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A proposal to detect an extraterrestrial intelligence radio broadcast







Today directional antennas are being pointed towards stars and star clusters of our galaxy, searching for radio broadcasts by alien civilizations.

This texts describes a method that would have some advantages:
And some drawbacks:
The principle is the radio signal broadcasted by the extraterrestrials must be indefinitely sub-modulated. Say the carrier signal would have a frequency of 1 GHz. Well it would be the modulation of a 1 MHz signal. That 1 MHz sub-carrier would in turn be the modulation of a 1 kHz signal. That 1 kHz sub-carrier would be the modulation of a 1 Hz signal. That 1 Hz sub-carrier would be the modulation of a 1 mHz signal. That 1 mHz sub-carrier would be the modulation of a 1 µHz signal. That 1 µHz sub-carrier would be the modulation of a 1 nHz signal (that is a signal with a period of 31 years). And so on. Down to signals with a period of millenniums, millions of years... Whatever period an extraterrestrial civilization would last (and ours).

The receiver, we Earthians build, would be an omnidirectional antenna that feeds a 1 GHz receiver. The output of that receiver would be fed to a 1 MHz receiver. The output of that second receiver would be fed to a 1 kHz receiver. The output of that third receiver would be fed to a 1 Hz receiver. And so on towards a 1 mHz receiver, 1 µHz, 1 nHz... The first stages of this system do transmit apparently pure noise to their next stage. Yet at at one stage - if the broadcast signal exists -a clear signal will appear. Depending of the broadcast strength and the distance. Each next stage produces a signal with a 30 times better signal/noise ratio. But maybe the signal will become detectable out of the noise only at the fifth or the tenth stage. That's why this experiment has to be carried out for a long time. To give the very low frequency receivers a chance. If the signal exists this system will detect it. But noone can tell wether it would be after an hour receiving or after a thousand years.

Of course there is weak probability the extraterrestrials would have chosen a sub-frequency division factor of 1000 like in the example above. Do they have ten fingers ? Most probable is they choose a power of two, like 128, 256, 512, 1024, 2048, 4096, 8192, 16384, 32768, 65536, 131072, 262144, 524288 or 1048576. The higher the factor, the better the transmission yield yet the more it can be disturbed by Doppler effects and the like. All this factors have to be tested out altogether.

Next, it is virtually impossible they choose a carrier frequency of 1 GHz. We will have to test a lot of frequencies below the system's maximum frequency. But not an infinity of frequencies. It depends on the frequency divider being tested. Using a 1000 divider for example, after a trial at 1 GHz gave no result, next frequency to test out would be 999 MHz. Testing 999,5 MHz is of no use. And so on down to say 100 MHz, each time trying out a frequency 1/1000th lower. That makes only 2300 frequencies. The higher the divide factor, the more carrier frequencies have to be tested out. Of course standard carrier frequencies like the hydrogen molecule oscillation should first be tested out. All those frequencies should be tested out at the same time.

Did they choose FM or AM ? At first hand both should be tried out. But AM has a severe drawback : the carrier signal is not of constant amplitude. It will be short pikes of pulsed signal, or imply too weak sub-carriers. FM frequency is a good choice yet with a question : what are the two extreme frequencies ? Probably the two closest frequencies that can best be differenciated. This depends of the sub-carrier divide factor. There need to be one period difference on a number of periods equal to the divide factor. That would be 1 GHz and 0.999 GHz in our base example. Other modulation techniques can also be used, like phase shift modulation. Each modulation stage outputs a constant frequency, yet with a 0° or 180° shift if the input signal is O or I. But I bet for FM.

For high frequencies, the first receiver, for the carrier signal, will have to be hardware. Maybe the second stage too. But lower frequency sub-carriers and of course very low frequency subcarriers can be chain received by computer calculations.

What if they choose different divide factors for each modulation stage ? This could be possible if the signal is intended for encrypted very long distance transmissions at very low baud rates. But in that case we have no chance to catch the signal. There are too many combinations to test out. By the way they would probably have used a directional emission too, with virtually no chance we would be located inside the beam. If the signal is intended to be received by other civilizations I suppose the divide factor will be the same for each modulation stage.

Once a signal would be found, Searching from where out of space it comes is easy. Directional antennas can be used to scan more and more narrow parts of the sky till the emitting star is found (the star around which the planet of the extraterrestrials orbits). The narrower the antenna sighting, the higher the detected subcarrier frequency will be. Till the carrier itself can directly be received. Perhaps carrying key information.

The only terrestrial interferences this system can receive are from similar receivers that would be operating elsewhere on Earth. Indeed its sensitivity is so high it could consider the calculation emissions of a twin system as an emitter. Another source would be an emitter we would build to broadcast towards potential extraterrestrials. So that emitter should emit on a randomly chosen carrier frequency. This implies maybe the potential extraterrestrials did choose their frequency purely at random. Should such modulation system be used for practical purposes like very low baud-rate transmissions of one bit per month using few energy or omnidirectional antennas, or for beacon localization, then chains of sub-frequency divider factors should be used that mathematically cannot disturb the systems trying to receive extraterrestrial signals.

None of the technical choices proposed in this text is for sure. They are just proposals that seemed most obvious. Maybe some are mathematically wrong.


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Eric Brasseur  -  30 July 2002