I don't quite understand this: Newcomers to the interference problem sometimes suggest monitoring the spectrum to find vacant frequencies available for use. Th

I don't quite understand this: Newcomers to the interference problem sometimes suggest monitoring the spectrum to find vacant frequencies available for use. This makes sense on the surface, but if you dig deeper you’ll discover that it’s surprisingly hard to do. ... The detectors might also miss activity in bands carrying low-power signals: GPS and other satellite downlinks, celestial sources of interest to radio astronomers, search-and-rescue beacons, distant TV stations, and more. To reliably detect all of these, a sensing device would have to be situated within the beams of directional transmitters, or, for other kinds of radio signals, be as large and sensitive as the receivers used. ---- * Directional emitters: If you don't detect it, then you aren't in its path and it doesn't affect you. The directional emitter could interfere with the destination with which you're communicating, but then the destination could detect it and inform your device. I suppose a third case is it could interfere with signal between source and destination but without affecting either end; I'm not sure what to about that. * Low-power emitters: If the signal is so low-power that your sensor can't detect it without a large, sensitive receiver, how much interference will it cause?