Skip to main content

Understanding what DSC can do

Call & Response


The distress button on this DSC-enabled Standard Horizon VHF is behind the spring-loaded red flap.

Most VHFs have life-saving features—if only users activated them.

Every DSC radio has a “panic button” that can transmit an automated distress message instantly, and will go on doing so until the message is acknowledged or cancelled—or the boat sinks. But only a small proportion of U.S. boaters have taken the two simple steps that are required to make the most of this potentially life-saving technology.

Until 1992, ships were required to carry radio equipment that was based on their size and the number of passengers they carried. So a large passenger ship needed to have equipment capable of worldwide communications and fully qualified radio officers, even if it never went out of sight of land, while a small cargo ship could set off across the Pacific with nothing more than an MF radio that would struggle to send a signal 200 miles. Indeed, if it had to send a distress message, it was largely a matter of luck that anyone would be within range. In principle, the system was virtually unchanged from the days of the Titanic and the Lusitania.

Technology could certainly do better, so the 1990s saw the appearance of the Global Maritime Distress and Safety System (GMDSS), rules that effectively forced all ships to carry at least two independent means of transmitting distress alerts directly to rescue agencies ashore. Private pleasure craft don’t have to carry GMDSS equipment, but we can if we want to—and the U.S. Coast Guard is eager that we do.

One of the technologies at the very heart of the GMDSS is Digital Selective Calling (DSC), which effectively automates many of the calling procedures that we would otherwise carry out by voice on VHF channels 9 or 16. It’s important because it allows distress alerts to be transmitted simply by pressing a button, but that’s not its only purpose: DSC can also speed up and simplify routine calls, such as those to your marina or to your fishing or cruising buddies, so long as both ends of the conversation have the right equipment, properly set up.

It’s easy to do, and you probably have the equipment onboard already: All you need is a VHF DSC radio and a GPS receiver. First, though, there’s the administrative bit: You need to register for a Maritime Mobile Service Identity or MMSI. It’s a nine-digit number that is unique to your boat and is issued by the FCC through SeaTow, the U.S. Power Squadron, and BoatUS, the most popular option by a wide margin.

Getting your GPS to talk to your radio involves making a simple two-wire connection between the NMEA 0183 output from the GPS and the NMEA 0183 input to the radio. You may need to check the manuals of both devices to find the color coding of the wires and to make sure the two are “speaking the same language,” but it’s not a long or complicated job. If you don’t care to do it yourself, any electronics installer should be able to do the job in less than an hour.

After the connection has been made you’ll have to enter your MMSI into the memory of your radio. You only get one shot at it, and the procedure varies from one make to another, but the manufacturers have made it difficult to get it wrong. And once you’ve done it, all the DSC functions of your radio come alive.


This Icom IC-M504 displays a GPS position on screen.

With your GPS talking to your radio, and your MMSI in its memory, you’re almost set to go. The only thing missing is storing the MMSIs of your local coast guard station, as well as your buddies, just as you store important contact numbers in your cell phone. With that done, if you want to call someone, you simply recall his MMSI from the radio’s memory, tell the radio that you want to make a routine call, and select the working channel you intend to use. Your DSC radio converts all that into a burst of digital code that takes only a fraction of a second to transmit on Channel 70 but which replaces all the “Big-Ben-Big-Ben-Big-Ben-this-is-Rubber-Duck-Rubber-Duck” business that usually happens on Channel 9. And to accept your call, a recipient simply has to press one button and pick up his microphone: His radio will switch to your chosen working channel automatically.

If you just want to know where he is, many radios offer a position-request function where the whole exchange is reduced to a short crackle of digital data that ends up with your friend’s position coordinates displayed on your radio screen, with no voice contact at all.

But DSC really comes into its own in distress procedures. Every DSC radio has a red “panic button,” protected by a spring-loaded flap. Lift the flap, press the button, hold it for five seconds, and the radio automatically broadcasts a coded distress message, including your position and basic vessel data, that sets off alarms on every DSC-equipped bridge and wheelhouse within range, as well as in Coast Guard stations.

Sounds great, doesn’t it? But the funny thing is how few of us seem to be taking advantage of this technology. David McLean, national marine sales manager of Icom America tells me that he has heard potential customers say, “Oh, I have a cell phone, I don’t need a radio.” McLean’s next question: If you are on the water, and your boat is sinking, and you’ve got ten boats floating around you, do you have their cell-phone numbers? “No!” he says. “Of course you don’t. That’s why you need a [DSC] radio!”

U.S.C.G. Lieutenant Commander Max Jenny further points out that even a distress call on Channel 16 may not be heard by those that are in the best position to help. “We maintain a Channel 16 listening watch,” he told me. “But other vessels might be just a few miles away, but listening or working on another channel. If the DSC message doesn’t go out, they’re not going to know about it.” Recent changes to the rules mean that all current DSC radios (and many older ones) will receive DSC messages even if they are operating on other channels at the time, and will prompt their operators to switch to Channel 16 to monitor any voice messages that follow.

Great as the technology is, the Coast Guard estimates that just 10 percent of boats have DSC radios connected to GPS. No one knows why so many of us are opting out of such a smart, free upgrade.

This article originally appeared in the November 2011 issue of Power & Motoryacht magazine.