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    Re: accuracy of automatic celestial navigation
    From: Paul Hirose
    Date: 2002 Dec 8, 23:07 -0800

    George Huxtable wrote:
    > Not so difficult to define the direction of a star to that precision,
    > perhaps, but what worries me more are the errors in establishing the
    > direction of the vertical reference. An aircraft is different from a
    > spacecraft in that it has some unavoidable degree of buffeting from the
    > atmosphere that it's flying in, small changes in engine output, tiny
    > movements of control surfaces, movement of the crew, together with
    > gravitational anomalies from the ground it's passing over.
    In the case of the B-2 AINS (astro-inertial navigation system) the
    star tracker is inside the same case as an inertial nav system. The
    inertial and stellar hardware are tightly integrated with each other.
    So the star tracker's short term pitch, roll, and heading
    stabilization comes from the inertial platform.
    Earth's lumpy gravitational field is indeed a factor in inertial
    navigation; it has to be stored in some sort of internal map or table.
    Through some magic not clear to me (I worked in maintenance, not
    engineering!) the AINS feeds the measured directions of stars back to
    the inertial portion to correct its errors. Giving the otherwise blind
    inertial system a look at the outside world is enormously helpful. As
    long as it's got a clear view of the sky the AINS will remain within
    [classified] meters of the correct position, worldwide and practically
    independent of mission duration. It doesn't drift away over time like
    a normal INS.
    In obscured skies the star tracker keeps trying unless you deselect
    stellar mode. It can shoot through a hole in the clouds. It lingers on
    a body only long enough to measure its position, then goes looking for
    others in its field of view (anything within 45 deg of the zenith).
    Initially the scope is placed on the star's predicted az/el. It
    searches outward in a "square spiral" pattern until it locks on or
    concludes the star is obscured. There are 61 stars in its catalog;
    unfortunately I don't know their identities. The 57 classic
    navigational stars plus 4?
    Another thing I never learned is how the AINS knows the offset between
    UT1 and UTC. An ATTU (airborne time transfer unit) was installed in
    the plane shortly before each flight to provide accurate time. Between
    flights we kept the ATTU connected to a rack-mounted rubidium time
    standard which in turn was synched to radio station WWV. Distance from
    WWV was set on thumbwheels to correct for propagation time. But as far
    as I know the whole rack was on UTC. I never saw any DUT1 readout
    provision or any other hint of UT1. My guess is that the offset was
    supplied as part of the computer-generated mission plan uploaded
    before flight.
    Physically, the AINS is fairly compact. It would fit in the passenger
    seat of a small car and weighs roughly as much as a man. Using it to
    navigate the car would take some doing, though. It needs a powerful
    flow of chilled air into its cooling duct, and gulps a fair amount of
    400 Hz 115 VAC 3-phase and 28 VDC power.
    In operation the system was quite reliable. I remember only a few
    times when the AINS had to changed out. Good thing, since it was
    removed with a crane. The location is easily visible in an overhead
    view of a B-2: the star tracker window looks like a round dark hole
    about the size of a dinner plate between the cockpit and the left hand
    engine air intake.
    I got to run it outdoors a time or two. It was started like a normal
    INS. You inserted the lat, lon, and elevation, then selected an align
    mode. There were several available. It depended on how much of a hurry
    you were in; of course the best nav performance was attained by using
    the most time consuming align mode.
    (It's not enough to simply know the airplane is parked at such and
    such coords when you align the navs. What *part* of the airplane? In
    the B-2 the reference point is the nose landing gear; our parking
    spots at Edwards AFB all had little brass survey marks set in the
    concrete at the spot where the nose tires would go. I bet you won't
    learn trivia like that from Discovery Channel!)
    When alignment was done, you saw a NAV READY message on your little
    screen in the cockpit. At that point the system was frozen to your
    starting coordinates, waiting for you to select a nav mode. Again
    there were choices, basically boiling down to what kind of assistance
    the inertial platform would receive. For example, you had pure
    inertial, stellar inertial, GPS inertial, etc.
    Being an enthusiast I liked to select pure inertial and watch the
    coordinates drift away from the correct values. Then I switched to
    stellar inertial and watched the star tracker pull it back to the
    right spot. Worked as advertised - in broad daylight!
    Although it was fascinating to do celestial at a superhuman level, I
    quickly found it boring. All it required of me was to push the correct
    buttons. There was no pride from mastering tables of mysterious
    figures, no "eye" or "touch" to develop, no satisfaction at seeing
    LOPs meet nicely. I found the pinnacle of high tech celestial
    navigation barren of personal satisfaction, and after a couple visits
    was satisfied that I had seen enough.

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