Even with EM, they could have a highly advanced encoding format which is barely detectable about the random noise floor (spread spectrum like). US army uses these kind of techniques to make the signal both hard to detect and hard to jam.
Who knows how far you can push this, maybe to the point where it's essentially impossible to detect the signal if you are not the intended target.
Who knows how far you can push this, maybe to the point where it's essentially impossible to detect the signal if you are not the intended target.