NavList:

Back in the early days of the SR71 reconnaissance plane, there was a star tracking system that could pick up stars while sitting on a runway on an overcast day. It was a very expensive system, and they no  longer use it (replace by GPS). I wonder if it was also IR based?

In any event, here’s the text from Wikipedia:

Astro-Inertial Navigation System[edit]

The USAF sought a precision navigation system for maintaining route accuracy and target tracking at very high speeds.^{[citation needed]} Nortronics, Northrop's electronics development division, had developed an astro-inertial navigation system (ANS), which could correct navigation errors with 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.^{[52][verification needed]}

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 (RSO)'s position, tracked stars through a circular quartz glass window on the upper fuselage.^[38] 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 56 (later 61) stars.^[53] The ANS could supply altitude and position to flight controls and other systems, including the Mission Data Recorder, Auto-Nav steering to preset destination points, automatic pointing and control of cameras and sensors, and optical or SLR sighting of fix 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 1,000 feet off the direction of travel at Mach 3.^[54]

Paul Dolkas