Quindar tones
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Quindar tones, most often referred to as the "beeps" that were heard during the American Apollo space missions, were a means by which remote transmitters on Earth were turned on and off so that the Capsule communicator (CapCom) could communicate with the crews of the spacecraft. It was a means of in-band signaling to simulate the action of the push to talk-release to listen (commonly referred to as PTT) button commonly found on 2-way radio systems and walkie-talkies.
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 thus subject to noise and crosstalk when the channel was quiet. A VOX-based keying system would result in false keyups that would annoy the astronauts or disturb their sleep.
This meant that the muting audio 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 keying.
Implementation of Quindar tones
Two tones were used in the Quindar system, named after its manufacturer, both being pure sine waves that were 250ms long. The "intro tone" was generated at 2,525 Hz and signaled the "key down" keypress of the PTT button and unmuted the audio. The "outro tone" was slightly lower at 2,475 Hz and signaled the release of the PTT button and muted the audio. The two tones were generated by special equipment located at Mission Control, and they were decoded by detectors located at the various tracking stations.
The selection of the tones allowed them to travel in the same passband as a human voice, which has a range from roughly 300 Hz to 3,000 Hz.
Common misconceptions about Quindar tones
There are two common misconceptions surrounding Quindar tones. The first is that one tone originated on Earth, while the second came from the transmitters used by the astronauts while in space. This confusion exists because many ground-to-space transmissions were initiated by Mission Control and responded to by the astronauts. In this sequence, the CapCom would press the PTT which would send the intro tone, then he would speak. When finished, the CapCom would release the PTT which would send the outro tone, and the astronauts would respond to Mission Control. Therefore, those transmissions would consist of a "beep", followed by Houston talking, then another "beep", and the voice of the astronauts.
Another misconception about Quindar tones is that they were designed to signal the end of a transmission, similar to a courtesy tone used on many half-duplex radio repeaters. This wasn't necessary because the Apollo Unified S-Band System was full-duplex. (This discussion does not apply to the distinct half-duplex VHF-AM voice system used during launch and landing.)
Like cellular phones, the ground and the spacecraft each transmitted continuously on separate frequencies. Both ends also continuously transmitted their FM voice subcarriers. Nothing (other than the desire to avoid confusion) kept an astronaut and a Capcom from speaking and being heard at the same time. This often happened because of the radio signals' 3 second round trip time between the earth and the moon.
The sole purpose of the Quindar tones was to mute the uplink audio to avoid annoying the astronauts when the Capcom had nothing to say. The tones were needed because the muting function had to be placed at the remote uplink transmitter to mute noise on the telephone circuit from Houston to the uplink transmitter as well as background noise in Mission Control. The astronauts didn't even hear the Quindar tones, as they were filtered out before transmission.
A modern digital replacement for the analog Apollo system would have no need for Quindar tones. A digital transmission circuit from Houston to the uplink transmitter would add no noise to a quiet channel, and background room noise in Houston could be cut off with a local switch. Even if there were a need for remote control of a transmitter function, it could be done with an inaudible out-of-band digital signal.
Origin of the name
Quindar tones were named for the manufacturer Quindar Electronics, Inc. Glen Swanson, historian at NASA's Johnson Space Center who edited the Mission Transcript Collection, and Steve Schindler, an engineer with voice systems engineering at NASA's Kennedy Space Center, confirmed the origin of the name. "Quindar tones, named after the manufacturer of the tone generation and detection equipment, are actually used to turn on and off, or 'key', the remote transmitters at the various tracking stations."[1]
References
- ^ "The Story Behind the Beep". The Mission Transcript Collection. 2006-01-01. Retrieved 2009-08-17.