The Electromechanical Angle Computer Inside the B-52 Bomber's Star Tracker

The B-52's Astro Compass is an electromechanical system from the pre-GPS era that tracked stars to calculate the heading and lines of position needed for navigation.

The core device, the Angle Computer, takes a star's declination, LHA, and the observer's latitude as inputs and solves for altitude and azimuth. Inputs include celestial data from the Air Almanac, time, latitude/longitude, and an approximate heading (BATH), with values converging through iterative calculation.

The device's calculation method is an analog mechanism that physically reproduces the celestial sphere.

• Models the celestial sphere using a hemisphere with a radius of 2 5/8 inches
• Mechanically reproduces star positions via a declination arm, latitude arm, and rotational input
• Uses differential gears to offset the effect of latitude changes on other axes
• A slider and azimuth arc output the final altitude / azimuth

On the rear, synchro transmitters, servo amplifiers, motors, and a gear train are connected to output the calculation results as electrical signals. It is a transitional design in which the computation is mechanical but the drive and output are electrical.

Operation is fully automatic but has clear constraints.

• To find stars, it searches roughly ±4° azimuth and ±2.5° altitude using a spiral search pattern
• Altitude is limited to 85° or less due to the azimuth discontinuity near the zenith
• Latitude range is -2° to +90°, with automatic switching between northern and southern hemispheres
• Provides 0.1° heading accuracy and can also be used for position determination via line of position

The coordinate system transitions through SHA, declination, GHA Aries, and LHA into a horizontal coordinate system referenced to the aircraft. This is in the opposite direction from astronomy's right ascension, and star coordinates change over the long term due to precession of the equinoxes.

Ultimately, this device stands as a prime example of a sophisticated electromechanical analog computer that solved the navigation triangle using gears and motors during an era when digital computers were still expensive and slow.