Safe Offshore Sailing


‘DSC 101A’ by Ron Trossbach

(updated Nov 2012)

The use of Digital Selective Calling (DSC) in VHF and SSB radios is relatively new and not widely understood. This paper will give the reader a broad view of what DSC is and how it applies to recreational boats. Web site links and references are included for more detailed explanations.

DSC definitely has a place in small boat operations and is simple to install with potentially huge safety benefits. It is no longer necessary to read latitude and longitude numbers over the radio in emergencies or when exchanging position information with another boat. DSC does this automatically in less than one second.

DSC capability should already be in VHF and SSB radios built/sold in the US after June 17, 1999. Class D DSC fixed VHF radios purchased after March 2011 will have improved features including two channel CH 70 reception and test calling.

The physical hookup is relatively simple - two wires between any model radio and any model GPS. The registration is FREE - a unique nine digit Mobile Maritime Service Identifier (MMSI) number comes from the FCC Ship Station License or from several US Agencies, if the boat is not leaving the United States.

"The Coast Guard urges, in the strongest terms possible, that you take the time to interconnect your GPS and DSC-equipped radios. Doing so may save your life in a distress situation!" See This USCG site also provides the background, history and description of how DSC fits into the Global Marine Distress and Safety System (GMDSS).

Boat U.S. Foundation has an online tutorial on Marine VHF titled 'Can You Hear Me?' which is an interactive run-down of everything a boater needs to know about using a marine VHF radio with Digital Selective Calling (DSC). The headline on this announcement is "Spending 35 Minutes Online Could Save Your Life". See

ICOM North America has a DSC Informational Video on Individual and Emergency Calls at

At a bare minimum, responsible boaters should be capable of sending a DSC Distress Call as well as taking the correct action when a DSC alarm sounds on their radio. DSC is in place and operating now, especially in US coastal area where RESCUE 21 is operational. This is a huge safety improvement that all boaters should understand and be prepared to use in case of emergency.

MAKING A DSC DISTRESS/MAYDAY CALL. This call is only made when personnel and/or property are in immediate danger and immediate assistance is requested. Press the Red Distress Button on your radio and hold it in for five seconds. Listen for a DSC Distress Acknowledgement. After it is received, or if it isn’t acknowledged, shift to the VHF Distress, Safety and Calling Frequency (VHF Ch-16) or a SSB Safety and Hailing Frequency (2182, 4125, 6215, 8291, 12290 or 16420 kHz, USB) and issue a voice MAYDAY, following the format on the DSC Distress Communication Form.

Persons in distress can use any frequency/means to alert other mariners/persons ashore to their plight.

IF YOU HEAR A DSC DISTRESS CALL. Shut the radio alarm off by pressing any button on your radio. Write down the MMSI and position information showing on your radio display screen. Wait 3-5 minutes for an authority to answer the call. If no other station replies attempt to verbally relay the MMSI and position information to USCG. Contact the station in distress if no one else does and go to their rescue, if you are able to do so.

IF YOU ACCIDENTALLY MAKE A DSC DISTRESS CALL. Shut the call off. Get on the VHF Distress, Safety and Calling Frequency (VHF Ch-16) or SSB Safety and Hailing Frequency (2182 kHz, USB) and make an all stations announcement to cancel the DSC Distress Call.

DSC DISTRESS COMMUNICATION (MAYDAY) FORM. A copy of this form should be filled out with boat description ahead of time and posted near each fixed DSC VHF Radio on board. This form can be filled out and used as a script when issuing voice MAYDAY Calls.

DSC Primer

The purpose of this paper is to describe the hookup of GPS receivers to DSC capable VHF radios and provide information about obtaining an MMSI for your Yacht.

BACKGROUND.  CCA now lists MMSI numbers in The Fleet section of the annual yearbook.   A DSC tutorial article and a detailed description of how DSC connects with Search and Rescue (SAR) are both posted on the CCA Web Site at  A review of these papers and a careful reading of the DSC section of the Owner’s Manual for each of your DSC capable VHF radios are recommended.

HOOKUP.  The actual wiring of DSC involves connecting two wires coming from the ‘NMEA Out’ section of the GPS to the ‘NMEA In’ section of the VHF Radio.  Locating these wires is slightly different for each radio and GPS so the manuals must be consulted.  Most units have connection plugs which must be wired together by the installer.  Once that is done, the GPS Interface must be adjusted to insure the proper position message is being sent to the VHF radio. The Interface tab is normally found in the GPS Setup Menu.

MMSI.  This nine digit number is like a phone number that identifies your boat.  It must be inserted into the VHF radio before DSC will work.  Be careful when entering your number into the radio because, by law, it can only be enabled twice before it must be returned to the factory to be reset.   You can obtain an MMSI from one of three sources in the US:

  1. From your vessel’s Ship Radio Station Authorization license issued by the Federal Communications Commission (FCC).  These licenses are obtained on-line or by mail and cost $160.  Vessels that go beyond domestic operations are required to have this license.
  2. Free MMSI numbers are issued by Boat U/S to vessels that will be operating domestically.  See
  3. SeaTow also issues free MMSI numbers to vessels that operate domestically.  See

SUMMARY.  Once the MMSI is installed and the VHF radio connected to the GPS Digital Selective Calling operations can be learned by carefully following the owner’s manual.  The best way to become familiar with the procedures is to practice with another boat.  There is no established way to test the Distress feature so the red “DISTRESS” button should be pressed only in a true emergency.   Similarly, calls to “All Ships” should be avoided unless they are Urgent, Safety or Routine in nature like a ’Pan Pan’ or ‘Securite’ call.

Frequently Asked Questions & Answers

Q.  How can I tell if the GPS is connected and DSC working on my radio?

A.  Look at the screen on the face of the radio to see if your current GPS position and time are displayed.  If they are the GPS is connected properly to the VHF Radio.

Q.  What does “operating domestically” mean?

A.  Vessels are considered as operating domestically when they do not travel to foreign ports or do not transmit radio communications to foreign stations.  Sailing in international waters is permitted, so long as the previous conditions are met.  If you travel to a foreign port a FCC issued Ship Station Authorization license is required.

Q.  Where do I get a connection plug for my radio or GPS?

A.  Check the Installation or Owners Manual Accessory List and order it from the manufacturer, if it isn’t a recognized type of connector available at Radio Shack or other electronics source.

Q.  Where can I find the Owners or Installers manuals for my VHF or GPS?

A.  Try to find them on the Internet and print them or download them into a computer, CD or Flash Drive.

Q.  Will DSC work without connecting the radio to the GPS?

A.  DSC will not work until an MMSI is entered into the VHF Radio.  Pressing the Red Emergency Button on a radio not connected to an active GPS will identify your boat but not your location. 

Q. Will people know my boat’s name when I call them using DSC?

A.  NO.  DSC messages only contain your MMSI, not your boat’s name or call sign.  This means you must know your MMSI.  Posting your MMSI number next to each VHF radio is highly recommended.

Q.  Do I need an MMSI for each VHF Radio on board?

A.  NO.  Only one MMSI is necessary for each vessel.  That MMSI should be entered in all VHF radios on board.

Q.  If I can’t wire my own setup how much will this cost?

A.  It should take less than an hour for a qualified technician to install and setup a VHF DSC capable radio.

Q.   Who can I talk to about this subject?

A.  Ron Trossbach prepared this paper.  You can reach him aboard SUNNESHINEin Southwest Harbor, Maine and at or (703) 403-8408.