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Quindar Tones

Copyright © 2004 by Markus Mehring and Bill Wood.
All rights reserved.
Last revised 21 January 2006.


Re: Apollo beeps

Journal Contributor Mark Burckhard writes:

"I've always wondered what purpose the 'beeps' served that one heard intermittently during the voice communications with the Command and Lunar Modules during the Apollo missions, as well as other space missions."

Journal Contributor Mike Dinn provides an MP3 clip ( 123k ) from a network audio check that includes numerous quindar tones.

Journal Contributor Markus Mehring replies:

"'Other space missions' is quite an accurate observation, since the 'beeps', in fact, are still in use today on Shuttle flights, at least on the UHF frequencies."

"These beeps are called 'Quindar-Tones'. Their purpose is to trigger the ground station transmitters when there is an outgoing transmission from Earth. The CapCom in the Mission Control Center, who is taking care of communications with the crew, uses his communication gear in a PTT mode exclusively. 'PTT' is short for Push-To-Talk, which means that the CapCom presses a button every time and as long as he wants to talk. (The crews back during Apollo - and also today - usually communicate via PTT as well, but they also have the so-called 'VOX mode' at their disposal, in which their microphones are voice-triggered by a certain adjustable threshold volume levels. VOX is used when they don't necessarily have their hands free.)

When the CapCom presses his PTT button to start a transmission, an intro tone (2.525KHz sine wave with a length of 250ms) is generated and triggers the ground station transmitters to send. And when he is finished talking and releases the button again, a slightly lower outro tone (2.475KHz, sine, 250ms) is generated to trigger the ground station transmitters to turn off. So in short, these are remote control trigger tones.

CU! Markus"


Journal Contributor Larry Turoski writes:

"A comment in the Apollo 8 Flight Journal indicates that the astronauts in flight could hear the Quindar tones in the ground transmissions. Specifically, Borman comments that the Quindars were clipping the CapCom's speech and requested that the CapCom wait a second or two after keying his mike before speaking to avoid the clipping. However, listening to the lunar surface mission audio, we can clearly hear the echo of the CapCom in the astronauts' headset, but no echo of the Quindars. My question is: could the astronauts hear the Quindar tones in flight?"

Journal Contributor - and Goldstone veteran - Bill Wood replies:

"I was able to find out following information from Frank Sullens, a former Mercury, Gemini and Apollo Air-to-Ground expert, which supplements that found in Markus Mehring's excellent Quindar description above."

"The same basic Quindar remote switching system used in Gemini days was used in Apollo as well. The 'clipping' described by Frank Borman was probably caused by the CapCom talking before the transmit tone was detected by the site Quindar Receiver. Detection of the tone closed a relay to connect the Net 1 voice circuit to the Unified S-band uplink voice subcarrier. This would cause anything said before the relay closed to be lost."

"Now why didn't you hear the unkey Quindar tone bounced back to the ground when the EVA astronauts' headsets keyed their VOX circuits? The Quindar receive modules had a notch filter that removed most of the keying tones from the S-band uplink. The 2525 and 2474 Hz tones were close enough together to be notched out with a single filter."

"While the Quindar filter removed nearly all of the tones, there were cases where you can still hear a little of the unkey tone turned around. This occurred mainly when the audio was distorted a little, causing more of the tone to be uplinked."


Journal Contributor - and former Honeysuckle Creek Deputy Director - Mike Dinn provides examples of Quindar checks done during Apollo 11.