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    Almanacs & UTC redefinition
    From: Paul Hirose
    Date: 2011 Dec 03, 19:05 -0800

    In October there was a "Decoupling Civil Timekeeping from Earth
    Rotation" colloquium to discuss the UTC time scale and the proposal to
    end leap seconds. Celestial navigation may be significantly affected if 
    the proposal is adopted.
    Papers from the colloquium proceedings are online:
    George Kaplan's paper looks at how a UTC redefinition will affect 
    almanacs and their users. In theory, there's no effect. "Since almanac 
    data are generally computed and displayed as a function of either UT1 or 
    TT, the initial, naive, answer is that nothing changes; UTC is not used 
    in the computation of the data and UTC is not the independent argument 
    of any of the tabulations. It has been the user’s responsibility to 
    convert between UTC (or whatever his external time scale is) and the 
    time scale(s) used for the almanac data, and we can simply assert that 
    this responsibility remains unchanged. In fact, the basic conversion 
    formulas from UTC to UT1 or TT won’t change if leap seconds no longer 
    In practice, few navigators have used the formula. "It is a safe bet 
    that fewer than one astronomer in ten, and a similarly small fraction of 
    navigators, know the difference between UTC and UT1, or even that there 
    are two kinds of Universal Time. That’s because, up to this point, the 
    difference has been bounded at 0.9 s, and for the vast majority of 
    purposes, we can set UT1=UTC to sufficient accuracy...
    "Obviously, this will not be the case if leap seconds end and UT1–UTC
    becomes unbounded. At a minimum, we will have to re-label our time
    argument to be explicitly “UT1” and make sure that the explanatory text
    is very clear about the conversion from UTC, something that most users
    have not had to worry about. That is, people who have never heard of the
    IERS or the difference between UT1 and UTC will have to come up to speed
    on these time conversions. In the U.S. Navy, celestial navigation is
    generally performed by enlisted quartermasters, and we expect that some
    new training will be required."
    On the other hand, he notes that "John Bangert of USNO has suggested
    that if leap seconds are dropped from UTC, we could switch our Earth
    rotation dependent tabulations [in the Almanac] from UT1 to UTC... That
    would undoubtedly be a great convenience for users. In the case of Earth
    rotation dependent data, however, the convenience would come at the
    price of some degradation in accuracy. It seems likely that the
    increased error (for tabulations computed about two years in advance)
    would not be worse than that from assuming UT1=UTC now, which is
    probably a common assumption. Furthermore, if necessary, the error could
    be removed by application of corrections based on the measured value of
    UT1–UTC for the date on which the data are needed."
    The ITU will put the matter to a vote next month.
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