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    Re: automatic celestial navigation
    From: Greg R_
    Date: 2008 Jan 9, 12:11 -0800

    Oops, forgot to add: Excellent article, thanks for posting it to the
    list.  :-)
    
    --
    GregR
    
    
    --- Dan Allen  wrote:
    
    > 
    > 
    > On Nov 28, 2007, at 11:25 PM, Paul Hirose wrote:
    > 
    > > Interestingly, that paper says the SR-71 astro-inertial unit had a
    > > catalog of 57 stars. I wonder if those were the same 57 stars
    > listed  
    > > in
    > > the nautical almanac.
    > 
    > I was reading today in Richard H. Graham's excellent "SR-71 Revealed:
    >  
    > The Inside Story" (Motorbooks, 1996) and came across this info where 
    > 
    > he -- an SR-71 pilot and squadron commander, and retired head of all 
    > 
    > SR-71s, so he is an authority -- states there were 61 stars in the  
    > SR-71 catalog.  Here is an extract from pages 65 and 66 of his book:
    > 
    > ---
    > 
    > Navigational Systems
    > 
    > 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 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 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 correctlv, and since Don had first-hand 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 lock-on, the SR71 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.
    > 
    > ---
    > 
    > Exciting!  I wish I had a pocket star tracker.
    > 
    > Dan
    > 
    > 
    > 
    > > 
    > 
    
    
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