Using the Moon as an Echo [video]
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The Arecibo’s site origins trace back to the 1950s, when Cornell University proposed its construction to the Department of Defense’s Advanced Research Projects Agency (ARPA), which is today known as the Defense Advanced Research Projects Agency (DARPA). A desire to better understand the composition of the ionosphere and how it might impact objects passing through, including ballistic missile reentry vehicles carrying nuclear warheads, was a key reason for its construction. At the time, ARPA was in charge of a broad ballistic missile defense program known as Project Defender. It was believed that nuclear warheads would produce a distinct signature when reentering the atmosphere, making it possible to distinguish them from decoys, so long as that signature could be quickly identified and categorized. The plan was to use the Arecibo Telescope to help gather general, but still valuable information about the ionosphere in support of this effort.
<https://www.twz.com/37898/collapsed-arecibo-radio-telescope-...>
Lunar bounce canons could become an entire form arising from that 2.7sec constraint.
What they are beaming at the moon are not mechanical sound waves (which obviously won't cross the vacuum that separates earth and moon) but radio waves (= electro magnetic radiation)
Since we are talking about audio waves at both input and output of the demonstration ...there is presumably a conversion of audio waves to radio waves -- possibly using some form of modulation like AM or FM to encode the audio information.
Assuming it is FM modulated, given that the return signal has all kinds of doppler effects due to relative velocities ... we can't simply tune into the base frequency of the FM Modulation carrier wave ... but need to correct it with each passing minute maybe.
And the frequency modulation encoded on top of the carrier wave should also be scaled back proportionally based on doppler correction.
Once we do that though...why wouldn't the echo sound identical to the origibal?
I didn't quite follow why there is still a change in pitch of the audio between input and output.
Did they simply skip the second step of error correction described above and are showing the resulting distortion in recovered audio after FM demodulation?
The amount of doppler depends on what frequency they are at and where the moon is. One can set an offset between RX and TX frequency, but one doesn't have to.
From the title I thought it might be about using lasers against lunar retroreflectors, passive arrangements of precise mirrors that (usually) reflect light back to its source.
https://en.wikipedia.org/wiki/List_of_retroreflectors_on_the...
It's a quite well know "ham radio thing", although it's mostly "empty", and few hams actually use those modes.
I haven't managed a two way contact because there is some broadband noise on 2m in the same direction that the moon rises that raises my noise floor in that direction by more than 10dB.
I've made a fair number of contacts via meteor scatter, however.
Did you connect with the Italy station before the EME to see if they could hear you, or did you just broadcast out into the void hoping for a listener somewhere else?
If I ever manage to squash my noise problem, I may bother trying to schedule some contacts as it's not unusual to do so for EME. But right now I don't want to waste people's time when I know that I'm deaf.
(It's really obvious that I'm deaf too, because I can just rotate the antenna and watch the noise floor drop).
But in short, you use a modulation technique where your short messages are sent very quickly, e.g. in 72ms. You then transmit in burts 15 seconds long, repeating your 72ms message over and over again. Hopefully a meteor zips by, if it does far away stations will get a signal from you.
Parties take turns transmitting in either even or odd timeslots and listening in the other. Communication goes pretty slowly but it goes. Every morning around sunrise the part of the earth you are on is facing into the earths orbit and there are a few meteors. It works better during showers.
The receiver for these modes is computationally expensive: there is no time for a fixed synchronization pattern like most digital communications methods use, so instead the rx side does a brute force search.
My first meteor scatter contacts were with a high gain vertical omni, but a directional antenna works much better.
The geometry of a meteor scatter path allow contacts that go about 2000 km but 1000km is easier and more common.
Participation is a limiting factor, when I'm working msk144 outside of a meteor shower I may only get one or two contact in a day or none at all. And it's not unusual to for a normal exchange (which is just CQ MYCALL MYGRID; MYCALL THEIRCALL THEIRGRID; THEIRCALL MYCALL ROGER; MYCALL THEIRCALL BESTREGARDS) to take an hour-- depending on the luck of the falling rocks.
Only later was it revealed that once in a blue moon (heh), things were aligned so that the US could use bounces off the moon to detect and analyze a certain class of Soviet radars.
Those periods of use for the radio telescopes in question were booked well in advance.