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    Accuracy of Nautical Almanacs and UT1/UTC
    From: Frank Reed
    Date: 2012 Feb 12, 22:22 -0800

    In the thread on the French Nautical Almanac, Dave Walden, you wrote:
    "It is my opinion, I believe shared by others, that the French are as good as or better than JPL at ephemeris generation based on recent publications."

    Well, there should be no difference whatsoever at the level of accuracy of the nautical almanacs. The idea that their calculations are "as good as or better" is simply irrelevant.

    But there is an issue that could make one almanac appear to have a slight difference in accuracy. The time variable in the nautical almanacs was traditionally been GMT which is essentially UT1 in modern terms. In other words, it is the time based on the rotation of the Earth, essentially the (observed) sidereal time scaled for solar days. By GMT/UT1, the GHA of any object and specifically, to keep things easy, the GHA of Aries should advance in a way that shows no discontinuities. In particular, when leap seconds are added to the calendar, their should be no 0.25 minute of arc jump since leap seconds do not affect GMT/UT1. On the other hand, if the independent time variable for a nautical almanac is UTC, from which all broadcast times and zone times are derived, then there will be a jump of 0.25' in the tables, assuming that the tables were published with foreknowledge of the leap second. And even if you don't have the day in question, you can detect this by linear extrapolation for dates around it.

    Apart from that, no modern nautical almanacs or equivalent should show any differences in the fundamental data larger than 0.1' and that only occasionally due to differences in rounding. Naturally this excludes cases where the data are modified, for example to give the position of the visual center of Venus instead of the geometric center, or cases where the fundamental positions are nudged up or down a bit to permit better results with linear interpolation.

    None of this matters for practical (traditional) navigation, of course. But if you're aiming your submarine's missiles updated by celestial observations, you better get it right.

    -FER
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