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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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