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What is Digital Selective Calling for VHF Radios?


aka That little red button on the marine radio

DSC on marine VHF radio display

If you're shopping for a new marine VHF radio for the first time in a while, you may notice a little red change. I'm talking about the little red distress button on any new VHF radio. The button is one indication that the radio is enabled for Digital Selective Calling (DSC). You may see this acronym repeatedly floating around, but what does it mean? In short, it's the 21st Century evolution of marine radio communication. Here's some info to explain how DSC benefits recreational boaters.

DSC compliant radios are easily recognizable by a distinctive red button marked "Distress". The DSC technology was first introduced on large ships and merchant vessels as part of the Global Marine Distress Safety System (GMDSS). It provides vessel identification and facilitates direct vessel to vessel communication. Recognizing the advantages to the recreational market, the advancement of DSC to the rest of us adds digital communications technology to your standard VHF marine radio. Now, with a Digital selective calling enabled radio, you can send traditional voice transmissions over VHF channels as well as data or digital transmissions on channel 70. Think of the messaging as a simple text message for common radio communications. But unlike text messages on mobile phones, a DSC message is a succinct purposeful message intended to initiate a response (More on this later). To participate in the digital messaging, your radio must be registered to get a 9 digit identity called Maritime Mobile Service Identity (MMSI). This becomes your boat's unique phone number. This is very cool technology for a multitude of reasons.

Perhaps the biggest benefit of DSC, it keeps channel 16 open. Think of a busy area like Long Island Sound on the 4th of July. With waters growing more crowded every year, the constant hailing and non-essential chatter over channel 16 creates a log jam. One of the biggest advantages with DSC radios, now you don't have to hail on channel 16. Instead, you send a digital message on channel 70 directly to your buddy's MMSI "phone number". Once the receiving party receives and accepts the digital message, both radios automatically turn to the working channel you've selected. The unit stores the message for future reference. This evolution lessens the traffic on channel 16, keeping it clear for essential USCG emergency calling.

Another giant advancement with DSC radios, they improve the process in which you call for help in a marine emergency. The radio should be interfaced with your GPS receiver. The GPS will constantly update vessel position and time position was recorded. This information is automatically relayed in your DSC distress message. The digital message also permits clear communication in situations where voice messages would be difficult or impossible to understand. Further, because it is a static message, it remains on the recipients radio display, whereas a spoken radio call chances going unheard. When you issue a digital distress message by pressing and holding the distress button for 5 seconds, your message goes directly to CG shore station and includes MMSI identity number and position. If time permits you can designate the nature of distress. The CG knows who you are, where you are and the nature of your distress. All other DSC equipped vessels in your vicinity also receive this valuable information.

The result is dramatically increased speed and reliability of distress calls. The CG has modernized it's national distress response system (NDRS) Rescue 21, the marine equivalent to land based 911. All DSC enabled radios in hailing range that receive a distress call immediately sound a distinctive alarm and immediately switch to channel 16. This alerts all vessels in signal range radius, called a "broadband transmission," making it far more effective than "narrowband" cellphones.

Non-Emergency Calls with your DSC Radio

channel 70 DSC on marine VHF radio display

Of course there's more advantages as well. For non-emergency use, DSC equipped radios enable the following:

  • routine calls to other vessels and shore stations
  • sending and receiving position reports
  • polling other vessels for position reports
  • group calling
  • Routine calls eliminate the need to hail another vessel on channel 16. It will automate the switch to your selected working channel. Receipt of the incoming call is shown on recipients radio, also displaying your MMSI and boat name. Receiving party can then select confirmation which is clearly announced back to your radio, and both radios switched to whichever working channel you've designated.

    If the vessel you are trying to reach has their radio set to "unattended" it will return a signal to inform you. It will also record your MMSI and the time of your call.

    When an attended radio responds it will automatically switch to the working channel you selected. Your radio will also switch to the same selected channel ready to communicate.

    Polling calls request a vessel's position without initiating voice communication. Response is automatic and requires no action by the called vessel. It is useful when sailing to meet another vessel and when keeping track of other vessels in a flotilla.

    Group calls to a number of pre-selected vessels can be made for groups with a special group MSSI. The call will alert all of the vessels in group that receive the DSC group hailing call. Position information can be sent to your chart plotter and/or radar to display location of all vessels in group.

    What do I need to install DSC VHF radio on my boat?

    The big question, how can I take advantage of this technology once I purchase my DSC equipped radio? Here are the five things required to make your DSC fully functional:

  • your unique Maritime Mobile Service Identity (MMSI) Number
  • VHF/DSC radio
  • GPS receiver
  • Two-wire connection to your GPS
  • Few minutes to learn the DSC system

  • How to Obtain your free Maritime Mobile Service Identity (MMSI) Number

    The following websites provide free registration services to get your unique MMSI for a DSC VHF Radio:

    The Recreational Class for DSC Radios

    Not all Digital Selective Calling radios are the same. DSC radios on ships have more capability and cost in excess of $800. For recreational boaters, a class D radio is the norm, and are now available in the $200 range. Class D VHF/DSC radios have 2 receivers so that the radio can receive DSC signals even when it is receiving on another channel. It is limited by not being able to receive DSC signals while transmitting. Only more expensive Class A radios required on ships can do this. This info from the from USCG website explains more:

    Class D:

    Minimum DSC capability for VHF marine radios carried by recreational boaters, commercial fishing vessels, and other non-SOLAS regulated vessels. Class D required capabilities include:

    • Distress call
    • All-ships call
    • Individual station call
    • Use of distress, urgency, safety and routine priorities
    • Nature of distress
    • Distress coordinates
    • Time for last (distress) position update
    • Type of subsequent communications
    • Radio VHF channel
    • Display
    • Receive distress relay and distress acknowledgment calls
    • Alarm
    • Distress acknowledgement (receive)
    • Geographical area call (receive)
    • Test call
    • Test acknowledgement

    The following links have more info should you so desire:

    Rescue 21 for boaters

    digital selective calling (DSC) fact sheet


    How to transmit a DSC distress call

    1. Press and hold distress button under red cover for 5 or more seconds until beep is heard.

    2. After beep is heard, pick up the mic and speak your standard MayDay call. This will alert non-DSC equipped boats in area

    3. DSC signal tells the CG your boat's info and nature of distress. It is not necessary to press the distress button again
    • If you mistakenly press the distress button, turn radio off and back on again to stop distress broadcast, and contact the USCG on channel 16 to alert them of faulty Distress transmission and providing them with your MMSI number.
    1. You have the option do designate the nature of distress. Options include:
      • Abandoning Ship
      • Fire
      • Capsizing
      • Flooding
      • Collsion
      • Grounding
      • Man Overboard
      • Listing
      • Disabled
      • Piracy
      • Explosion
      • Sinking
    2. Undesignated alerts may also be sent, and suggest the most dire emergency.

    3. A radio receiving a DSC distress call will beep and automatically switch to channel 16. It will display calling station's MMSI, lat/long and time of call. The radio will store these details of the incoming mayday call. Any receiving radio can also relays that signal automatically, until a shore based unit confirms receipt of distress.

    4. Once the USCG shore station receives and responds to the DSC distress call, the DSC alarm will stop automatically and CG personnel will direct you with further instructions.


    Sources of information for this article from:

    "Can You Hear Me", a BoatUS presentation

    Rescue 21 for boaters

    digital selective calling (DSC) fact sheet
    ICOM America Digital Selective Calling Video

    - written by Michael Reardon

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