Tuesday, May 1, 1962 – Digital coding, Gemini rendezvous and radar schemes

A working group made of representatives from Goddard Space Flight Center and the Manned Spacecraft Center has formed to study making Project Gemini telemetry be transmitted fully by Pulse Code Modulation (PCM) systems. This follows a briefing from Lockheed on the system. Pulse Code Modulation is a method of numerical representations of samplings of an original analog signal. Human speech has been transmitted on such systems in experimental equipment as long ago as 1926, and was used — in conjunction with a vocoder, developed by Bell Labs, and with randomized thermal noise recorded by the Muzak Corporation — to provide secure high-level Allied communications during the Second World War.

Goddard has previously examined PCM systems for ground stations, and believes that such a system could in future handle NASA’s needs for more channels of information, higher data rates, greater accuracy, and lower-weight equipment. Based on Lockhed’s briefing it is thought a full PCM telemetry system, for both ground and flight equipment, could be completed and put into place for the Gemini Program. The Gemini Project Office approves a PCM Instrumentation Working Group, formed between the Manned Spacecraft Center and the Gemini Project Office, to take responsibility for implementing the PCM system.

Another major objective of the Gemini Project will be the rendezvous with Agena Target Vehicles. The Air Force Space Systems Division has awarded a letter contract to Lockheed Missiles and Space Company for eight Agena vehicles to be modified into Gemini Agena Target Vehicles. The mission requirements are that the target vehicle be able to establish a circular orbit, to provide a stable target for the Gemini spacecraft to rendezvous and dock, to respond to commands from either ground stations or the spacecraft, to perform complex orbital maneuvers by either real-time or by stored commands, and to have an active life on orbit of five days.

Lockheed believes that these mission requirements will require several adaptations to the Agena vehicle: modification of the propulsion system and the addition of a secondary system — two 16-pound and two 200-pound thrusters — for ullage and minor orbital changes; the design of a digital command and communications subsystem which will need to include a programmer, controller, telemetry system, and onboard tape recorder; changes for the guidance and control functions unique to the Gemini Agena Target Vehicle; and the addition of an auxiliary forward equipment rack, which will support the target docking adaptor. Space Systems Division had authorized Lockheed on March 19 to proceed with the Gemini-Agena program.

Finally, McDonnell has proposd a rendezvous evaluation pod, ejected from the Gemini spacecraft on orbit, to be a tool for evaluating the Gemini rendezvous radar and maneuvering spacecraft on early flights, before the Agena Target Vehicle is ready. A decision from the Manned Spacecraft Center about whether to proceed with designing and developing such a rendezvous pod is expected soon.

Author: Project Gemini Chronology

The Project Gemini Chronology is drawn from multiple sources, but most heavily from NASA's Special Publication-4002, Project Gemini Technology and Operations: A Chronology, prepared by James M Grimwood and Barton C Hacker, with Peter J Vorzimmer. Other chronologies will be used where fitting, such as (particularly) those for Project Mercury and Project Apollo.

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