The Need For Quindar Tones
For Mission Control (in Houston, Texas) to stay in continuous contact with the astronauts as they travelled to and from the Moon, NASA used several tracking stations around the world, switching from one to the next as the planet turned. Each station had an Apollo Unified S-Band (USB) earth station connected to Houston with dedicated telephone lines. The USB system was full duplex but microphone switching was still needed to block local conversations and background noise from being transmitted. The astronauts needed only push-to-talk or VOX switching, but this was insufficient at Houston because the telephone lines connecting it to the tracking stations were analog and subject to noise and crosstalk when the channel was quiet. This could annoy the astronauts or disturb their sleep.
This meant that the mute switch that controlled the Houston-to-spacecraft audio had to be located at the tracking station transmitter, and the purpose of the Quindar tones was to operate this switch remotely. The same system was used in Project Gemini and were still in use with half duplex UHF Space Shuttle communications for transmitter RF switching.
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