Beep This is Houston beep

One iconic symbol of the 'space programme' was the strange 'beep' that constantly seemed to punctuate the conversation between mission control and the spacecraft. Anyone mimicking or lampooning the spacemen felt the need to pepper their speech with the curious tone that came on the public feed of audio to the press and broadcasters from NASA. Despite their association with something hi-tech, their purpose was rather prosaic. They were called Quindar tones and their purpose was simply to control a switch.

The USB radio system required that a radio carrier was sent to the spacecraft at all times for tracking purposes (see Chapter 6) in contrast to the situation with a walkie-talkie where the carrier is transmitted only when the push-to-talk button is pressed. However, it was not desirable for Capcom's microphone to be constantly live on the uplink to the spacecraft. He had to talk to others in the mission operations control room (MOCR) and indeed to any other site on the net, and his microphone could also pick up nearby conversation. Nor was it desirable for the long-distance line from Houston to the ground station to reach the spacecraft as it was prone to interference. A decision was made to only allow the line to be fed to the spacecraft when Capcom wanted to speak, and so a method had to be found to tell the ground station when he had pressed his push-to-talk button and when he had released it. However, to have arranged a separate circuit just to carry a signal to tell the ground station to switch would have been expensive, so engineers sent Quindar tones on the same line as the one used by Capcom.

When he wanted to speak to the crew and pressed his push-to-talk button, a quarter-second burst of 2.525 kHz tone was produced. This was detected at the ground station, where it operated an electronic switch to place Capcom's line onto the uplink. When Capcom had finished speaking and released his push-to-talk button, another quarter-second burst of tone, this time of a slightly lower pitch of 2.475 kHz, signalled the switch to remove the Houston line from the uplink.

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