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    Re: FOG's, was Re: automatic celestial navigation
    From: Paul Hirose
    Date: 2008 Jan 27, 15:46 -0800

    George Huxtable wrote:
    >
    > Three axes,
    > fixed in space could be- Toward the North celestial pole, with Dec = 90:
    > toward Aries (in the Earth's equatorial plane), with Dec = 0,
    > Right-Ascension = 0: and (also in that equatorial plane) 90 degrees from
    > Aries, with Dec = 0, R.A. = 90. These axes remain (nearly) unchanging, in
    > space.
    
    That's correct, and such an orientation might be used in space
    applications. I'm not sure; all my experience has been with aircraft
    INS. As far as I have ever seen (except for strapdown systems), these
    keep the gyro and accelerometer platform horizontal. In the horizontal
    plane the platform may be oriented to true north. Other systems don't
    attempt to stabilize the platform about the vertical axis. Instead, they
    keep book on the "wander angle": the difference between true north and
    the platform azimuth. Then there's the Litton Carousel INS, which keeps
    the platform horizontal but constantly rotating at 1 rpm.
    
    In all these systems, the initial azimuth orientation is precisely
    established during INS alignment, which is necessary every time the
    system is turned on. Alignment is almost completely automatic; the
    hardest part for the operator is entering latitude, longitude, and
    altitude. At military bases the parking spots have geodetic survey marks
    embedded in the concrete to facilitate this. (Some aircraft with
    integrated avionics have an "align to GPS" option to eliminate manual
    coordinate entry if the GPS is running.)
    
    After the alignment is started there isn't anything to do but monitor
    its progress and wait. This can take 20 minutes or more. A full
    alignment on a B-1 bomber takes 90 minutes, though you can terminate
    early and accept reduced accuracy.
    
    Some military aircraft systems let you "cock" the INS by doing a ground
    alignment, then shutting down. If you don't move the plane, the system
    remembers its orientation and location, making INS available for a
    scramble take-off without a lengthy (possibly fatal) delay for
    alignment. The stored alignment isn't as good as a normal alignment, though.
    
    
    > 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.
    
    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."
    
    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
    
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