Man Overboard Electronics: DSC Radios

 red button is for an automated Digital Selective Service distress call

The red button is for an automated Digital Selective Service distress call on Channel 70.

My friend Lee came back from a boat show recently, and started asking me questions about VHF marine radios he’d seen there — ones with with “man-overboard” buttons. I explained that these radios, when hooked up to your GPS, can transmit a latitude and longitude when you push and hold the red emergency button—and that it was for any emergency, not just MOB situations. It led to a broader discussion about how these radios, outfitted with Digital Selective Calling (DSC), actually work. Lee also wanted to know how other electronic devices such as Emergency Position Indicating Radio Beacons (EPIRBs) and Personal Locator Beacons (PLBs) complemented the new radio capabilities.

First a little background. Since the sinking of the Titanic in 1912, use of radio has been growing as a way to save lives at sea.  The signing of the international SOLAS (Safety of Life as Sea) treaty in the 1970s called  for a Global Marine Distress Signaling System or GMDSS. GMDSS was designed to have multiple ways of sending or receiving distress signals automatically, without having to rely solely on a radio operator trained in Morse Code.

Satellites relay EPIRB and Navtex signals. Navtex is an international, automated system for instantly distributing maritime safety information (MSI) which includes navigational warnings, weather forecasts and weather warnings, search and rescue notices, and similar information to ships. Traditional radio communications and Digital Selective Calling (DSC) are also components of GMDSS.

Personal locator Beacon

Personal Locator Beacons should be worn by boaters going offshore or boating alone.

DSC has two forms: Category A for commercial shipping and Category D for recreational boaters. Both category A and D radios can send and receive digitally encoded distress calls on Channel 70, which is reserved exclusively for emergency digital communications. Category A radios have more features and are much more expensive.  For the purpose of this blog, we’ll be discussing type D or recreational DSC.

VHF radios equipped with DSC are capable of transmitting a distress call that includes your position (if your radio is connected to your GPS or has its own GPS receiver) and vessel particulars such as type, color, and so on, provided you have registered your radio with the maritime mobile service. All VHF DSC radios have a unique ID number (MMSI). When you send a distress call, your radio’s ID is matched to the database of information, and rescuers can be on their way with your position, vessel particulars, and emergency contact info in hand. The registration also helps the Coast Guard identify false alarms.


Emergency Position Indicating Radio Beacons, or EPIRBs, send position reports via satellite--all part of the GMDSS system.

To send for help, all you or one of your crew needs to do is manually lift the protective cover, then push and hold the emergency button on your radio for a few seconds. The transmission will continue automatically until manually shut off (or the power fails) which might happen if your vessel was sinking—then your EPIRB would subsequently send out a distress signal, too. Finally, if you did end up in the water, hopefully you are wearing a Personal Locator Beacon (with built-in GPS). Of course, you would have to be conscious to be able to manually set off the PLB. But if you are, the PLB signal bounces off a COSPAS-SARSAT satellite, and again,you’re saved.

It is amazing how technology has improved safety globally for boaters. For a free online registration of your new DSC radio with Maritime Mobile Service, visit BoatUS for the forms you need. For more information on the proper use of marine radio, read my blog on VHF radio protocol.

For my friend Lee and other boaters, the long-term direction of Digital Selective Calling should lead to the elimination of the need to monitor Channel 16. Stay tuned for more on EPIRBs and PLBs in future blogs.

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