By Frank Lanier
Chesapeake Bay Magazine (2005)

Imagine sending a mayday over your VHF radio telling not only where you are but what the problem is with the touch of a button – or how about using your VHF to call a specific vessel and pass hot fishing information to your buddies without everyone else listening in?  Such are the promises of DSC (Digital Selective Calling) VHF radio and while some of its advertised advantages are still in the future a bit, others are available now and can greatly enhance your boating experience.

DSC is one of the latest advances in marine radio communications and while currently being used with both MF (Medium Frequency) and HF (High Frequency) radios, VHF DSC in this focus of this article.  When properly registered with a Mobile Maritime Service Identity (MMSI) number (more on this later) and interfaced with a GPS or LORAN unit, DSC radios have the capability to operate much like an EPIRB in an emergency.  They transmit encoded vessel and owner information, position, and the nature of distress (if entered) at the push of a button and continue to do so automatically until answered by another DSC equipped shore unit or vessel.

Another popular feature of DSC radio is the ability to make private “direct dial” calls to other DSC radios.  Enter the MMSI number of the radio you want to call and only that radio will receive your message – just like a phone call.  Calls can also be directed to a specific group or set of DSC radios (all Coast Guard units, for example), receivers in a specific geographic area, or all receivers in radio range, a useful feature when the individual identity of a unit being called is unknown.


As of February 1, 1999 the Safety of Life at Sea (SOLAS) Convention (an international treaty document) required all passenger ships, cargo ships, and most other vessels 300 gross registered tons (GRT) and larger on international voyages to carry DSC- equipped radios.  DSC is currently used on VHF, MF and HF maritime radios as part of the GMDSS (global marine distress and safety) system, with Channel 70 (VHF) and 2187.5 KHz (MF) being reserved exclusively for DSC safety and distress calls.

          DSC radios are currently required on all SOLAS regulated ships, who consequently are no longer required to maintain a live 2182 kHz radio listening watch – they were supposed to be able to secure their VHF channel 16 watch as well by February 1st, 2005, however the International Maritime Organization recently suspended plans for this indefinitely as DSC communications upgrades for many search and rescue organizations (U.S. and Canadian Coast Guards for example) will not be in place by then. Although the long-term goal is elimination of the requirement to monitor VHF Channel 16, it will continue to be the primary hailing and distress channel for the some time to come.  Even after the US Coast Guard completes the required upgrades to their VHF system (an effort dubbed RESCUE 21) and begins monitoring Channel 70, they will continue to monitor Channel 16 for the foreseeable future.

What this means to current DSC VHF users in the U.S. is that while other DSC radio equipped vessels can receive VHF DSC alerts (there have been recent incidences of commercial ships picking up “mayday” calls on Channel 70 and relaying them to the USCG) Channel 70 will not be fully monitored for distress signals by the U.S. Coast Guard until its communications systems upgrades are completed (sometime in 2006 if current predictions hold true).

          Because of potential safety problems stemming from a lack of communications between SOLAS-regulated ships and other vessels (recreational boaters, commercial fishing vessels, etc.), the Coast Guard petitioned the Federal Communications Commission in 1992 to require all marine radios made or sold in the U.S. be DSC ready.  DSC VHF radios built to SC-101 specifications can transmit a distress call providing position, time, and MMSI to any station within range and monitoring DSC Channel 70.  Search and rescue organizations can then reference an MMSI database for a description of the vessel and additional owner information provided at time of registration.

          The two main flavors of VHF DSC radios pertaining to recreational boaters are Class A and Class D. Both can send and receive digitally encoded distress calls on channel 70, however Class A radios meet all GMDSS DSC requirements for SOLAS-regulated ships and have all the bells and whistles, such as two separate receivers (which allows constant monitoring of channel 70 even while monitoring normal working channels or weather channels).
Class D radios and those advertising compliance with specification SC-101 meet the bare DSC essentials and while less expensive, are also less functional (meaning their use should be limited to smaller, recreational vessels).

Each VHF DSC transmission to channel 70 is typically less than ½ a second in duration and (depending on the unit) includes the purpose of the call (emergency routine communications, safety items, etc), the MMSI number of the caller, position (if connected to a compatible GPS or LORAN unit), and time of location.  Channel 70 is a hailing and distress frequency for DSC only and upon receiving and answering a call, the radio will automatically switch both radios to a working voice channel (which in the case of a mayday is VHF channel 16).  Other DSC radio features can include the ability to receive and store messages, “caller-ID” functions, storage of MMSI numbers and even call waiting.  Another neat trick (if connected to a suitable chart plotter) is the ability to locate another DSC equipped vessel at the push of a button.  Your friend is slaying the fish and sends you his coordinates via DSC – it appears on your chartplotter (privately and encoded) followed by a dialog box that pops up, asking if you want to navigate there.  If part of a group or fleet of boats, some DSC radios can be set to answer an automatic “polling” mode by sending constant updates on vessel position so you know just where everyone is – does it get any easier?


While most recreational boaters are no longer required to have an FCC issued ship’s station license to operate a VHF radio, DSC radios must be properly registered to deliver their advertised benefits, particularly those pertaining to emergency situations. DSC radios are encoded with a unique nine digit FCC identification number that allows the ship-to-ship calling feature (the MMSI number mentioned earlier).  Once the radio is registered with the FCC, that information (along with your boat information) is entered into the US Coast Guard’s national distress database.  If your vessel requires an FCC license, you can obtain an MMSI during the application process for it.  If you don’t need an FCC licenses you can obtain an MMSI free by contacting visiting websites for either BoatUS, Sea Tow Service International, Inc, or MariTel, all of which have signed Memorandums of Understanding (MOUs) with the Coast Guard and the FCC permitting them to issue MMSI to vessels not requiring an FCC license.