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    Re: GPS as a time authority
    From: Richard B. Langley
    Date: 2009 Sep 15, 14:14 -0300

    All these references to a GPS receiver knowing UT1, should say UTC. Granted, the
    difference between UT1 and UTC is never as large as 0.9 seconds, but there is a
    difference. The GPS navigation message contains the current offset between GPS (System)
    Time and UTC as well as advance notice of upcoming leap seconds when they are
    announced.
    -- Richard Langley
    
    Quoting Lu Abel :
    
    >
    > Antoine:
    >
    > Most GPS receivers are, however, aware of the difference between "GPS
    > time" and UT1 and automatically correct for it in their display.   If
    > one digs down down into the setup parameters in the average GPS receiver
    > one will find a choice of time displays, including both GPS time and
    > UT1.   The factory default setting is usually UT1, since that is the
    > time we all live by.
    >
    > But you do raise an interesting point -- is it possible that the time
    > difference observed by the original poster, John Parsons, was due to
    > someone having their GPS set to display GPS time rather than UT1???
    >
    > Lu Abel
    >
    > antoine.m.couette{at}club-internet.fr wrote:
    > > I think that the reason brought up by "M. Lunav" is quite valid : processing times
    > may vary according to types of GPS receivers.
    > >
    > > There is also an ESSENTIAL REASON, which may have been skipped in this thread.
    > >
    > > GPS cannot accomodate any time discontinuities. Therefore, some 15 years ago, GPS
    > time - i.e. the one used by the GPS System - was the same as UT1.
    > >
    > > Now with the extra seconds added now and then to take in account the Earth rotation
    > changes, GPS time itself which is much more stable than UT1 has become different from
    > UT1 by a number of seconds amounting to 8 seconds (this amount to be confirmed)
    > to-day.
    > >
    > > Antoine M. Cou?tte
    > >
    > >
    > > >
    > >
    >
    > >
    >
    
    
    ===============================================================================
     Richard B. Langley                            E-mail: lang---.ca
     Geodetic Research Laboratory                  Web: http://www.unb.ca/GGE/
     Dept. of Geodesy and Geomatics Engineering    Phone:    +1 506 453-5142
     University of New Brunswick                   Fax:      +1 506 453-4943
     Fredericton, N.B., Canada  E3B 5A3
         Fredericton?  Where's that?  See: http://www.city.fredericton.nb.ca/
    ===============================================================================
    
    
    
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