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    Re: More Space Navigation
    From: Steven Grobler
    Date: 2003 Feb 19, 14:48 +0800

    To answer the Apollo navigation question, Apollo astronauts used a
    computer (AGC - Apollo Guidance Computer) that processed readings from a
    stabilized "inertial platform", comprising gyros and accelerometers.
    They would also have checked and updated the guidance computer's
    calculated position using star sights.
    The gyro's and accelerometers essentially allow you to plot the vehicles
    trajectory in 3-D from the known starting point.
    Fred Hebard wrote:
    > I acquired a copy of Space Navigation Handbook (Navpers 92988) from
    > Amazon.com. The book has enough differential equations to gag me,
    > without getting into Hamiltonians and quaternions.  Most of the math
    > was applied to orbital determinations.
    > The book concentrated on near-earth navigation, as that was where the
    > authors (chief of whom was Capt. P.V.H. Weems) expected most people
    > to be in the near future; very practical.  The basic method advanced
    > for fixing a position was essentially what earth-bound types use.
    > They advocated use of spherical coordinates with an origin at the
    > center of the earth.  Range to the spacecraft could be determined by
    > marine sextant measurement of the apparent diameter of the earth.
    > Altitudes of stars could be determined with reference to the earth's
    > edge, using the semi-diameter obtained from the range measurement to
    > bring the measurement to the center of the earth.  They also
    > discussed some other instruments and optical systems, but it was cool
    > to see reference to marine sextants.
    > I don't know that marine sextants would be too useful peering through
    > the thick windows of a spacecraft, but still....
    > One big problem earth-bound types have is that our planet is
    > rotating.  So one needs to know the time to locate the first point of
    > Aries.  That is not necessary for a spacecraft, unless it wants to
    > know what part of the earth it's over, which might be useful
    > information at re-entry or other times.
    > I'm still wondering what it would be like to navigate in interstellar
    > space.  There, position (parallactic) shifts of nearbye stars would
    > need to be accounted for.  I suppose until one got very far away, a
    > coordinate system centered on the sun could be used.  It doesn't
    > appear one could use a marine sextant to determine range to the sun
    > by diameter measurement after one was very far away.
    > I hope others will expand on some of these points.  Perhaps Dan Allen
    > could summarize more of Richard Battin's book for us poor souls too
    > far from research librarires.   This is all very slow going for me,
    > but fascinating.  I also wonder how the Apollo astronauts and
    > engineers actually navigated their craft.
    > Fred

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