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Components and layout of a typical Saturn IB/V instrument unitThe instrument unit was designed by NASA's George C. Marshall Space Flight Center in Huntsville, Ala. IBM manufactured and assembled the instrument unit at its Space Systems Center in Huntsville.

Located between the Saturn S-IVB stage and the spacecraft.

The structure consisted of three 120-degree sections of thin-walled aluminum alloy face sheets bonded over sections of aluminum honeycomb one inch thick.

Major Components: digital computer and data adapter; analog flight control computer; inertial guidance platform; emergency detection system control rate gyros and control accelerometers. Together, they measured acceleration and vehicle attitude, determined velocity and position, and calculated and issued control commands to engine actuators.

The inertial guidance platform sensed the vehicle's acceleration and flight attitude. It sent measurements to the digital computer through the data adapter. The digital computer used the measurements to determine the vehicle's position and velocity. Any corrections required to keep the vehicle on course were calculated by the digital computer.

Correction signals, with outputs from control rate gyros and control accelerometers, went to a switch selector. The switch selector - one in the instrument unit and one in each propulsion stage - decoded the correction signals and passed them to the flight control computer. The flight control computer issued commands to steer the vehicle by gimbaling the engines.

Heat from electronic components dissipated through 16 cold plates lining the interior wall of the instrument unit. An antifreeze-like coolant circulated from a reservoir through the cold plates.

Measurements: several hundred measurements were made by sensors to monitor performance and environmental data during flight. Results were telemetered to the nearest ground station. Measurements were taken of:

• Physical qualities, such as mechanical movements, atmospheric pressures, sound levels, temperatures and vibrations; all of which were transformed into electrical signals.
• Electrical values, such as voltage, current and frequency, which determined stage separation, engine cutoff and other flight control functions.

Telemetry: measurements were routed to telemetry equipment for transmission to the ground. Multiplexing - transmitting messages one after the other, but so fast that they appear to be transmitted simultaneously - was used on most channels. Multiplexing permitted many transmissions on just a few channels.

Tracking: radar transponders onboard the launch vehicle increased the range and accuracy of ground-based tracking systems by replying to pulses of radar energy aimed at the Saturn.

Radio Command: received and interpreted data transmitted from the ground for the digital computer; followed a series of error control steps to verify that the message received agreed with the message transmitted.

Power Supplies:

• Main supply: 20-30 seconds before liftoff, the launch control center transferred power to four 28-volt alkaline silver-zinc batteries.
• Special supplies: five-volt master supply converted the main supply to a reference voltage for the measuring system's signal conditioning modules. The 56-volt power supply provided the voltage for the guidance and control system's inertial guidance platform.

Emergency Detection System: detected any abnormal conditions in pitch and engine thrust during launch.

Each major component was tested individually and as part of its system (guidance and control, electrical, etc.) before the instrument unit was shipped to the John F. Kennedy Space Center.

After testing, the digital computer, data adapter, flight control computer, inertial guidance platform and the power supplies were removed and packaged separately for shipment. The other components remained in the instrument unit during shipment.