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Chapter 5: Sociology

Detectability of Spread-Spectrum ETI Signals

Dear Dr. SETI:
As you may or may not be aware, the military ( and increasingly the business community ) are using Spread Spectrum type modulation to transmit voice, data, etc. The point being that the intelligence in the signal is spread widely over a broad range of frequencies rather than concentrated on one frequency ( Amplitude Modulation ) or a very narrow band of frequencies ( Frequency Modulation.) Therefore, you can be listening on one or even several frequencies and still not comprehend an intelligible signal. Also, this technique allows signal detection well below the noise threshhold. Indeed, I am given to understand that with frequency-hopping techniques it is well nigh impossible to detect or at least demodulate the intelligence from the signal. You probably are well aware of all this, but as an amateur organization it is always possible that with time and money pressures an area of signal detection is not yet utilized.
Wishing you the best,


The Doctor Responds:
The point you raise about spread-spectrum is indeed valid. This is an area which has received considerable attention in the SETI community. An editorial on our Web site deals with the issue. See:

Thanks for your thoughts, and interest in SETI. Perhaps you will be one of the people to help find solutions to the signal detectability problems associated with exotic modulation modes.

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