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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.

George.

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.





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