> It's fairly hard to hide when your entire purpose is to transmit a signal.
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).
If you have a directional antenna (which would be needed to actually locate the transmitters location anyway) it's actually quite easy to spot GPS signals. It's only because conventional GPS antennas need to receive signals from all directions at once (or at least half of a sphere) that the signal drops below the noise floor. Any signal which can communicate a reasonable amount of data will have to be high powered enough to be easily detected and located by third parties.
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.
