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    Re: astrocompass still in use
    From: Dan Allen
    Date: 2002 Sep 25, 21:24 -0700

    On Wednesday, September 25, 2002, at 09:44 PM, Ken Gebhart wrote:
    > BTW a list member mentioned the MD-1 on the B-52.  I think this was an
    > astro
    > TRACKER not an astro compass.  It would lock onto stars automatically
    > and
    > translate to a lat/long for the navigator.  It also gave the azimuth
    > of each
    > body observed, and that would translate automatically into a true
    > heading for
    > the airplane in order to check its gyro compass if desired, thus
    > performing
    > the function of an astrocompass.
    Do you have any of astro trackers in stock?  I'll definitely buy one!
    The SR-71 Blackbird also had a very sophisticated star tracker.  Here
    is an extract from "SR-71 Revealed, the Inside Story", by Richard H.
    Graham, pages 65-68.  Richard Graham was an SR-71 pilot for many years.
      This is his own description of the system.
    Navigational Systems
    The SR-71�s high speed and sensitive missions demanded a navi-gational
    system that was highly accurate, reliable, and didn�t depend on inputs
    from other sources subject to electronic jamming. Patterned after
    navigational systems used on ICBMs, the SR-71�s Astro-inertial
    Navigation System (ANS) filled those requirements.
    Simplistically, the ANS was a star tracking navigation system. At least
    two different stars had to be tracked for optimum navigation
    performance. With a highly accurate chronometer (to the 100th of a
    second) supplying Greenwich Mean Time (GMT) and the Julian date, along
    with a 61-star catalog stored inside the ANS computer, it was possible
    to know precisely where SR-71 was over the ground.
    Selection of which star to track was made by the ANS computer a
    function of latitude, longitude, day of year, time of day, aircraft
    pitch and roll, and location of the sun. The computer selected a star
    by going through its star catalog, which was arranged in decreasing
    star brightness until it found a star. A telescope-like star tracker
    looked for the stars in an expanding rectangular spiral search pattern.
    The ANS window was located on top of the fuselage, just forward of the
    air refueling door and consisted of a round piece of distortion-free
    quartz glass (about 9 inch diameter) that allowed the star tracker to
    see through.
    On the cockpit ANS panel a star �ON� light indicated that a mini-mum of
    two different stars had been tracked within the last five minutes. Star
    tracking was automatic. However, the RSO could assist the system in
    overcoming conditions such as overcasts, changes of sky background
    brightness, long periods of ground time, and air refueling when the
    boom obscures the tracking window. Former RSO, Col. Phil Loignon (Ret),
    recalls a sortie he flew over North Vietnam that changed future ANS
    Jim Watkins and I launched on a operational sortie. We had solid cloud
    cover to 60,000 feet and no star lock on at coast in. A viewsight fix
    revealed a position error, so I updated the ANS. After exiting North
    Vietnam, the �STAR� light came on, and our track showed a 10 nautical
    mile error. The inquisition hy the 15th Air Force following that was
    some-thing to hehold. We had flown over Hanoi instead of 10 miles away.
    Our error had allowed intelligence to determine that a new device on
    the North Vietnam radar sites was actually an optical device for
    tracking low level fighters. Although I was thought to have �screwed
    up,� Lockheed came through with the determinations that the ANS tracked
    a light bulb in the hangar and had induced a heading error. We changed
    our ANS turn-on procedures as of that date.
    By comparing the position of the stars to their known location, and
    with the exact time of day, the ANS could then compute the aircraft�s
    precise position. A normal gyro compass alignment of the ANS required
    36 minutes of warm-up time and provided the SR-71 with great-circle
    navigational accuracy of 1,885 feet (0.3 nautical mile) for up to ten
    hours of flying time. It still amazes me even today that astronomers
    have chart-ed our solar system so accurately that it allows the ANS to
    calculate the SR-71�s position so precisely. Things may change here on
    Earth from cen-tury to century, but the same stars guided both
    Christopher Columbus and Habus.
    The heart of the ANS was a large, self-contained unit�about half the
    size of a large refrigerator�called the Guidance Group. A computer
    inside the Guidance Group computed auto-navigation, guidance and
    avionics control, and maintained a continuously updated account of
    nav-igational status and coordinate values. The computer also stored
    instru-ment and mathematical coefficients, predetermined data
    references that defined the stars, and the mission flight plan. For
    continuous accuracy. the computer initiated and evaluated self-tests
    periodically throughout the flight. Software corrections to the star
    data were provided for the supersonic shock wave over the star tracker
    window that refracts the star light and for pressure and temperature
    gradients acting on the window causing optical lens effects.
    The aircraft�s flight plan and sensor operation for the entire mis-sion
    were contained on a wide tape punched with holes and loaded inside the
    Guidance Group computer memory. The tape was made by the 9th SRW�s
    Mission Planning Branch, a group of highly experienced Air Force
    officers who knew how to plan SR-71 missions down to the finest detail.
    Many former SR-71 RSOs worked as mission planners to provide
    exper-tise. As the tape ran inside the Guidance Group, the pattern of
    holes �told� the aircraft where to navigate, what bank angle for turns,
    when var-ious sensors were to turn ON/OFF, and where to have the
    sensors �look� for intelligence gathering.
    Prior to every flight, ANS maintenance personnel loaded the tape and
    ran the Guidance Group in their shop to insure the programming was
    correct. The Guidance Group was delivered to the aircraft several hours
    before flight. It was hoisted up by a crane assembly and slowly
    low-ered into its air conditioned bay located directly in front of the
    air refuel-ing door. Once inside its bay, numerous electrical, air
    conditioning, and computer connections were completed, mating the
    Guidance Group to the aircraft. An exterior aircraft panel containing
    the star tracker window bolted over the Guidance Group.
    The RSO had all the ANS controls in his cockpit. On the ANS panel, the
    RSO had a constant digital readout of longitude and latitude, wind
    direction and velocity, time to turn, and distance to the next turn
    point. By use of his keyboard a variety of other information was
    available from the ANS display panel, such as ground speed and true air
    speed. As long as everything was working satisfactorily, the RSO
    monitored the readouts to insure their accuracy. At any time, the RSO
    could manually override the ANS�s preprogrammed flight path and sensor
    action points, if required. It was an automatic abort if the ANS wasn�t
    working correct-lv, and since Don had first-hand knowledge of that, he
    had total respon-sibility in making abort decisions concerning our
    navigational accuracy. If we were in clouds or couldn�t achieve a
    satisfactory star lock-on, the SR-71 navigated by an inertial-only
    guidance system. The inertial system had to be aligned and was updated
    automatically by the ANS when it was navigating normally. By using fix
    points every hour, the inertial-only system maintained a navigational
    accuracy of two nautical miles per hour.
    On one occasion Don flew with another pilot, then Lt. Col. Bob Crowder
    (Ret), to take the SR-71 to a remote island in the Indian Ocean, called
    Diego Garcia. It was a test of Det l�s capability to support and fly
    reconnaissance missions out of a bare-base, remote island. Diego
    Garcia�s strategic naval location was also gaining popularity with the
    Air Force as a staging base to fly B-52s and tankers from. We wanted to
    prove our capability to fly the SR-71 from there as well. One of
    Beale�s SR-71 shel-ters was secretly torn down overnight and erected on
    the island days later, JP-7 was shipped in and stored, and large
    ground-handling equipment was flown in from Det 1 on C-141s. An advance
    party of Det 1 maintenance people were sent to receive the aircraft.
    Don really didn�t want to go on the trip because he had just planned
    his big Lieutenant Colonel promotion party about the same time he was
    due to return from Diego Garcia. His room was all ready for the party,
    so he left me with last minute instructions to have his promotion party
    regardless of whether he was back in time or not.
    After several air refuelings, the aircraft arrived in good shape.
    Although no sorties were flown out of Diego Garcia, maintenance
    prac-ticed all the necessary routines in order to prepare the aircraft
    for flight. After everyone was confident that the right equipment and
    supplies were in place to carry out numerous sorties, the return flight
    back to Okinawa was planned. Shortly after takeoff, Don�s ANS went
    haywire, and they returned to Diego Garcia. Navigation to each air
    refueling track was over thousands of miles of open ocean and required
    a high degree of reliance solely on the ANS. On their second attempt
    they had to abort on the ground, again because of a bad ANS. On the
    third try, Bob and Don were prepared for an alternative means of
    navigation in case the ANS failed a third time. It was called dead
    reckoning! The ANS did fail inflight; however, they successfully
    proceeded on to all their air refuelings by basically pointing the nose
    of the aircraft straight ahead. All the crews met Don at the aircraft
    and told him his party was �gang-busters!� He was one day late.
    Neat stuff!

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