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    Re: UTC - GPS Time
    From: James H. Maynard
    Date: 1996 Jun 26, 01:43 GMT

    In article 6BT8FkgxonB{at}gps_gear.mayn.de,
    	guido{at}gps_gear.mayn.de (Guido Lenz) said:
    >
    >Does anybody have the exact time delay in GPS to UTC time? I
    >heard rumours of about 11 seconds, but from experience this
    >seems to be 11.21 seconds. Does anybody have exact data?
    >
    >Best regards
    >Guido
    >--
    >And in the end, it's not the years in your life that count.  It's
    >the life in your years. -Abraham Lincoln
    >
    >
    
    GPS time is currently FAST (that is, ahead) of UTC time by
    exactly 11 seconds.  The number of seconds by which the
    GPS time for a given event is greater than the UTC time
    for that event is the same as the number of leap seconds
    that have been inserted into the UTC time scale since
    the GPS time scale began at 1980-01-06 00:00:00.0 UTC.
    
    Leap seconds are inserted as necessary to keep UTC in
    reasonably close agreement with the time scale that uses
    the rotating earth as a clock. Each time a leap second
    is inserted into the UTC time scale, the difference between
    the GPS time and UTC time increases by one second.
    Therefore,
       (GPS time) - (UTC time) = (number of leap seconds),
    where
       (GPS time) and (UTC time) are the readings on the
    two time scales for the same event, and
       (number of leap seconds) is the number of
    leap seconds that have been inserted into the
    UTC time scale since zero hours, zero minutes,
    zero seconds UTC on Sunday, the 6th of January,
    1980.
    
    I don't know when the next leap second will be
    inserted, as the International Earth Rotation Service
    has not yet promulgated that information.  Tyhpically,
    leap seconds have been inserted from a year to a
    year and a half apart.  They are usually inserted at
    the very end of June or of December, UTC, but
    if necessary to insert one sooner, the IERS might
    choose the end of March or of December.
    
    Jim Maynard, K7KK -- Salem, Oregon, USA
    jmaynard{at}teleport.com (Home), iim1jhm{at}iim.ups.com (Work)
    "Every vessel shall at all times maintain a proper lookout...."
    (COLREGS Rule 5)
    
    
    
    From mail Mon May 19 13:40 EDT 1997
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    Date: Mon, 19 May 1997 12:23:36 -0400
    From: Pierre H Boucher 
    Subject: [Nml] GPS Satellites Not 2000 Compliant?
    To: "(inconnu)" 
    Message-Id: <199705191223_MC2-16E1-6F1A---.com>
    Sender: owner-navigation{at}gomoku.ronin.com
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    Reply-To: Pierre H Boucher 
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    David Deratzian wrote :
    
    >Has anyone heard that the GPS satellites are not year 2000 compliant?
    >This was suggested to me recently.  Anyone know if it is true?
    
    The following was a Notice to Mariners issued by the CCG
    
    L3182/96  LAURANTIAN REGION
    GPS SYSTEM TIME ROLLOVER
    WARNING TO THE USER
    
    GPS system time will roll over at midnight 21-22 August 1999.
    In fact, GPS accounts for time using a Week Number and the number of
    seconds in that week.  It counts weeks using a starting point of midnight
    (0000) on the evening of 5 january 1980/ morning of 6 january 1980 (UTC),
    and has increase its count by 1 for each week since then.  Both week and
    seconds are broadcast as part of GPS message provided by satellites and are
    used by receivers in their computations.  The GPS Week Number field message
    can only provide for numbers up to 1024 wich means that, at completion of
    week 1023, the Week Number will roll over from 1023 back to 0:
    
    Week beginning at midnight (0000) on :  GPS Week Number satellites
    broadcast :
    
    08 August 1999                          1022
    15 August 1999                          1023
    22 August 1999                          0
    29 August 1999                          1
    
    It will be the responsability of the user to account for this changeover,
    the satellites themselves will simply start broadcasting the new Week
    Number.  How it will affect your particular GPS unit will depend on what
    brand and model of receiver you have.  Some receivers may merely display
    inaccurate date information, but other may also calculate incorrect
    navigation solutions or might stop providing positions.  If the roll over
    hasn�t been taken into account during the conception of your GPS receiver,
    the unit might have problems.  Some units will require a software update.
    Mariners should ask the manufacturer or distributor of the receiver wheter
    this change will affect their equipment.
    -------------------------------------------------
    
    Pierre Boucher N
    CPS code 16-32
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