Radio Message Construction
The lack of an established communications protocol is a challenge for METI.
First of all, while trying to synthesize an Interstellar Radio Message (IRM), we should bear in mind that Extraterrestrials will first deal with a physical phenomenon and, only after that, perceive the information. At first, ET's receiving system will detect the radio signal; then, the issue of extraction of the received information and comprehension of the obtained message will arise. Therefore, above all, the Constructor of an IRM should be concerned about the ease of signal determination. In other words, the signal should have maximum openness, which is understood here as an antonym of the term security. This branch of signal synthesis can be named anticryptography.
Also characteristics of the radio signal such as wavelength, type of polarization, and modulation have to be considered.
Over galactic distances, the interstellar medium induces some scintillation effects and artificial modulation of electromagnetic signals. This modulation is higher at lower frequencies and is a function of the sky direction. Over large distances, the depth of the modulation can exceed 100%, making any METI signal very difficult to decode.
Read more about this topic: Active SETI
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