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    Re: Digital Sextant
    From: Dan Allen
    Date: 2002 Apr 2, 09:44 -0800

    -----Original Message-----
    From: Navigation Mailing List
    [mailto:NAVIGATION-L{at}LISTSERV.WEBKAHUNA.COM]On Behalf Of Paul Hirose
    Sent: Sunday, March 31, 2002 10:16 PM
    Subject: Re: Digital Sextant
    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---.net (Paul Hirose)
    B-2 flight test maintenance, 1987 - 97
    Dan Allen replies:
    Great information - thanks Paul!
    From "SR-71 Revealed, the Inside Story",
    by Richard H. Graham (a SR-71 pilot), p. 65f are more details
    about the astro-inertial navigation system given below:
    The SR-71's high speed and sensitive
    missions demanded a navigational 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
    minimum 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 procedures.
    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 something 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
    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 warmup
    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 charted 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 century 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 navigational
    status and coordinate values. The computer also stored
    instrument 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 mission 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
    expertise. As the tape ran inside the Guidance Group, the
    pattern of holes "told" the aircraft where to navigate, what
    bank angle for turns, when various 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 lowered into its air
    conditioned bay located directly in front of the air
    refueling 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 correctly, and since Don had firsthand knowledge of
    that, he had total responsibility in making abort decisions
    concerning our navigational accuracy. If we were in clouds
    or couldn't achieve a satisfactory star lockon, 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 barebase, 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 shelters 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 practiced 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 "gangbusters!" He was one day late.
    Great stuff!  We need a portable star-tracker!

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