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    Re: De Lurk
    From: Steven Grobler
    Date: 2003 Feb 13, 11:02 +0800

    Space (and aircraft) inertial navigation systems comprise a stabilized
    platform with accelerometers and gyroscopes. These track the vehicles
    acceleration in three axes and attitude in three axes, relative to some
    known starting point. The accelerometer signals are integrated to get
    velocity and displacement in three axes and the gyros give the
    directional info (rotations about 3 axes), so one can track the
    vehicle's trajectory in 3D. Don't ask me about the math - I haven't been
    able to get my mind around it!
    The trouble is that gyros and accelerometers have some drift error that
    means the position error increases with time. So the platform needs to
    be updated periodically. Astronauts would do this with some kind of
    sextant device and key in the corrected position to the nav computer
    that looks after the inertial platform. (This was shown in the movie
    Appollo 13, when the inertial platform orientation data was transferred
    from the command module to the lunar module computer, and a check had to
    be done to ensure it was right).
    Space grade systems can hold position accuracy for much longer than
    aircraft systems, purely because of the quality of the gyros,
    accelerometers used.
    On aircraft, a GPS can be used to automatically update the inertial
    system in lieu of star sights. The inertial system provides position
    data between GPS updates or in the event that GPS data is not available.
    Steve G
    Brooke Clarke wrote:
    > Hi Robert:
    > I would like to look up the patent, but you can not search on the inventor name prior to 1975.
    > Do you have a patent number?
    > Thanks,
    > Brooke Clarke
    > Robert Eno wrote:
    > > The United States Naval Academy produced a book called: "Space Navigation
    > > Handbook" in about 1961. It covers such topics as three dimensional
    > > navigation and even includes discussions on several mechanical/electrical
    > > devises for position fixing in space. This includes a diagram of an
    > > invention developed by Capt. P.V.H. Weems, whose name should be very
    > > familiar to all of you old salts out there.
    > >
    > > I have to confess that I have not read this book cover to cover. It was
    > > generously given to me by a fellow from the US in response to an enquiry
    > > that I made about "space sextants".
    > >
    > > Other than that, I can't expand on the topic. A mathematician/genius, I am
    > > not!
    > >
    > > Robert
    > >

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