Quindar tones
Encyclopedia
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 walkie-talkies.
) 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.
The selection of the tones allowed them to travel in the same passband
as a human voice
, which has a range from 300 Hz to 3,000 Hz.
, 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.
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
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
In-band signaling
In telecommunications, in-band signaling is the sending of metadata and control information in the same band or channel used for data.-Telephone:...
to simulate the action of the push to talk
Push to talk
Push-to-talk , also known as Press-to-Transmit, is a method of conversing on half-duplex communication lines, including two-way radio, using a momentary button to switch from voice reception mode to transmit mode....
-release to listen (commonly referred to as PTT) button commonly found on walkie-talkies.
The need for Quindar Tones
For Mission Control (in Houston, TexasHouston, Texas
Houston is the fourth-largest city in the United States, and the largest city in the state of Texas. According to the 2010 U.S. Census, the city had a population of 2.1 million people within an area of . Houston is the seat of Harris County and the economic center of , which is the ...
) to stay in continuous contact with the astronauts as they travelled to and from the Moon, NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...
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
Unified S-Band
The Unified S-band system was a tracking and communication system developed for the Apollo program by NASA and the Jet Propulsion Laboratory . It operated in the S band portion of the microwave spectrum, combining voice communications, television, telemetry, command, tracking and ranging into a...
(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
Project Gemini
Project Gemini was the second human spaceflight program of NASA, the civilian space agency of the United States government. Project Gemini was conducted between projects Mercury and Apollo, with ten manned flights occurring in 1965 and 1966....
and were still in use with half duplex UHF Space Shuttle
Space Shuttle
The Space Shuttle was a manned orbital rocket and spacecraft system operated by NASA on 135 missions from 1981 to 2011. The system combined rocket launch, orbital spacecraft, and re-entry spaceplane with modular add-ons...
communications for transmitter RF switching.
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. The "outro tone" was slightly lower at 2,475 Hz and signaled the release of the PTT button. The 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
Passband
A passband is the range of frequencies or wavelengths that can pass through a filter without being attenuated.A bandpass filtered signal , is known as a bandpass signal, as opposed to a baseband signal....
as a human voice
Human voice
The human voice consists of sound made by a human being using the vocal folds for talking, singing, laughing, crying, screaming, etc. Its frequency ranges from about 60 to 7000 Hz. The human voice is specifically that part of human sound production in which the vocal folds are the primary...
, which has a range from 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 EarthEarth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
, 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
Unified S-Band
The Unified S-band system was a tracking and communication system developed for the Apollo program by NASA and the Jet Propulsion Laboratory . It operated in the S band portion of the microwave spectrum, combining voice communications, television, telemetry, command, tracking and ranging into a...
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.