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    Re: Celestal navigation on a CD
    From: Frank Reed CT
    Date: 2007 Mar 26, 15:45 -0700

    George H, you wrote:
    "One of the morsels available on this IoN disc is "Lunar parallax
    method of astro navigation", by J S Thompson
    That may strike a chord with some of us on NavList. There's very
    little that's really new..."
    Yes. Perhaps you missed it -- I brought up this article on the list
    back in November. Here's my brief review in NavList message #1784:
    Of course, he does not mention that this is just plain old "line of
    position" navigation using the Moon as a horizon instead of the Earth,
    but then again, the article is really about implementation of this
    concept in an automated system, rather than theory.
    And you added:
    " Of course, in a ballistic missile, there's no way of sensing the
    horizontal or the direction of gravity."
    This was clearly the thinking at the time, but in fact, the same
    automated, analog light sensing technology that he is describing could
    have been used to determine the location of the horizon. To do this,
    you put a light sensor on a rotating platform. It sweeps around
    looking at a great circle. Assuming we're no more than a couple of
    hundred miles up, any random great circle will intersect the Earth.
    The sensor will alternately see light and dark as it rotates around.
    The platform automatically wobbles about, changing the orientation of
    its axis, until the light signal becomes uniform. At that point,
    you're looking at a great circle parallel to the horizon. That
    observation, combined with ordinary altitude measurements of stars,
    would have yielded a better position fix --and one that worked every
    day of the month (but only in daylight).
    And you asked:
    "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?"
    The trick here is that the name "Nautical Almanac" has shifted. The
    title "The Nautical Almanac" finally reverted to the actual nautical
    publication only in 1960. For several decades before that, the
    official navigator's almanac (the one that we would think of as a
    "nautical almanac" today) was published under the title "Abridged
    Nautical Almanac" in the UK and under the title "American Nautical
    Almanac" in the US. In this same period, the almanac for astronomers
    (which today is published under the title "Astronomical Almanac") was
    known as the "American Ephemeris & Nautical Almanac" in the US and
    "The Nautical Almanac & Astronomical Ephemeris" in the UK. There's a
    long history to this that I won't go into right now. The key is that
    the book informally known as "THE Nautical Almanac" worldwide was
    actually the astronomer's almanac. And that almanac gave the Moon's
    position to a precision of 0.1 seconds of arc.
    You wrote:
    "How sad, all that ingenuity was turned to such horrific ends"
    I think this comment makes more sense if you say 'potentially horrific
    ends' since Thompson was writing *after* the last use of nuclear
    weapons. Of course, pretty much all of the major developments in
    navigation have come about because of war and/or "commercial greed".
    Even those vikings with their sun stones were known to have done a
    little pillaging here and there. :-)
    You concluded:
    "but no doubt equally ingenious scientists are working on similar
    schemes, even today."
    Back in the fall, I described on the list an automated navigation
    system that was developed about ten years ago for a spacecraft
    exploring in the asteroid belt. Digital cameras have replaced angle-
    measuring sextants, but the principle is the same: you measure the
    exact location of some relatively nearby object (like a small
    asteroid) and plot against the background stars yielding the exact
    right ascension and declination of the asteroid. Since we know very
    accurately the x,y,z position of the asteroid from previously
    calculated orbits, the observed RA and Dec places you on a line of
    position that passes through the small asteroid and extends into space
    towards the opposite RA and Dec. Do this for two or more known
    asteroids and you have a position fix. The reason for automating this
    is money. The vast majority of spacecraft are navigated by
    communicating with ground tracking stations. These are big radio
    telescope dishes scattered all around the globe. They are expensive to
    maintain and operate, and they have limited bandwidth. So as computing
    power increases and computer memory prices plummet, it becomes more
    practical to offload as much navigation to the spacecraft as possible.
    To post to this group, send email to NavList@fer3.com
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