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The Need for Quindar Tones
In order for Mission Control (located in Houston, TX) to stay in contact with the astronauts as they travelled to and from the Moon, NASA used several tracking stations worldwide including the Goldstone Deep Space Communications Complex in California and the Canberra Deep Space Communications Complex in Australia as part of the Manned Space Flight Network. These stations were connected to Houston using dedicated telephone lines, which were very expensive to operate at that time. NASA had the option to build two separate systems for operating the transmitters - one to carry the audio from the CAPCOM and another to carry the control signal for the PTT button - but instead chose to combine these two systems together into a single system to keep down the cost of operating the network.
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 2,525Hz and signaled the "key down" keypress of the PTT button. The "outro tone" was slightly lower at 2,475Hz 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 as a human voice, which has a range from 300Hz to 3,000Hz.
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 so that the other party would know that the transmission was complete, similar to a courtesy tone used on many half-duplex radio repeaters. While a Quindar Tone did indeed serve this function for ground-based stations, it did not do the same for those in flight.