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WHAT KIND OF SIGNALS DO WE LOOK FOR?
The best communication method known to us is through radio waves.
Radio waves have the information carrying capacity and they can be
transmitted using equipment that is cheap to build. The information also
travels at the speed of light. Moreover we can receive signals from various
directions and communicate simultaneously with many different
civilizations.
The electromagnetic spectrum is very large and the signals can be sent at
any frequency. So we should find a reasonably small region of it to begin
our search.
The galaxy produces noise at low frequencies and the atmosphere at
higher frequencies. Between the two noisy regions we have a relatively
quiet region -- from about 1 GHz to about 10 GHz.
Neutral hydrogen gas emits radio signals at 1.42 GHz and the hydroxyl
emits signals at around 1.64 GHz. We know that the water molecule is
composed of these two species. Accepting the fundamental need of water
for life to exist, we find ourselves with a frequency range between these
two emissions which is a quiet region of the spectrum and is termed "the
water hole." This gives us a nice limited frequency range to start our
search.
Nature of the spectrum of the signals we look for:
A message transmitted on many frequencies is not efficient as it consumes lots of power. However, if one selects a very narrow frequency bandwidth to concentrate the power of the signal, then the signal can be transmitted easier through the background noise. Considering the distances over which these signals must travel before they reach our planet, we expect intelligent extra-terrestrials to send a very specific frequency message.
To distinguish these signals from local earth-based signals, we recognize the fact that local signals maintain more or less constant intensities with time. However in case of a SETI telescope, it is the sky that is seen to drift past the focus of the telescope. In 12 seconds a target typically crosses the focus (target beam) of the telescope. Thus an extraterrestrial signal is expected to grow in intensity and then diminish over the 12 second time period. This shape is described by the Gaussian curve.
Due to the relative motion of planets, we are likely to observe a Doppler shifting or frequency shifting of the signal due to relative motion of the telescope during the 12 second period. This would cause the signal to rise or fall in frequency slightly over the time length of 12 seconds. These signals are called chirped signals. Also if alien civilizations are sending information our way then the signals are surely going to be pulsed in nature. Thus we may refine our search by checking for chirped signals containing pulses.
