Most owners of VHF radios equipped with Digital Selective Calling are familiar with the primary benefit DSC offers: transmitting an automated distress signal at the push of a button.

But your DSC radio can do a lot of other things, too, and I get the sense that virtually none of us use these features. For example, it allows you to "hail" another DSC-equipped vessel or shore station specifically, like placing a person-to-person call. When you initiate a call, it also identifies your vessel to receiving stations, like caller I-D. But we’ll get to all that.

The Standard Horizon Eclipse GX1000S fixed-mount VHF radio, like all current models, includes Digital Selective Calling functionality.
[The Standard Horizon Eclipse GX1000S fixed-mount VHF radio, like all current models, includes Digital Selective Calling functionality.]

First, the basics. So what is DSC? Digital Selective Calling is one component of the Global Maritime Distress & Safety System (GMDSS). DSC-equipped radios have all the features of conventional VHF radios, but DSC technology allows them to work something like a telephone, as well, conveying information with every call.

But it’s not enough to simply mount your DSC VHF and connect it to power and an antenna. If you stop there, it will work in the conventional way, but you won’t get any DSC functionality. You need to set it up correctly to enable your radio to operate as intended.

So what do you need to do? There are two ways to ensure that your vessel’s position data will be included in any DSC distress message you transmit: connect your VHF directly to a GPS receiver outputting NMEA, or you connect it to an NMEA output from a chartplotter. Ideally, you would connect the VHF to the chartplotter, because that will also enable the DSC position information that you receive to appear right on your screen. This would be very helpful if, for instance, a nearby vessel were to transmit a DSC distress message. It should be pretty simple, but depending on your boat’s electronics installation, it can be a tricky figuring out how to connect it all up.

DSC uses MMSI numbers to operate. MMSI stands for Maritime Mobile Service Identity, a unique nine-digit number associated with a specific vessel or shore station. In the United States, BoatUS manages the MMSI registration database for most recreational vessels, issuing MMSI numbers to all boaters free of charge. You can register online and you don’t need to be a BoatUS member to take advantage of this service. When you complete the online application, be as thorough as possible.

BoatUS’s MMSI page allows you to apply for a free MMSI online, and also provides links to information about DSC and the Coast Guard’s Rescue 21 system.
[BoatUS’s MMSI page allows you to apply for a free MMSI online, and also provides links to information about DSC and the Coast Guard’s Rescue 21 system.]

However, if you intend to travel outside the United States, into Canadian or Mexican waters, for example, you should instead obtain your MMSI number from the FCC, by applying for an FCC Ship Station License.

The FCC’s Ship Radio Stations homepage provides information and tools to enable you to apply for a Ship Station License online.
[The FCC’s Ship Radio Stations homepage provides information and tools to enable you to apply for a Ship Station License online.]

Your radio’s instruction manual will explain how to enter an MMSI number, which is crucial. Many radios will only allow you to enter a MMSI number two or three times, and some only once, before you have to return the unit to the manufacturer to have it reset. So it’s important to obtain a genuine MMSI number from the appropriate authority and program it into your radio carefully.

As unlikely as it sounds, people have programmed their DSC radios with everything from social security numbers to telephone numbers to vessel registration and hull numbers. None of those will work. Only genuine MMSI numbers should be programmed into DSC radios.

In an emergency, when the skipper decides it’s appropriate to do so, activate the VHF radio’s "distress" feature. On my radio, I can hold the "distress" button down for three seconds to initiate a generic distress call. Or I can press it once and select from ten slightly more specific distress calls, including "fire," "grounding," "sinking," "man overboard," and "piracy."

Detail of the “distress” menu options on the screen of BoatUS’s DSC VHF simulator.
[Detail of the "distress" menu options on the screen of BoatUS’s DSC VHF simulator.]

When you initiate a DSC-enabled distress call, your radio transmits your MMSI number, your GPS coordinates and the nature of the distress. It’s not a lot of information, but the GPS coordinates alone are tremendously valuable. Furthermore, the MMSI number allows the Coast Guard to look up the data you provided when you applied for your MMSI number; now they know what kind of boat to look for, how many persons are normally aboard, and who to contact in an emergency. Even if you are unable to hold up your end of a conventional radio dialog, the Coast Guard has a great deal of information to work with.

In my cruising area – Puget Sound and the Strait of Juan de Fuca – the Coast Guard’s Joint Harbor Operations Command is equipped to receive and interpret DSC transmissions, but this only became true early in 2007 with the deployment of the Coast Guard’s Rescue 21 upgraded communication technology here. Many boaters aren’t aware of it, but currently Rescue 21 only covers about half of the country’s coastal water. For an excellent account of a successful mayday response facilitated by Rescue 21 technology, read Petty Officer 3rd Class Kelly Parker’s article.

The multi-year rollout schedule for Rescue 21, combined with the limited information included in a DSC distress call, means that it will remain important for mariners to attempt to establish voice contact on Channel 16 the conventional way in an emergency. DSC has the potential to improve the odds in lots of maritime emergencies, but it’s only part of the picture.

But wait . . . there’s more.

The main reason to install a DSC-enabled radio is to enhance your chance of preventing property damage, injury, and loss of life. But as I indicated earlier, that’s not all you can do with DSC.

For one thing, you can hail a specific vessel if you know her MMSI number. Many of us do this the conventional way, "in public" on Channel 16, and then switch to a working channel. This works fine, although it’s not uncommon to miss calls if you don’t happen to be monitoring Channel 16 when your vessel is hailed.

Still, channel 16 gets pretty busy around here during the summer. It might be handy to share MMSI numbers with your cruising buddies. When one of your friends calls you this way, your DSC radio will sound an alarm tone, and you can either ignore or accept the call. If you accept the call, your radio will automatically tune itself to whatever working channel the caller specified, and you can talk over the airwaves in the usual way.

OK, so what else can you do with DSC? Well, you can hail a whole fleet directly, without knowing the names or MMSI numbers of the boats. All you need to know is the Group MMSI number your fleet uses, and every vessel within range will receive your hail. Those vessels need to have this Group MMSI number programmed into their radios, of course. As with a station-to-station call, an alarm sounds, and each recipient is free to accept or ignore the call.

While DSC individual and group call capabilities can be useful to commercial vessel operators, it remains to be seen whether recreational boaters make use of these features in a significant way. One of the obstacles is the array of inconsistent and extremely rudimentary user-interfaces available on most VHF radios, but I expect this to change over the next few years.