Two-thirds of its 4.5m subscribers receive the channel through a scrambled, over-the-air, analogue signal, not as part of a broader cable or satellite package.
The project directed by the National Geographic Channel and Arecibo Observatory is sending a response to the " Wow signal" some 35 years after the event.
However, if there is a lot of noise (interference) on the channel the receiver is listening to, it cannot detect the intended signal because it is covered by noise.