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    Re: Celestal navigation on a CD
    From: George Huxtable
    Date: 2007 Mar 26, 20:06 +0100

    One of the morsels available on this IoN disc is "Lunar parallax method of 
    astro navigation", by J S Thompson of the Rand
    Corporation, in "Navigation" vol 3, Sept-Dec 1951, nos 1 & 2.
    That may strike a chord with some of us on NavList. There's very little that's really new...
    He investigates a proposal to use Lunar parallax, by measuring very precisely 
    the angular distance between a limb of the Moon and
    two fixed stars which are about 90 degrees apart in azimuth in the sky. This 
    is for determining, from on board, the position of a
    ballistic missile, to 1-2 miles, during its journey. Presumably that 
    requirement relates to the blast radius of a fission bomb of
    those cold-war days, and perhaps to the size of the Kremlin.
    The main virtue of the proposal was that it would not be jammable, unlike 
    radio methods. Of course, in a ballistic missile, there's
    no way of sensing the horizontal or the direction of gravity.
    The aim was to measure the angles with an overall precision of no worse than 
    an arc-second, which corresponds to 1 mile when the
    Moon is overhead, and 2 miles when 60 degrees away from the position under the 
    Moon. That precision would require some very
    sophisticated optics, using photomultipliers as light-sensors. In those days 
    any electronics used vacuum-tubes, and only analogue
    computing was even conceivable.
    The author goes into interesting matters that have to be taken into account 
    when working to such accuracy, such as displacement of
    the Moon's centre-of-mass from the centre of its disc, profiles of 
    Moon-mountains, and aberration of light resulting from the speed
    of the projectile.
    He says "The position of the moon's centre of mass as seen from the centre of 
    the earth is given as a function of time in the
    Nautical Almanac to about 0.1 sec of arc". Not in more recent Nautical 
    Almanacs, which give it only to within 0.1 minutes. Perhaps,
    in those days, there was no separate US Astronomical Almanac, and that precise 
    information was supplied for astronomers rather than
    navigators. Does any list member know?
    How sad, all that ingenuity was turned to such horrific ends, but no doubt 
    equally ingenious scientists are working on similar
    schemes, even today.
    contact George Huxtable at george@huxtable.u-net.com
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    To post to this group, send email to NavList@fer3.com
    To unsubscribe, send email to NavList-unsubscribe@fer3.com

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