Why You Need To Know About DSC and MMSI


Every boat should have an MMSI number! There are safety and convenience reasons, and it’s easy.


In summary, what DSC and an MMSI number buys you is:


1)          A simple way for a completely inexperienced person to put out a detailed emergency call on your VHF radio.

2)          A more reliable and simpler way to initiate ship-to-ship VHF transmission.


MMSI stands for Maritime Mobile Service Identity. It is a 9 digit unique number that is associated with your specific VHF installation, and really like a digital “Call Sign”.


DSC stands for Digital Selective Calling. It is an added digital capability available on all fixed-mount VHF radios sold since 1999 (FCC requirement).


Here is how to get it working for you:

1)          Make sure your VHF has DSC capability (if not, buy a new one)

2)          Have your VHF connected to your GPS

3)          Get an MMSI number (see below for details on how)

4)          Program your MMSI number into your VHF

5)          (Optional) Program in the MMSI numbers of boats you frequently call


Notice that your DSC-equipped VHF has an “Emergency” button. After the setup is complete, if that button is pressed your VHF will send out a general emergency call including details of your MMSI number and your exact position all automatically. Even if everyone on board becomes incapacitated, the VHF continues to broadcast the emergency with all relevant details.


The Coastguard does (or will soon) monitor for these broadcasts. Already, commercial ships are required to monitor for digital distress calls and have relayed maydays to the Coast Guard. Because your MMSI number is unique, the Coast Guard has all the details of your boat, so it knows who you are, exactly where you are, and that you are in trouble.


So that is the “Emergency” part. It’s as easy as setting it up then showing anyone on board where the “Emergency” button on the VHF is.


Still, none of us hope to be in an emergency, so what are the day-to-day advantages?


Imagine any shared experience (cruise, salmon derby) when you (the skipper) want to call another boat. Try him on 16 – no response. See if he’s listening on 9 – no response. What channel is he on??? The cruise channel? Some other channel? We all know how frustrating it is to try to hail someone you are sure is out there.


DSC solves all of that. It is a digital replacement for a calling frequency. If you have your friend’s MMSI number in your VHF, all you do is choose the working channel you want to use, then ask your radio to call him by choosing his name from a menu. If he is in range his VHF will automatically switch to the working channel you have chosen and will ring like a phone. He just answers you using his VHF mike.


What else? DSC uses channel 70 to send the digital signal, so stay off that channel.


What are the disadvantages?

1)          Most VHF radios are very clumsy for entering this info, so it is a slow and tedious process. You will probably also find that scrolling through a large list will be awkward in a seaway, so practicalities dictate that your list of addresses will be really limited to a small number of commonly called boats

2)          There appears to be an “Enhanced Group Calling (EGC)” capability which would let a single call be made to, say, all boats in a cruise group. However, I haven’t figured out how to do that yet, so in the meantime we have to call each individually. EGC numbers are an individually assigned special form of MMSI. My initial research provides scant information on EGC and it looks like it may be too hard to set up for what it would provide. Any further research welcomed.


If the marine electronics manufacturers could get their collective act together, it potentially could be easy to maintain your list from, say, a laptop in the nav station. However, the initiative 25 years ago to create the NMEA common communications system has not progressed, and all manufacturers are pursing proprietary inter-device communications. This is a dreadful situation for all of us, and will be a subject for another paper some time in the future.


To register for your MMSI, and for more information, go to:



For more information on MMSI go to:



Final Note: In case you were still in doubt, all INMARSAT satellite terminals use an MMSI number as the heart of their identification string, and the numbers are also used by 406Mhz EPIRB's (Emergency Position Indicating Radio Beacons).


Get and use this new technology. It’s easy, free, safer and more reliable.