Apollo Command Module (CM) Guidance and Navigation System (GNS) Coupling Data Unit (CDU), part of the GNS's inertial subsystem (ISS) that was composed of an inertial measurement unit (IMU), part of the power and servo assembly, part of the controls and displays, and three inertial CDUs. The CDU, which measures 20" x 11.5" x 5" and weighs roughly 39 pounds, features a NASA label on the back: "Apollo G & N System Coupling Data Unit Assembly C/M, Part No. 2007222, Serial No. ACSK-…, Cont. No. NAS 9-497, Designed by MIT Instrumentation Lab, Mfg. by AC Electronics Div. GMC." Bears two "Eval Test" stamps.
During the Apollo missions, astronauts steered the spacecraft using position data radioed from earth, an on-board telescope, and a sextant that measured angles between the earth, sun, moon, and stars. These angles were measured by precision instruments (resolvers) and converted into digital numbers by the CDU that the on-board computer could use. The spacecraft could be pointed in any direction by a series of small compressed gas jets controlled by the computer through the CDU. This positioning, or attitude control, was used to steer the spacecraft toward the moon or the earth, and control the critical attitude during re-entry into the earth's atmosphere.
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