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Re: Extraterrestrial Radar

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Posted by Joseph Lazio/">Joseph Lazio on August 9, 1998 09:55:54 UTC

: I have a question as to the practical approach to listening for extraterrestrial radar. Deep space radar signals would operate at short wave lengths, use polarization methods and transmit short pulses. Can SETI listen for these signals?

It depends. There's not one single SETI program. There are a number of different SETI programs, run by different institutions. For instance, the Planetary Society runs one, and the SETI Institute runs another. These different programs search for different kinds of signals.

You're right that there are a number of different parameters about which SETI researchers must worry. Since we don't know what ETs (if any exist!) have chosen, SETI researchers look at various examples from terrestrial communication systems, consider the limits imposed by interstellar communication, and try to pick parameters that make sense. That doesn't mean that they've picked correctly, but in the absence of any other information, we don't have any better options.

Most SETI programs do not look for short pulses. Rather they look for very narrowband signals. This choice is made by analogy with terrestrial signals. Terrestrial signals are often transmitted with a carrier wave, which can be quite narrow in frequency. Because the signal is narrowband, it transmits very little information in and of itself. However, we know of no natural celestial sources that produce signals as narrow as typical terrestrial carrier wave signals. Thus, if a SETI program ever finds a narrowband signal, that is in and of itself strong evidence for an intelligent origin of that signal.

: Also Considering that radar systems range over a spectrum of 1.2 GHZ to 10 GHZ or perhaps even higher, could SETI bandwidth be too narrow to find such a signal?

Again, the exact amount of the spectrum that is searched depends upon the SETI program. Many programs focus on the part of the spectrum between 1.4 GHz (near where hydrogen emits a particular kind of radiation)and 1.7 GHz (where the OH radical emits radiation). This particular part of the spectrum is called the "waterhole" because H and OH are the components of water, which is thought to be essential for life.

There are other programs which consider even larger parts of the frequency spectrum. For instance, the now defunct NASA program was going to search between 1 and 10 GHz.

However, there are no guarantees. ETs could be transmitting at entirely different frequencies than what we think. : Finally could the search for a signal with a intelligent message be the wrong approach?

Again, as I indicate above, that really isn't the approach that's taken.

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