If you're really concerned about someone being able to tell that there's a sub within 100 nm of the buoy, the solution's simple: drop several buoys, only one of which broadcasts a real message, the other ones just send 'This is a test' or something innocuous. Or drop several buoys all of which send the same message; that way not even the air crew doing the dropping know which location was the real one. Or, better yet, drop multiple buoys... and drop some of them where _attack_ submarines hang out, and give the attack boys pre-arranged orders to go hunting for anyone who might be disposed to go looking for the recipients of the message. If you drop 10 buoys, and one is near a missile sub, and one is near three or four attack subs, and the rest are dropped in empty water, then 80% of the time the opposition is busy searching for something that's not there, and 10% more he's walking into a trap. If you tell the missile sub crew to get clear as soon as they pick up the message, then even the remaining 10% is likely to be a dry hole by the time that the bad guys get around to it. If they ever do.