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    Re: Timekeeping and sight time records
    From: Richard B. Langley
    Date: 2005 Mar 18, 08:16 -0400

    All what you say is correct, except for the "rollover" which refers to the
    rolling over of the GPS week cycle every 18 years or so and nothing to do
    with leap seconds. The GPS receiver can determine UTC using the UTC
    - GPS Time offset which is included in the navigation message. Depending on
    the receiver model, the time displayed on the receiver screen might not be
    updated on the exact second. Some are known to lag slightly in their update
    cycle. If the receiver has a 1 p.p.s. output, it should be spot on.
    -- Richard Langley
    P.S. My ears are burning. I think a flame about too much GPS on the list is
    imminent! ;-)
    
    On Fri, 18 Mar 2005, Bill wrote:
    
    >> You are thinking about the difference between GPS (System) Time (which is not
    >> updated for leap seconds) and UTC which is compensated for leap seconds to
    >> keep it within 0.9 seconds of UT1. Currently the difference is 13 seconds.
    >> Nothing to do with "satellite rollover."
    >
    >I get deeply confused on this subject at times.  My Garmin 76 appears to
    >display the same time as other online government sources (nanoseconds
    >between glances at one or the other noted ;-)
    >
    >Somehow I thought the GPS difference was set back to zero when the
    >satellites rolled over.  Perhaps my recollection, or source (a GPS list
    >years ago around the Y2K scare) is/was in error.
    >
    >I did read at:
    >
    >http://tycho.usno.navy.mil/gpstt.html
    >
    >"The number remains constant until the next leap second occurs. This offset
    >is also given in the navigation (NAV) message and your receiver should apply
    >the correction automatically. As of January 1, 1999, GPS time is ahead of
    >UTC by thirteen (13) seconds."
    >
    >I enjoy the use of the word "should" when dealing in multiple seconds or
    >nanoseconds. 
    >
    >If I understand, the II/IIA satellite containing two cesium (Cs) and two
    >rubidium (Rb) atomic clocks, and Block IIR satellite containing three Rb
    >atomic clocks all report to a "paper" Composite Clock (CC).  Then some black
    >box factors in the missed leap seconds and viola, my 76 has the time to the
    >nearest .9 seconds of some UTx standard.  How does magic work?
    >
    >Bill
    >
    
    
    ===============================================================================
     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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