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Digital Selective Calling (DSC) for Marine VHF Radios

A Critical Safety Function on Your Boat

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Digital Selective Calling (DSC) for Marine VHF Radios

Digital Selective Calling (DSC) is an emergency calling system that is now built into most fixed-mount marine VHF radios and some handheld VHF models. DSC allows a boater in an emergency to simply press one button to initiate a distress (mayday) call that provides rescuers with the exact GPS location of the boat.

DSC is an important and possibly live-saving emergency system that boaters should understand - but it must be activated by the boat owner. This can be one of the most important types of safety equipment on your boat.

Many boat owners have purchased and installed VHF radios with DSC capability but have not gone to the trouble of activating the DSC function. Unless correctly activated, the DSC function simply does not work. Although the VHF radio will still work and allow you to make a mayday call in an emergency, the DSC function provides better protection and is essential in emergencies where the boater is not able to complete a mayday call and inform the Coast Guard of the boat's location.

Because only one emergency button on the radio is required to make a DSC distress call, the captain can easily show guests or crew what to do in an emergency if the captain is incapacitated.

Why Use DSC?

Before the advent of DSC, a boater in an emergency had to manually make a mayday call on VHF channel 16. Since many other boaters use channel 16 to hail others (against Coast Guard rules, which dictate hailing on channel 9), there is a chance that a manual emergency call on 16 simply may not be heard because of other radio noise or interference. The DSC signal, however, is digital and is better communicated in poor conditions.

Additionally, in many emergencies the radio caller is in a state of panic and having to act very quickly to save another person or the boat. It takes time to place a call, transmit the emergency, and read the longitude and latitude numbers from a GPS display into the microphone. There is some risk that the transmission may be unclear and the location not correctly understood.

And in many cases the nature of the emergency causes the boater to drop the radio mic before giving all information, in order to attend to the emergency. The tragic result can be that even if a rescue effort is launched, without the exact location of the boat, help may be delayed too long.

DSC eliminates this risk with its automatic function on a dedicated emergency marine channel (70).

How Does DSC Work?

Every DSC-activated VHF radio is issued a unique ID number, called an MMSI number (Maritime Mobile Service Identity). That number provides rescuers with additional information about the boat and the owner (including the owner's cell number and telephone numbers for emergency contacts). If an automatic distress call is made, rescuers will know what type of boat and whom they are looking for.

All DSC radios can also be connected to a GPS unit (a chartplotter or dedicated GPS receiver). Although this is not required in order to make a distress call, it is very strongly recommended, because with it the emergency automatic distress call will inform the Coast Guard (and other nearby, DSC-equipped boats by sounding an alarm) of your exact location.

By pressing the one emergency button on your DSC VHF, you start the radio making automatic distress calls that provide your MMSI identity and location (if GPS is connected). This call is continually repeated until the Coast Guard answers or it is manually stopped, allowing you to attend to the emergency.

How to Set Up and Activate DSC

Two key steps are required to set up and activate a DSC radio. First, apply for a free MMSI number for your boat. (Note that the number is assigned to the boat itself, not to the radio, so you use the same MMSI with a handheld radio used on the same boat.) BoatU.S. provides a free online service to obtain your MMSI for boats in U.S. waters. U.S. boaters in international waters require an FCC license and an MMSI from the FCC.

The MMSI number is entered into the DSC radio following the instructions in the owner's manual.

Second, connect the DSC radio to a GPS receiver (except for some newer radios with a GPS receiver built in). Unfortunately this step is somewhat more complicated with first-generation DSC radios, since there are many different types and brands of equipment that may be combined here. For newer DSC VHF radios and GPS chartplotters, the wiring is usually explained in the equipment manuals. Consult a marine technician if needed.

Then it's a good idea to test the equipment with a DSC routine call to another DSC radio. If your radio has a "test call" function, you can call the Coast Guard for an automatic call acknowledgment. Never place a distress call to test the equipment!

Other DSC functions

DSC-equipped and activated radios can use the DSC system for making calls other than distress, emergency calls. If you know the MMSI of another boater, you can establish initial contact with that boat's radio directly without having to hail the boat by name on an open channel; after the DSC sound signal, you communicate as usual on regular channels.

Similarly, you can use DSC to obtain the location of a known boat by polling its unique MMSI number.

While all DSC-equipped radios have a single emergency switch to initiate a distress call, operating these other functions varies among different brands. See your VHF owner's manual for steps to follow.

For more informationabout DSC classes and functions (important to know before you buy), go here. To apply for a free MMSI number online, go to this website.

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