Noticed this abbreviation on my chart next to the fog horn location at the entrance to my harbor, so I looked it up. Its a system that allows boaters to turn the horn on by way of VHF radio. Makes sense since you might be in a fog bank off shore when its clear at the harbor. Look up the VHF channel for the horn and key the mike five times to turn it on. Its like some airstrips that let the pilot turn on the landing lights via radio. Cool stuff. Half of you probably already know this but I'm posting it for the other half.