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Re: Extraterrestrial Radar

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Posted by dime/">dime on May 16, 1999 19:15:00 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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