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    Re: automatic celestial navigation
    From: Frank Reed
    Date: 2007 Dec 06, 23:22 -0500

    Gary, you asked:
    "If I understand you correctly you will get the altitude of the satellite by
    calculating or measuring the altitude of a star that it passes
    nearby......Why not just use the altitude of the star in the first place,
    kinda like normal celestial navigation?"
    No measured altitudes. You watch the satellite crossing the sky with
    binoculars (and the binoculars are not really necessary, but we may as well
    use a tool that's probably available). As it glides along, it will pass very
    close to at least one 2nd or 3rd magnitude star. When it does, you draw a
    quick little sketch of the star pattern and mark it "Sat X passed less than
    0.1 degrees from this star," for example. Then you call up some satellite
    prediction software on your computer. You let it calculate the path of
    satellite X among the stars for various locations around your estimated
    position. Modern satellite prediction software will usually draw in the star
    patterns so you could do this visually and get an immediate result. You find
    any pair of points where the predicted motion matches what you saw. Draw a
    line through those two points and you get a line of position as good as a
    sextant sight but without using a sextant. One could even dispense with the
    binoculars under many conditions. You observe a second satellite for a
    second LOP, or if you have exact GMT and fast fingers on a stopwatch, you
    could record the instant when "Sat X" passed near that star. Then you get a
    position from a single observation.
    This method of position-finding would have worked even thirty years ago,
    when navigators might still have had some practical use for it. The
    calculations would have required a programmable calculator, but the ones
    just becoming available back then would have been fully capable of running
    predictions. The plotting would have required a star atlas showing stars
    down to about sixth magnitude (there were several good ones available back
    then). But the real problem was acquiring satellite orbital elements. Today
    they're online. But back then, satellite observers published orbital
    elements in newsletters distributed by postal mail, and very few people knew
    about them.
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