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The main feature distinguishing signals produced by a transmitter from those produced by natural processes is their spectral width, i.e. how much room on the radio dial do they take up? Any signal less than about 300 Hz wide must be, as far as we know, artificially produced. Such narrow-band signals are what all SETI experiments look for. Other tell-tale characteristics include a signal that is completely polarised or the existence of coded information on the signal.
Unfortunately, SETI searches are burdened with confusion caused by narrow-band signals from our own planet. Military radar and telecommunications satellites produce such signals. Project Phoenix uses a second telescope to sort out this unwanted interference. Since the second telescope is hundreds of miles away from the main instrument, an extraterrestrial signal will have a slightly different frequency at the two sites. This is because of the Earth's rotation and the effect of Doppler shift. Looking for the expected slight shift in frequency at the two telescopes is a good way to judge which signals are local, and which are truly extraterrestrial.
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