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    Re: automatic celestial navigation
    From: Gary LaPook
    Date: 2007 Dec 06, 23:09 -0800

    Gary writes:
    O.K. but there are only  limited periods during the day when satellites
    are visible, shortly before sunrise and  shortly after sunset, say about
    an hour each. During sunlight you can't see them against the bright sky
    (or the stars either) and after the sun is well down the satellite is in
    the shadow of the earth and is not illuminated.
    frankreed@HistoricalAtlas.net wrote:
    >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.
    > -FER
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