- Before GPS, aircraft used celestial navigation, tracking stars, planets, or the sun manually.
- In the early 1960s, an automated system for the B-52 bomber, the Astro Compass, tracked stars using an electromechanical analog computer called the Angle Computer.
- The Angle Computer physically modeled the celestial sphere with a half-sphere mechanism, solving the navigational triangle to compute a star's altitude and azimuth.
- The system required inputs from the Air Almanac, a book updated every four months, providing celestial coordinates.
- The Astro Compass could also determine the aircraft's position using the line of position technique, drawing circles from sub-stellar points.
- The Angle Computer was a hybrid of mechanical, electrical, and early electronic components, soon replaced by digital computers.