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    Re: automatic celestial navigation
    From: Frank Reed
    Date: 2008 Jan 10, 12:57 -0500

    Greg R., you wrote:
    " Although I think the inertial part of it might be a little pragmatic on a
    sailboat... ;-) "
    The inertial part is the principal navigation system. Though the description
    as posted makes it sound like this system is doing something akin to common
    celestial navigation, that's really unlikely. Consider: to do standard
    celestial navigation, you measure altitudes relative to a horizon. At the
    altitude where the SR-71 would fly, there is no visible horizon, so that
    means an artificial horizon. Now there's been no great advance in all these
    decades in artificial horizons. There's simply no way to get a vertical that
    is not influenced by the motion of the airplane. You're limited to measuring
    angles between celestial objects or angles relative to components within the
    airplane's structure.
    So why would that navigation system need the stars if not for a standard
    celestial fix? The key here is that it's an "inertial" navigation system.
    This is a sophisticated form of dead reckoning. It uses accelerations
    (including rotations) to calculate the current position by integration:
    acceleration integrated over time yields net change in velocity, velocity
    integrated over time yields net change in position. But an inertial system
    drifts, and the biggest drift is in the orientation of the platform. If we
    can compare the platform with a known inertial frame of reference, we can
    largely eliminate that drift. The distant stars, of course, constitute an
    excellent measuring standard for an inertial frame of reference. So, more
    likely than not, the SR-71 system used the stars to re-set the inertial
    system. In effect, the stars in this system are being used AS A COMPASS! The
    primary navigation is inertial.
    The paragraph from the Wikipedia article on the SR-71 strikes me as
    reasonably accurate: "The ANS primary alignment was done on the ground and
    was time consuming, but brought the inertial components to a high degree of
    level and accuracy for the start of a mission. A "blue light" source star
    tracker, which could detect and find stars during day or night, would then
    continuously track stars selected from the system's digital computer
    ephemeris as the changing aircraft position would bring them into view.
    Originally equipped with data on 56 selected stars, the system would correct
    inertial orientation errors with celestial observations. The resulting
    leveling accuracies obtained limited accelerometer errors and/or position
    Incidentally, there's a way to dispense with the stars for inertial
    navigation. An inertial frame of reference can also be accurately maintained
    using a ring laser. Those supposedly began to replace astro-compass systems
    in the 1980s (so they say!).
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