Skip to main content

Understanding DSC and MMSI

What you need to know about the latest in marine VHF radio technology.

It may not rank up there with the great mysteries in life, but it is definitely a puzzlement: Why don’t more boaters set up and understand the use of the Digital Selective Calling function on their VHF radios?

Next to the invention of the EPRIB, the single-button DSC feature is the most valuable safety feature developed for boaters. In some ways, it’s even better than an EPRIB. Unfortunately, based on random surveys taken during United States Coast Guard Auxiliary safety checks, fewer than half the DSC-equipped VHF radios on recreational boats are programmed for use.

When properly used in near-coastal and inland waters, DSC, with the press of a single button, can alert more people to the need for help than any other method. DSC digitally sends a distress call to rescue personnel over the Global Maritime Distress and Safety System. When a radio is programmed with a Maritime Mobile Service Identity number, DSC transmits the vessel name and description, and an emergency contact’s information, to the authorities. When the radio is connected to a GPS receiver, DSC also transmits the vessel’s position.

The DSC alert is also received by other boaters within radio range. And, DSC lets the boater identify the exact nature of his emergency. All DSC radios are programmed with the following list of conditions: undesignated, explosion, flooding, collision, grounding, capsizing, sinking, adrift, abandoning ship, piracy attack and man overboard.

Obtaining MMSI Numbers

MMSI numbers issued for recreational craft in U.S. waters are available from the United States Power Squadrons at and from BoatUS at

Recreational MMSI numbers designated for use in U.S. waters differ from numbers the Federal Communications Commission issues for use in international waters. If you plan on taking your boat into international waters and contacting a foreign government agency by VHF radio, then you are required to obtain the appropriate FCC station license and associated international MMSI number. Doing this also lists your boat’s information in an international search-and-rescue database.

Whether for U.S. or international use, MMSI numbers are assigned to a VHF radio, not to a person. The numbers are intended to stay with the radio; when you sell a boat, you must log in and cancel that MMSI registration, so the boat’s new owner can update the database with his information.

Some VHF radios allow you to delete the original MMSI number and enter a second number; others require the radio to be returned to the manufacturer or an authorized service technician to delete the number from the radio, allowing a new number to be programmed into it.

Recent Changes in VHF Channels

Some VHF radio channels used to have the letters A or B assigned to them, meaning they were duplex channels with two frequencies available for simultaneous conversation. Channel 22A, for instance, was what the U.S. Coast Guard used for official communications and broadcasts. Only the A frequencies were used in the United States. Canada and other countries used the B frequency.

As of January 1, 2017, the letter designations were changed to four-digit designations, with either 10 or 20 preceding the channel number. Channel 22A became 1022; 22B became 2022. Older radios with the original channel designations will continue to work, as they still place the radio onto the same frequency. The Coast Guard published a list of the channel designations here:

Though older radios will continue to work using the same frequency (only the channel designator is different), VHF manufacturers will eventually update their radios to display these new channel designations.

Help the Coast Guard help you. Connect your radio to your GPS and program it with an appropriate MMSI number.

Information on maritime communications and VHF radio use from the U.S. Coast Guard:

The United States Power Squadrons and BoatUS offer a two-hour online course in VHF marine radio operation.