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    Re: Digital Sextant
    From: Paul Hirose
    Date: 2002 Mar 31, 22:15 -0800

    Hal Mueller wrote:
    > In the round of SBIR topics that closed back in January, US Navy's
    > SPAWAR was looking for proposals for developing an automatic
    > celestial navigation system that would be able to image stars in the
    > daytime.  There was also some work by SPAWAR a few years back to
    > adapt space/aviation star trackers to shipboard use.
    For many years military airplanes have had star trackers. For example,
    the B-52 had (it was deleted when inertial nav was installed) an
    astrocompass which would give an accurate heading even in polar
    regions. The business end was a prism protruding a little above the
    top of the fuselage and protected by a small (5"?) glass dome. You set
    the star's SHA and declination into the system via mechanical drum
    counters, similar to odometers. The system clock also had to be set to
    GHA Aries. The present position required to solve the celestial
    triangle came through a tie in to the bomb-nav system. The horizontal
    reference was gyro-stabilized.
    It was called an astrocompass, but the system actually did a full
    sight reduction continuously in real time. Counters displayed
    heading to a tenth degree as well as the intercept (to 1 minute? -
    can't remember anymore).
    My memory is hazy, but I believe there were two sets of SHA/dec
    counters and a knob so the navigator could switch back and forth
    between stars quickly.
    The system could use the sun or stars. Can't remember if it was good
    enough to track stars in daylight. There was an "aurora filter" which
    could be swung into the optical path via a switch throw.
    My specialty was maintaining the bomb-nav system, but the astrocompass
    was of similar vintage, so I assume it used the same technology:
    vacuum tube analog summing amplifiers, mechanical differentials,
    synchros, etc.
    Years later, in the B-2 program, I encountered an AINS (astro
    inertial navigation system). As the name implies, it married an
    inertial platform with a star tracker. They were physically in the
    same unit. The star tracker was a little telescope (looked like a
    Schmidt Cassegrain) on an alt azimuth mount. Instead of being shaped
    like a dome, the optical window was flush with the aircraft skin.
    Field of view was a cone centered about the zenith. It could see up to
    45 degrees away from the zenith.
    If you look at an overhead shot of a B-2, there's a round dark spot
    about the size of a dinner plate between the left hand air intake and
    the cockpit. That's the AINS window.
    This system absolutely would track stars in broad daylight, even under
    a hazy sky. I don't think it observed the sun, just stars. It had a
    catalog of 61 stars. Unfortunately I never asked an engineer for a
    list of those stars.
    In the early years of the flight test program, GPS was not yet
    integrated into the B-2 avionics, so system time came from an ATTU
    (airborne time transfer unit). Setting the time manually into the
    plane wouldn't give enough accuracy. Shortly before flight, we'd put
    fresh batteries in the ATTU, "hack" it electronically from a lab time
    standard, then install the ATTU on the plane.
    AINS operation was totally automatic. It went from star to star on its
    own schedule, tracking each for less than a minute. This system had
    the advantages of an inertial nav system, except that the drift was
    bounded by the star tracker. I.e., the nav solution stayed within x
    meters of the truth no matter how long the flight, or where in the
    world you were. The "x" is classified, but I wouldn't bet on beating
    it with a sextant!
    On one occasion I was cockpit operator and had enough slack time to
    play with the nav system on a B-2 parked outdoors. With the AINS in
    "free inertial" mode (star tracker disabled) I could see its position
    slowly drift away from the GPS. But when I switched to stellar
    inertial mode, it pulled right back in. Very impressive, and this was
    in bright sunlight.
    I was told this astro inertial platform was nearly the same device the
    SR-71 carried. You can see the same round port on top of the '71. I
    also heard Northrop's expertise with this type of navigation was an
    outgrowth of the old Snark intercontinental cruise missile. They had
    to develop astro inertial because a pure inertial system would drift
    too far during the missile's flight, much longer in duration than an
    paulhirose@earthlink.net (Paul Hirose)
    B-2 flight test maintenance, 1987 - 97

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