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VHF/GPS/DSC Interface

THE VHF/DSC/GPS INTERFACE

By Capt Ed Wiser for Coastal Angler Magazine. April 2013.

The most valuable electronic safety device on your boat is the simple VHF radio with DSC interfaced with your GPS/chartplotter. The VHF operates on a discrete band width and is dedicated for marine use. It is commonly used for calls for assistance, current weather reports, and casual conversation with other boats. Although it is widely used by hunters and contractors in some areas, it is only intended for terrestrial usage by licensed stations. You can tell if your VHF radio has DSC capability by the presence of a red emergency button on the front. No button means you have an antiquated analog unit and it should be replaced. Fixed mount analog units are no longer available. VHF radios are inexpensive and this is no place to cut costs.

Digital Selective Calling (DSC) is an emergency calling system built into 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 call that provides rescuers with the exact GPS location of the boat.

When the VHF is interfaced with a GPS/chartplotter and the unit is properly wired and “enabled” the radio is being fed a constant update of the vessel position. When the operator presses the red “panic” button the VHF instantaneously broadcasts a distress signal with the GPS supplied position data. The signal goes to all other properly installed VHF/DSC units including the Coast Guard, towing companies, and most recreational vessels. If the boater has previously registered his unit with BOAT/US or some other designated agency, he will have received an MMSI and placed his boat data in a centralized data base. Thus, rescuers know who they are looking for and critical information about the boat.

This is an incredible improvement over the old voice contact system. People in distress are liable to be panicked, have a difficult time communicating, and may be in a situation with a weak or intermittent signal. They are trying to handle the emergency, while answering questions from the Coast Guard and supplying position information. After all, the only time you are going to call the USCG is when you have an extreme emergency like the boat is on fire or sinking, or your best friend is having a heart attack. The last thing you need to do is have a lengthy conversation with someone ashore describing the nature of the emergency, boat position, vessel description, etc. This is extremely important. USCG reports that 90% of distress calls lack position data!!!

To utilize DSC apply for a free MMSI number for your boat. The number is assigned to the boat, 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 VHF/DSC radio following the instructions in the owner’s manual.

Then test your radio with a call to another DSC radio. Towing companies routinely do this as a public service. 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.

Please note, to utilize this life saving technology, you must have a VHF radio with DSC capability, it must be properly interfaced with a working GPS/chartplotter, and the GPS must be “enabled” to transmit position data to the VHF. If in doubt about your situation, contact a certified marine electronics technician.