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    Re: accuracy of automatic celestial navigation
    From: Paul Hirose
    Date: 2002 Dec 9, 11:42 -0800

    George Huxtable wrote:
    > down to affordable levels. And on a rough sea-surface, the disturbing
    > accelerations are orders of magnitude greater than a high-altitude aircraft
    > has to contend with.
    I don't remember the price tag on the B-2 AINS, but it must be
    staggering. The thing is an optical, mechanical, and electronic
    marvel. We never opened it - if there was a malfunction it went back
    to the factory to be repaired in a "clean room".
    An INS can handle motion far more violent than a seagoing vessel will
    experience. For many years fighter and attack aircraft have carried
    INS. The older ones, at least, were not "strap-down" systems, but
    mounted the gyro and accelerometer platform on gimbals. But since the
    platform maintains a fixed orientation in space as the plane gyrates
    around it, inertia is your friend. The gimbal torque motors need only
    compensate for the friction in the gimbal bearings, and of course this
    is reduced to a very low level.
    > In the days before GPS, how could the instaneous position in flight be
    > independently determined sufficiently accurately, for testing purposes? Is
    > Loran accurate enough, and fast-responding enough? The aircraft will
    > presumably pass through 15 metres in less than 30 milliseconds, so it's a
    > demanding requirement.
    A tracking instrument on the ground called a cinetheodolite is the
    old-tech method. You often see these in documentaries about the space
    program. The operator manually follows the target as a motion picture
    camera records the scene. Azimuth and elevation readings are optically
    superimposed on each frame, along with the time. With simultaneous
    film from two or more of these instruments it's possible to
    reconstruct the target's flight path.
    The "kinetos" I saw at Edwards AFB were so big, they were motorized
    and the operator rode the instrument.
    Rodney Myrvaagnes wrote:
    > I don't understand how the earth's lumpy gravitation could affect
    > inertia in a B2. I know it goes pretty fast, but it would have to get
    > close to the speed of light for relativistic effects to become
    > comparable to the drift of the ANS. Am I missing something?
    The 1984 Bowditch explains this better than I can:
    "Since an accelerometer cannot distinguish between a kinematic
    acceleration and a gravitational acceleration, any uncertainty in the
    gravitational environment manifests itself as a system error. in the
    case of marine inertial navigation in locally level coordinates, it is
    the horizontal components of gravity that cause significant errors.
    These are directly due to deflections of the vertical (art. X4), which
    are tilts of the actual (plumb bob) vertical vector relative to the
    presumed reference vertical."

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