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    Re: GPS as a time authority
    From: Lu Abel
    Date: 2009 Sep 15, 10:18 -0700
    Thanks for the correction, Richard.   I should have used the correct terminology.


    Richard B. Langley wrote:
    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
    -- Richard Langley
    Quoting Lu Abel <lunav@abelhome.net>:
    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@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)
    Antoine M. Cou?tte
     Richard B. Langley                            E-mail: lang@unb.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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