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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 and the Julian date, along with a 61 star catalog stored inside the ANS computer, it was possible to know precisely where the SR-71 was over the ground. Selection of which star to track was made by the ANS computer as a function of latitude, longitude, day of year, time of day, aircraft pitch and roll, and location of the sun. The computer selected the 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. It consisted of a round piece of distortion free quartz glass, 9 inches in diameter, that allowed the star tracker to scan efficiently. | |
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| © Copyright Richard Graham | |
