I wonder if this would be possible to solve by going "the GPS way". As far as I understand how the system works, GPS signal as received on the ground is 20dB below thermal noise. We can still find and amplify it because we know the seeds and algorithms of random number generators on the satellites. But if you were an alien that just came to Earth, you wouldn't be able to find those transmissions. To read them, you need to know that they are there and what to look for (i.e. have a synced RNG).

I'm not sure if these move fast enough to chase each other down; I bet it takes a while to get up to elevation. Even if that works it's still the same cost to shoot down as it is to replace.

    when your entire purpose is to transmit a signal