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 Lone that came on the audio feed 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 all they did was control a switch.

The USB radio system required that a radio carrier be sent to the spacecraft at all times for tracking purposes (sec Chapter 6). 4’his is in contrast to the situation with a walkie-talkie where the carrier is transmitted only w’hen 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 communications network, and his microphone could also pick up nearby conversation. Nor w as 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 allow’ the line from Houston to be fed to the spacecraft only 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. As it would have been expensive to have arranged a separate circuit just to carry a signal to tell the ground station to switch, engineers used a technique called in-band signalling w’hereby the signals to switch w ere not only sent on the same line as the Capcom’s words, they occupied the same audio band. These signals w’ere the Quindar tones.

When he w’anted Lo speak Lo the crew7. Capcom pressed his push-to-talk button. This produced a quarter-second burst of 2.525 kHz tone that w;as placed on the line to the ground station. There, it operated an electronic switch Lo connect the line from Houston to 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.