Understanding VHF-DSC Radio

Most marine radios (except handhelds) sold in the last decade or so have digital selective calling (DSC) capability. If you're thinking of upgrading an older radio, here's the very short version of what you need to know about DSC.


DSC radios have the ability to transmit and receive short digital messages on a dedicated channel (usually VHF 70, MF 2187.5 kHz or HF 8414.5 kHz). These messages can include:

  • Distress (Mayday) calls that set off an alarm on every DSC radio in range, and send a message including your exact GPS position and the type of emergency to all those ships.
  • All-ships (Panpan and Securite) calls that alert all ships in range that an urgency or safety message is about to follow on a specified channel.
  • Ship-to-ship hails to a specific vessel, which raise an alert on the other ship's radio saying "Voice call from ___ on channel ___." This removes the usual flurry of opening hails from channel 16, making that channel easier to use for emergencies.

The net effect is to make distress calls much easier and more reliable, and to move chatter such as opening hails away from channel 16. This makes the Coast Guard's job a lot easier and increases the probability that your Mayday call will be received and quickly acted upon by nearby boats.


DSC radios come in several types, and are available for the VHF (short range) and MF/HF (medium range) bands.

Class A is for use on commercial ships. Class A sets have all the bells and whistles, including a second antenna and receiver to provide continuous DSC monitoring.

Class D is for recreational vessels. Class D sets have the same core functionality as Class A sets, including a dedicated DSC receiver, but with some limitations:

  • They usually cannot transmit group, geographic area or "mayday relay" messages over DSC.
  • They have only one antenna, so most of them cannot receive DSC messages while transmitting.
  • They are simplex only (one person talks at a time, like a walkie-talkie), with no duplex (telephone-like) mode.

SC101 is an outdated type of VHF-DSC radio that is no longer legal to make (although existing SC101 radios are still legal to use). SC101 was a cheapskate version of Class D that omitted the dedicated receiver. The problem with that is obvious: if you're listening to a voice transmission, the radio will ignore distress calls.

ATIS (Automatic Transmitter Identification System) is an added feature that appends a digital identfying tag to the end of each transmission. It is required on European inland waterways.

AIS (Automatic Identification System) is a separate system, used for sharing position, course and speed data between vessels. Some VHF sets include integrated AIS receivers, but it is usually implemented as a stand-alone transceiver linked to your boat's chartplotter.

The VHF-DSC Class D is the standard radio for all modern recreational boats. You'll need ATIS if you cruise the European canals; other than that, your choice can be based on the radio's user interface, features, build quality and price. All VHF-DSC sets can communicate with all other VHF-DSC sets of any class.


If you haven't taken a radio operator's course this century, you'd be wise to do so; the current courses include training on how to use DSC calling and, in Canada, will add a corresponding endorsement to your ROC(M) card.

Once a DSC radio is installed and working, there are two more steps before all its features can be used:

  • Your MMSI (Mobile Maritime Service Identifier) number must be registered and programmed into it. The MMSI is the radio equivalent of a telephone number. If you hit the Distress button, the Coast Guard can identify your transmission by the MMSI. If you program your friends' MMSIs into your radio, you can call them directly like you would on a cellphone.
  • You must connect it to your GPS, if the radio doesn't have its own internal GPS. The radio keeps a short-term log of your last known GPS position fix and includes this with any distress call you make.

Further reading

US Coast Guard's DSC recommendations

Detailed DSC overview by Dunstan & Associates

Canadian Power & Sail Squadron's VHF-DSC licence course



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