space geek today.

i always wondered the exact technical reason for the 'beep' you always hear in space communications. i first nocited it on the Apollo missions but i believe nasa still uses it to this day. you know the sound, it sounds like this. call me an audio geek, but that sounds like .5 seconds or so of a 2.5Khz sine wave. swell. i know what it is, but what is it for? this sure answered my question:

Subject: Apollo era space communications

Question
While growing up watching the Apollo missions, I noticed that there was always this beep that could be heard as the astronauts communicated with the ground. I was periodic but I don't recall if it occurred at regular intervals or not. You could hear them talking and then hear this "beep". Do you know what this classic "beep" was all about? I've always wondered but never heard the answer. Thanks.


Answer
Hi Jeff.

Thanks for the question. What a great question!

An HF or High Frequency Radio draws quite a bit of power, to give you an idea, I intercepted a radio transmission from Houston to the ISS just the other day on an Airplane equipped with this HF radio. This radio draws about 5.5 amps just to "power it up" if the signal draws even throughout the transmission the reciever on the other end recieves it at 5.5 amps, this is where you get the term, " I have you five by five".
Now to answer your question, when drawing this amperage the HF has a tendency to just draw a lot of power in the first second, The signal at this point is weak, and when the power is full this would indicate it would be okay to talk. However, if you just pressed the button and started talking it would be hard to reach you, even in space.
So, To indicate full power in the HF, ALL spacecraft have this beep installed, just to indicate when the radio has reached its full potential, and in your question, when the radios reached their full potential, on the moon, the beep would sound off just about 2-3 seconds delayed due to the small power that the landing module could provide.

I hope that this explains your question.

let us know if you need any other questions asked, and once again thanks for the question, and dont forget to rate us!!!


Joel Osborne

(thanks to joel osbourne and about.com)