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    Differences between GPS and WWV time
    From: Dan Allen
    Date: 2001 Jun 22, 11:23 AM

    I can only speak to Garmin GPS receivers, of which I have had several.
    Garmin GPSes have a very crummy built-in clock.  If you put one on a shelf
    for
    a month or two, you can easily be off by many minutes.  I have often
    wondered if this was the reason it seems that such receivers have a
    hard time acquiring satellites the first time after a long period of
    unuse... but I digress.
    
    As soon as the GPS receiver locks on to several satellites and has
    a 2D or 3D fix, then I have seen the time jump to the correct time.  Well,
    almost.
    
    Internally there is no question that the GPS receivers have the correct time
    once they have a fix, since the entire system is based on the notion of
    absolutely correct time in order to fix your position.  The apparent problem
    lies in the software that displays time.  So sometimes on Garmin GPS 40, GPS
    12XL,
    GPS III, GPS 230, and GPS eTrex units the displayed time can be off by 1-2
    seconds from
    what WWV is broadcasting.  I have checked this on several occasions.
    
    Another empirical result: these delays appear to be minimized once the GPS
    receivers have fully caught up with several tasks:
    
    1) locking on to all visible satellites (this takes many minutes after
    boot-up)
    2) downloading the latest almanac from the satellites (this takes several
    minutes)
    3) displaying the position on a map
    
    Once these tasks are all done then the displayed time is usually within a
    half a
    second of WWV, perhaps even better.
    
    My gripe is that there is no indication of how accurate the time display is
    at any moment.  I've just figured this all out by comparing my GPS to WWV
    many times.
    
    Another digression: a good way of determining correct time is NTP, the
    Network Time Protocol used to sync computer's clocks automatically with
    various atomic clocks around the country.  Mac OS 9.1, Mac OS X, and Windows
    2000 all support this, as well as many varieties of Unix, Linux, and
    FreeBSD.  NTP can be setup automatically to do this every day or even every
    few minutes, and it is quite good.
    
    If you are interested more in NTP, go to
    http://www.eecis.udel.edu/~mills/ntp/
    
    Dan
    
    -----Original Message-----
    From  Navigation Mailing List
    [mailto:NAVIGATION-L{at}LISTSERV.WEBKAHUNA.COM]On Behalf Of Robert Owens
    Sent: Friday, June 22, 2001 9:36 AM
    To: NAVIGATION-L{at}LISTSERV.WEBKAHUNA.COM
    Subject: Re: Sextant accuracy with short distance to horizon
    
    Has anyone noticed a difference between GPS and WWV time. I hear some units
    display a different time depending on software.
    
    Bob Owens
    TUGLY
    26' Nordic Tug
    

       
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