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    Re: GPS as a time authority
    From: George Huxtable
    Date: 2009 Sep 15, 19:01 +0100

    Antoine is correct, except that I think that GPS time diverged from our
    civil clocks as early as 1980,  and the missing leap seconds have added up
    since 1980 to put GPS time ahead of GMT (and UT1) by 15 seconds; not 8
    seconds, as he (rather diffidently) suggested.
    
    However, it seems rather more complex than that. It depends on what exactly
    one means by "GPS Time".
    
    GPS time is the time-scale used internally by the GPS system, and, as
    Antoine says, does not allow for leap seconds, so it diverges from GMT. That
    means it can be used for precise measurement of time intervals, without
    needing to know if and when leap seconds have occurred in the past, or are
    predicted for the future. For many special applications, it's just what's
    needed. But not for "telling the time", by civil-time standards; the time a
    normal clock shows at Greenwich.
    
    Few navigational (i.e. not special-purpose) GPS receivers, actually show GPS
    time, I suggest. Instead, a GPS receiver which does its job properly allows
    for the offset from its internal GPS time by the delta-t change since 1980,
    to show GMT instead. How does it know what that offset is, seeing that leap
    seconds are unpredictable more than  six months in advance? Because the
    satellites are informed in advance from the ground station about the offset,
    and any shortly-forthcoming leap, and inform receivers accordingly. That
    message is broadcast to receivers at intervals of around 12 minutes.
    
    So, if you switch on a GPS receiver from cold, it won't know, for up to 12
    minutes, what the true offset actually is, to provide correct GMT. Best it
    could do, if it is well designed, is to remember what offset was used last
    time (and ideally, any predicted leap, then), and work on that assumption.
    But if it's more than 6 months since it was last used, even the cleverest
    receiver will be unable to know what exactly is the current offset value.
    None of this affects the actual navigational workings of the receiver, just
    its display of the civil time.
    
    Lu suggested that any delay in displaying the calculated time would have an
    effect, but it would call for a remarkably clumsy design to introduce such a
    delay without allowing for it appropriately. However, clumsy designers
    exist...
    
    ====================
    
    So, when the race authority declared that "GPS time" would be used, did they
    really mean GPS internal time, or just GMT as shown on a GPS receiver?
    Elapsed time would be identical either way, except in the case of a
    remarkably close finish in a handicap race that spanned the moment of New
    Year.
    
    George.
    
    contact George Huxtable, at  george@hux.me.uk
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    
    ----- Original Message -----
    From: 
    To: 
    Sent: Tuesday, September 15, 2009 4:42 PM
    Subject: [NavList 9731] Re: GPS as a time authority
    
    
    
    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
    
    
    
    
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