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Suppose there was a civilization around a nearby star that was deliberately taking steps to make itself less detectable (for whatever reason). How hard would it be to conceal itself from present SETI searches, assuming said civilization had technology no more advanced than ours? Looking around various papers on the internet, it looks like the most detectable component of our radio emissions would be high power radars, like those used for air traffic control. According to one paper I found, "decades ago, it was thought that ETI would have an easier time spotting signals from over-the-horizon radars built during the Cold War... But those have since been superceded by frequency-hopping 'spread spectrum' broadband radars that are undetectable by ETI." Is this accurate? If such a civilization used frequency hopping radars, would such signals be undetectable to SETI? I would guess that present SETI searches need long integration times, so a detectable signal would have to be long duration, and a signal that kept changing frequencies wouldn't be detected. Online Reader The Doctor Responds: You raise a very good question, Reader, and I'm pleased to see you've done your homework! The fact is, there are many things an extraterrestrial civilization can do to make themselves undetectable, should they wish to. But, spread spectrum itself is not a sufficient answer. It is a common misconception that direct-sequence spread spectrum (DSSS) signals cannot be detected without having the sequence key. To show that this is untrue, take a look at this image of a DSSS 1227.6 MHz GPS L2 P code signal, modulated on a +8.1 dBW carrier at a 10.23 MHz chip rate, with a resultant bandwidth of 20.46 MHz. This is the "secure" military GPS signal that's supposedly undetectable. Yet, you see it here, received by my Project Argus station! Truth be told, there is no way for me to decode this signal without the proper algorithm, but detection is another matter altogether. And, this detection was done with a standard Fast Fourier Transform (FFT) algorithm. For even more complex (and more widely spread) modulation modes, a nonlinear adaptive transform such as the KLT would have to be used. These are very computationally intensive, but certainly not beyond our capabilities, even on backward Planet Earth. Yes, the decommissioning of our old cold-war over-the-horizon search radars, plus the transition from high power terrestrial TV to cable and low-power digital satellite distribution, have made Earth less visible than we once were. Other civilizations may be getting dimmer in the RF spectrum, which may make our job of detection harder -- but not impossible. Now, what if a civilization decided tomorrow to hide under a rock? If they ceased all electromagnetic emissions, they would indeed become radio-silent. But, if they did so right now, and were, let's say, 100 LY distant, they wouldn't go quiet in our receivers for another 100 years! So in SETI, as in comedy and so many things, timing is everything.
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