The electromechanical angle computer inside the B-52 bomber's star tracker

Before GPS, how did aircraft navigate? One important technique was celestial navigation: navigating from the positions of the stars, planets, or the sun. While celestial navigation is accurate, cannot be jammed, and doesn't require any broadcast infrastructure, it is a difficult and time-consuming process to perform manually. In the early 1960s, an automated system was developed for the B-52 bomber to automatically track stars and compute navigation information. Digital computers weren't suitable at the time, so the star tracking system performed trigonometric calculations with an electromechanical analog computer called the Angle Computer.

The mechanism inside the Angle Computer physically models the "celestial sphere", with a complicated mechanism inside that moves a pointer that represents the position of a star. The corresponding angles (the azimuth and altitude) are read out electrically through devices called synchros, providing information to the navigation system through bundles of wires.

The Astro Compass system

The Angle Computer is one piece of the Astro Compass, a system that locked onto a star and produced a highly accurate heading, accurate to a tenth of a degree. The Astro Compass navigation system was built around the "Astro Tracker", the optical system that tracks a star. The Astro Tracker was mounted on the aircraft with a 4-inch glass dome protruding from the top of the fuselage. This unit contains a tracking telescope, which used a photomultiplier tube to detect the light from a star. A gyroscope and a complicated system of motors provided a "stable platform", keeping the telescope precisely vertical even as the aircraft tilted and moved.

The Astro Compass system consists of 19 components to support the Astro Tracker. These include amplifier and computer components that controlled the system, and control and indicator panels used by the B-52's navigator.

Controlling the Astro Compass

The Astro Compass has an interesting user interface, letting you input one value at a time by rotating a knob. First, you use the Master Control Panel to select a data value such as the clock time, SHA (Sidereal Hour Angle), or Declination for a star. Each knob on the Master Control Panel has a different geometrical shape, allowing the user to distinguish the knobs by feel.

Each data value has a separate electromechanical display. The system has three Star Data displays, so it can hold the positions of three stars at a time. To obtain the information to put into the Astro Compass, navigators used the Air Almanac, which provided celestial data on 10-minute intervals for the sun, planets, moon, and stars.

The navigational triangle: Computing a star's position

The Air Almanac provides star coordinates in a global coordinate system, but the Astro Compass needed to know star coordinates in the aircraft's local coordinate system. Determining the star's position requires changing the coordinate system by using spherical trigonometry and the navigational triangle.

The Astro Tracker is aimed by using azimuth and altitude. Azimuth is the horizontal rotation from north, and altitude is the angle up from the horizon. This system is called the horizontal coordinate system. Because the Earth's rotation makes the star appear to move, these angles constantly vary. The equations for altitude and azimuth are complicated, involving sines and cosines. This computation is performed electromechanically by the Angle Computer by modeling the celestial sphere, where stars are stationary on the surface while the Earth rotates in the middle.