What is a Quindar Tone?
Ever watched footage of the Mercury, Gemini or Apollo space projects? When Houston talks to the astronauts, there is a beep, then some talking then another beep? Yep – those beeps are Quindar Tones. If you listen carefully, the tones are not the same pitch: there are two distinct tones, one at 2525Hz and the other at 2475Hz. They are both 250ms in length…. like these:
What are these tones for? What’s going on? Why the beeps? Well, it all boils down to older technology. Back when they were shooting astronauts into space on top of missiles (some more controlled than others), eventually they got people into orbit. As astronauts orbited the Earth, they needed some way to talk to them, even when their space capsules were not within the line of sight of Mission Control in Houston, Texas. Communications centers and tracking stations were built around the world, each with the ability to talk directly to the space capsule as it orbited on by. Mission Control then had telephone lines to each of these stations around the world. These lines were dedicated lines, and expensive. The tones were used as a method to control when the remotely located transmitter was transmitting, and used the phone lines to send these remote control tones as audible beeps. Both tones originated at Mission Control…. like this:
- Mission control needs to say something to the astronauts in space. They push the push-to-talk switch.
- This send a 2525Hz intro tone to the system.
- The remote communications station receives the intro tone, and turns on the transmitter to the radio antenna aimed at the space capsule.
- Voice communications takes place.
- When done, Mission control releases the PTT switch, and the 2475Hz outro tone is sent, thus turning off the system. The remote transmitter is off.
An example for you is below. Note that the Quindar tones only take place just before and after Mission Control speaks. The astronauts do not initiate any of the tones. They make all radio calls into the “blind” so to speak, hoping that some ground tracking station is picking them up.
Now, you might wonder about the issues here. If an astronaut were to also talk at the same time, they might pick up a Quindar tone on their audio and retransmit it back to the ground and cause all sorts of troubles down on the Earth side of things. Yep – that was a problem(!) so engineers made their best effort to prevent the tones from even reaching the astronauts by placing a filter into the stream of all uplinked audio sent to the capsule. These filters were simple notch filters centered on the tone frequencies…. not perfect, by any means, but it worked, generally.
The name “Quindar”? That came from the organization that invented the system, Quindar Electronics. You can visit their site at: http://www.qeiinc.com/History.aspx to see some of their excellent history.
What now? Quindar tones were used from the early flights of Merucry through the Space Shuttle program. With new methods of telecommunications (i.e. fiber optics, satellite feeds, etc), sending command and control statements to remotely located transmitter sites is a lot easier. There is no need for audible tones these days.