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    Re: FOG's, was Re: automatic celestial navigation
    From: George Huxtable
    Date: 2008 Jan 28, 14:34 -0000

    Paul Hirose's contributions are always full of interest and knowledge, and
    his latest is no exception.
    
    However, I wonder whether there's a bit of misunderstanding between us.
    
    I had written-
    
    | > Although inertial navigation systems always show some drift, as a result
    of
    | > having to integrate (twice) the accelerations, which always have some
    | > zero-error, I doubt (from my inexpert perspective) whether any of that
    drift
    | > derives from the pseudo-gyros, which sense the orientation.
    
    And Paul replied, in [4459]-
    
    | Actually, I've read that gyros have always been the limiting
    | factor in INS performance. The B-1 navigator's manual says, "The
    | navigation accuracy of the INS inertial platform is most dependent on
    | gyro performance. Accelerometer accuracy is less critical unless major
    | malfunctions occur."
    
    But I wonder whether Paul and I are discussing the same type of gyro, here.
    My words referred to that modern development, which I christened the
    "pseudo-gyro", without spinning parts, and relying on the interactions of
    two counter-rotating light-beams. I suspect, though I can't be sure, that
    the words Paul quoted from the manual were from the days of the real-gyro,
    with spinning rotor, and I would readily agree that in that situation, an
    important source of drift would arise in the orientation-sensing of those
    gyros. Perhaps Paul will confirm whether or not I have got that right.
    
    Frank Reed mentioned that "RLGs in real navigation systems are equipped with
    little vibrating devices to keep them lively. These tend to prevent the
    counter-rotating beams from locking into the same state."
    
    Indeed, a "dithering" mechanism is required to avoid the tendency of the two
    light-beams to lock into phase together at very low rotation rates, and that
    dither has to be averaged out or allowed for, and provides a possible source
    of some short-term error. Nevertheless, isn't it the case that there is
    effectively zero long-term drift in the orientation sensing of these
    devices; quite different from the behaviour of their mechanical
    predecessors?
    
    ==========================
    
    Paul added the following interesting information, some of which I had read
    about elsewhere-
    
    | The B-52 SPN/GEANS INS had the most interesting gyro I've encountered.
    | The rotor was a hollow beryllium sphere roughly the size of a golf ball.
    | It rotated at about 650 revolutions per second, suspended by
    | electrostatic force in a spherical vacuum chamber. The gap between
    | sphere and wall was on the order of .002 inch, as I recall. Spin-up
    | torque was applied only at startup. After that the ball coasted in
    | vacuum with almost zero friction. To power down, the system braked the
    | sphere, then shut off the suspension voltage (the electrodes were on the
    | inner wall of the chamber). A power interruption would crash the sphere,
    | so a "rotor support battery" ensured backup power long enough for a
    | proper shutdown.
    
    | Since there was no contact between the rotor and its envelope, an
    | optical sensor observed a mark on the ball's "north pole" to keep the
    | INS platform aligned with the spin axis.
    |
    | A couple of these spheres are depicted here:
    | http://periodictable.com/Elements/004/index.html
    
    With a spacing of only .002 inch, and the ball held there in place against
    its weight entirely by electrostatic forces, I wonder how "stiff" that
    positioning could be, and how well it could resist any sudden accelerations
    caused by vibration, or by buffeting as the aircraft flew through
    turbulence. I bet the assembly had to be carefully cosseted within
    spring-mounting and padding.
    
    George.
    
    contact George Huxtable at george---.u-net.com
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    
    
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