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    Re: GPS as a time authority
    From: Douglas Denny
    Date: 2009 Sep 16, 16:58 -0700

    I am surprised that the software involved in these GPS receivers is as bad as 
    is reported.  It is a terrible inditement of the technology that having a 
    signal available accurate to within a fraction of a microsecond they cannot 
    display the correct time to within eight seconds!
    The GPS signal itself is unbelievably accurate, and contains within it the 
    full information necessary to indicate exactly which rise-time of the second 
    square-wave pulse is a particular second in UTC.
    It should be possible to indicate on a display at least to the nearest second 
    allowing for an update of the display screen and an allowance for the 
    processing time.
    My first-generation GPS Trimble Transpak II is one second behind UTC for a 
    screen update. The updates can be variable depending on the processing; the 
    indicated time jumping by two or three seconds sometimes instead of a 
    continueous count, but the update is within one second behind UTC.
    It being produced pre-2000 though, it does not contain in the software the 
    ability to distinguish the correct year! It indicates presently 31 Jan 09 - 
    but the 24 hour time is fine to within one second.
    Another anomaly which is software dependent with some GPS receivers is the 
    ability to display the velocity of the observer:  some devices only start to 
    indicate if more than a certain velocity is achieved, say five MPH.  The 
    Trimble displays what it finds down to fractions of an MPH or Knot or 
    Metres/sec as set for display.
    Douglas Denny.
    Chichester.  England.
    Original Post:-
    I own among other an older Magellan GPS 300.
    Once I checked my radio clock against it and found a difference of
    several seconds (!). Even more disturbing was, that the difference was
    gaining visually observable.
    Finally I realized, that whenever the time display page of the unit was
    opened, the time was in accordance with the the radio clock.
    My conclusion is, there must be a very odd internal mechanism to display
    the time (software counter or similar), that is started with the
    original reading of the GPS signal every time one goes to this page.
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