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Wonder if those ANS units have been declassified and are now available
on the surplus market? Although I think the inertial part of it might
be a little pragmatic on a sailboat... ;-) (Though maybe it actually
*could* actually track the various pitching/rolling/etc. movements?).

--
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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