They have a distance filter on warnings, but does not seem to affect display of targets. I had a bit of trouble getting it to receive anything at all, so I really need to check out the cabling. Can't do a good job of that until the mizzen is stepped again next summer. If the VHF and AIS receivers where installed in the same place, a guy could switch antenna leads and see if there was a difference, but its not so easy in my installation. If I put in a new one, I will move it to the VHF location - that would give me redundancy on the VHF antenna as well.