Editing Lockheed SR-71 Blackbird (section)

===Astro-inertial navigation system{{Anchor|Astro-Inertial Navigation System|Astro-inertial navigation system}}===
Nortronics, [[Northrop Corporation]]'s electronics development division, had developed an [[astro-inertial guidance]] system (ANS), which could correct [[inertial navigation system]] errors with [[Celestial navigation|celestial observations]], for the [[SM-62 Snark]] missile, and a separate system for the ill-fated [[AGM-48 Skybolt]] missile, the latter of which was adapted for the SR-71.<ref>Morrison, Bill, SR-71 contributors, Feedback column, [[Aviation Week and Space Technology]], 9 December 2013, p. 10</ref>{{verify source|date=February 2012}}

Before takeoff, a primary alignment brought the ANS's inertial components to a high degree of accuracy. In flight, the ANS, which sat behind the reconnaissance systems officer's (RSO's), position, tracked stars through a circular quartz glass window on the upper fuselage.<ref name="Shul"/> Its "blue light" source [[star tracker]], which could see stars during both day and night, would continuously track a variety of stars as the aircraft's changing position brought them into view. The system's digital computer [[ephemeris]] contained data on [[List of selected stars for navigation|a list of stars used for celestial navigation]]: the list first included 56 stars and was later expanded to 61.<ref name="manual4-3">[http://www.sr-71.org/blackbird/manual/4/4-3.php "SR-71A-1 Flight Manual, Section IV, p. 3."] ''sr-71.org''. Retrieved: 13 December 2011.</ref> The ANS could supply altitude and position to flight controls and other systems, including the mission data recorder, automatic navigation to preset destination points, automatic pointing and control of cameras and sensors, and optical or SLR sighting of fixed points loaded into the ANS before takeoff. According to Richard Graham, a former SR-71 pilot, the navigation system was good enough to limit drift to {{cvt|1000|ft|m}} off the direction of travel at Mach&nbsp;3.<ref>{{cite web|url=https://www.youtube.com/watch?v=CeBu6mRDaro|title=SR-71 Pilot Interview Richard Graham Veteran Tales|work=YouTube|date=6 August 2013 }}</ref>