It’s called DSC (Digital Selective Calling), and it allows today’s VHFs to send an automated mayday that alerts the Coast Guard and other vessels to your emergency. The idea is that the red button is pre-programmed with information about your boat so that, in the event of a life-threatening situation, you can send an effective mayday distress call while you’re busy with other tasks.
When you flip up the plastic shield and press the DSC Distress button for a few seconds, your radio will broadcast an alarm and your boat information to other vessels and authorities within 4 or 5 miles of your location. If your VHF radio has a built-in GPS, or is networked to onboard GPS, your position will also appear on any receiving radio’s display. After sending a DSC distress call, your radio will stand by for an acknowledgement on Ch. 70 and also receive calls on channel 16. If an acknowledgement is received, you should select Ch. 16 and advise the caller of your emergency situation. Your VHF display will show the name of the ship responding. If no acknowledgement is received, the DSC alert is repeated every four minutes.
So, here’s the really important part. None of this works unless you program an MMSI (Maritime Mobile Service Identity) into your radio before you need it. Registration is free at http://www.usps.org/php/mmsi_new/rules.php. You’ll enter your boat information, home port info and owner and emergency contact identity, and that information will be recorded and unique to your new MMSI. You then enter this number into your VHF radio and you’re good to go.
A properly-installed DSC Distress feature can save lives and precious crunch time. If you have an old VHF radio without DSC, buy a new one.