A bulldozer or lawnmower nearby, however, obliterates the guiding echoes. That's the difficult part. And the universe is noisy. Most SETI searches, including Project Phoenix, have concentrated their efforts on one slice of the spectrum in the microwave region where the universe is relatively quiet. But even here the radio signals of alien civilizations would be buried in background noise.
Computers filter through the noise looking for patterns indicating an intelligent origin.
Naturally produced radio signals mix in a wider range of frequencies. Of course, lots of narrow band, intelligent radio signals are bouncing around from TV stations, cellular phones, aircraft radar, satellites. The next step is sifting out all the human-made signals from on and around Earth. Project Phoenix uses several methods for that. One is to use two widely separated radio telescopes to observe.