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    Tai, Gps, Utc.
    From: Jim Easton
    Date: 1997 May 21, 13:40 EDT

    Found the following:
    Date: Mon, 19 May 97 21:59:54 -0700
    From: William Lewis <wiml@XXX.XXX>
    Subject: Re: ~2K (Ladkin, RISKS-19.16)
    Actually, GPS uses a measurement of time that has the same definition of a
    second as UTC and TAI, but is offset a constant 19 seconds from TAI. This
    was the same as UTC in 1980 (the GPS epoch), but leap seconds have increased
    UTC's offset from TAI to 30 seconds (soon to be 31) while GPS time has
    remained unaffected. Anyone trying to reconcile GPS time with local civil
    time (based indirectly on UTC) has to take this into account. (Personally, I
    think computers should keep time in TAI and have a table of leap seconds
    along with the time zones and other human-generated time cruft.)
    The GPS clocks do take numerous relativistic effects into account;
    presumably TAI and UTC are canonically measured at some particular location,
    with its particular dilations and whatnot. Astronomers have time scales such
    as TDB (Barycentric Dynamic Time) which account for relativistic effects on
    the Solar-system scale, and have ways to deal with the fact that
    simultaneity isn't a well defined concept in the first place.
    Jim Easton
    4364 Bonita Rd., No. 166
    Bonita, CA 91902-1421
    Tel: 619-548-0138
    Fax: 619-470-8616
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