Short answer: It's automated By Melissa Klein
The U.S. Coast Guard, which is responsible for maintaining the equipment in roughly 400 lighthouses across the country, has been using the same fog detector for more than two decades. It uses a projector to shine light across a given optical path, then measures and interprets the back scattered light. When the detector senses a drop in visibility, the unit sends a signal to the lighthouse’s electronic equipment, which then signals the foghorn to blow.
But many of the old projectors are breaking down, and spare parts are no longer being made, according to the Coast Guard. They are also prone to misfiring, causing the horn to sound continuously in clear weather. So lighthouses are switching over to the Mariner Radio Activated Signal System, which allows boaters to activate foghorns themselves when they need help navigating during inclement weather. Using a standard VHF radio, the mariner goes to a designated channel and keys (or taps on) the microphone five times in a row. A receiver on the foghorn in the nearest lighthouse will then trigger its distinctive moan to sound for 60 minutes. That earsplitting signal can be heard for up to a half mile.
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