The VHF Radio Distress Call: Digital Selective Calling (DSC)

Digital Selective Calling helps the rescue services help you. Photo: Master Sergeant Rick Cowan, United States Coast Guard (USCG)

When a vessel is in distress, the faster that search and rescue authorities can be alerted the quicker that a coordinated search and rescue operation can begin. That premise was the impetus behind the International Maritime Organization’s call for the development of a Global Maritime Distress and Safety System (GMDSS) in 1979.

Today, GMDSS is a reality. It allows the mariner to use an amalgam of communications equipment, both satellite and radio, in order to have the best chance of alerting authorities in the event of a life threatening situation. When far out at sea the EPIRB (Emergency Position Indicating Radio Beacon) is the perfect method to alert search and rescue services since it transmits directly to a constellation of satellites. Closer to shore, within about 20 nautical miles, the VHF radio is the go to device to use in the event of a Mayday situation.

However, when a yacht is rapidly taking on water and all efforts are aimed at keeping the vessel afloat, the captain may be unable to transmit a timely Mayday call. Wouldn’t it be nice, during this high stress moment, to just push one button to quickly alert the Coast Guard? That is exactly what Digital Selective Calling (DSC) is all about.

All new VHF radios have a red ‘distress’ button that will automatically establish initial contact with a coast radio station and nearby vessels equipped with a DSC VHF radio, and will continue to repeat the distress call until acknowledged by the coast authorities.

DSC VHF radios must first be programmed with the vessel’s registered Maritime Mobile Service Identity (MMSI), a unique identifying number that can be looked up in a worldwide database to find the boat’s emergency contact information. Next, the radio must be connected to the boat’s GPS receiver. This will allow the vessel’s latitude and longitude, as well as the current time, to be transmitted with the automated distress call. The latest generation of DSC VHF radios have an internal GPS that eliminates the need to hard wire the radio to the boat’s GPS.

One of ten pre-programmed ‘nature of distress’ options can be transmitted to assist in marshaling the proper resources by the search and rescue team. These predefined distress designations are: Fire or explosion, flooding, collision, grounding, listing, sinking, disabled and adrift, abandoning ship, man overboard and piracy. If time is of the essence, then the message can be immediately sent in the default mode with an ‘undesignated’ nature of the distress.

The VHF Radio Distress Call: Digital Selective Calling helps the rescue services help you. Photo: Master Sergeant Rick Cowan, United States Coast Guard (USCG)
Digital Selective Calling helps the rescue services help you. Photo: Master Sergeant Rick Cowan, United States Coast Guard (USCG)

Until a coast station acknowledges the DSC alert, the vessel’s VHF radio will continue to broadcast the alert approximately every four minutes. Once acknowledged, the boat’s distress alert is automatically stopped and simultaneously switched to channel 16. At this point, the boat’s radio operator can begin transmitting a standard voice Mayday call and message to start the detailed conversation needed with the search and rescue coast station.

A marine VHF DSC radio has a built-in dedicated receiver that constantly monitors channel 70, the sole digital channel on the radio. This allows the boat’s radio to receive any DSC distress alerts from nearby vessels. When sending a DSC distress alert, the radio is automatically switched to channel 70 when the distress button is pushed. It then broadcasts an ‘all stations’ coded digital Mayday message over Channel 70 to any VHF DSC radios within range. The radios receiving the alert hear a loud two-tone warble as the details of the alert are shown on the radio’s visual display. Channel 70 is only used for DSC communication, and voice communication is not allowed on that frequency.

To avoid a false alert the distress button is protected by a plastic cover, and the button itself must be pushed and held down for several seconds before the transmission will begin. In the event of an accidental false distress alert, the radio should first be turned off to stop the repetition of the alert. Next, broadcast an all stations voice message on channel 16 cancelling the distress alert, making sure to include the boat’s MMSI number.

While VHF DSC technology is a boon to safety, the United States Coast Guard tracks major issues that cause digital distress alerts to be less effective. The biggest problem is the lack of a follow-up voice Mayday call after the distress alert is acknowledged. Next are distress alerts that are sent without accurate location information, followed by calls using unregistered MMSI numbers.

 

Capt. Jeff Werner is a 23 year veteran of the yachting industry. In addition to working as a captain on private and charter yachts, both sail and power, he is a certified instructor for the RYA, MCA, USCG and US Sailing.

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