@Peter wrote The argument for certified OUT v. IN, in the absence of any regulatory requirement for either, is interesting. Very little GA uses either. I agree, but that’s not the whole point. The question, as I see it, is what proportion of light GA threats has ADSB (or similar). I have certified ADSB-out and a cheap portable uncertified “-in” device, which detects ADSB (including SIL=0),...