of 
    two passes.
    IIRC that string contains the latest correction/update information and "how 
    necessary" it is depends on what has changed since the last time the GPS was 
    running, if anything. But in the case of my antique Garmin 12XL, I make a 
    habit to check the displayed GPS time versus my chronometer time, which is 
    usually set to the USNO master source via the internet the night or day 
    before. They both hack--exactly--within the 1 second that I consider to just 
    be "push the button" timing error.
    Given a "known good" GPS and the proper initialization time, I think the 
    accuracy of GPS display time is going to be immaterial as compared to 
    anything less than having an actual master time source with you. After all, 
    it IS obtaining that time display from 16 atomic clocks.

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