To detect polar bears that may approiach camp, I'm building a tripwire syustem that triggers from one to three 12-guage shotgun blanks -- hopefully this will warn me of an approaching bear, give the bear something to think about, and give me time to get the shotgun (loaded with a variety of deterrent and lethal rounds) and get outside the tent -- the worst thing is to be attacked inside the tent. You can see the distance from the tent to the tripwire poles in here.