The U.S. Coast Guard offers MF/HF radiotelephone service to mariners as part of the Global Maritime Distress and Safety System. This service, called digital selective calling (DSC), allows mariners to instantly send an automatically formatted distress alert to the Coast Guard or other rescue authority anywhere in the world. Digital selective calling also allows mariners to initiate or receive distress, urgency, safety and routine radiotelephone calls to or from any similarly equipped vessel or shore station, without requiring either party to be near a radio loudspeaker. DSC acts like the dial and bell of a telephone, allowing you to "direct dial" and "ring" other radios, or allow others to "ring" you, without having to listen to a speaker. New VHF and HF radiotelephones have DSC capability.

History

On February 1, 1999, the Safety of Life at Sea (SOLAS) Convention, a treaty document, required all passenger ships and most other ships 300 grt and larger on international voyages, including all cargo ships, to carry DSC- equipped radios. Ships were allowed to turn off their 2182 kHz radio listening watch on that date. The International Maritime Organization has postponed indefinitely plans to suspend this VHF watch on ships.  It had originally planned to suspend this watch on February 1, 2005.

Because of the safety problems that lack of communications interoperability would cause between SOLAS-regulated vessels (mostly cargo ships) and other vessels (recreational boaters, commercial fishing vessels, etc.), the Coast Guard petitioned the Federal Communications Commission in 1992 to require all marine radios made or sold in the U.S. have a DSC capability. The Coast Guard had also asked the Radio Technical Commission for Maritime Services (RTCM), a non-profit professional organization, to develop a standard which would allow incorporation of DSC in a marine radio without affecting the low-end market price of that radio. The FCC solicited comments on that petition in 1992 and 1993, and prepared a Notice of Proposed Rulemaking on that and other maritime radiocommunications matters in early 1994. The FCC requested comments concerning that rulemaking from May to November 1995.  On 27 June 1997, the FCC adopted a Report and Order requiring radios type accepted on or after 17 June 1999 to include this minimum DSC capability.

Recommendations On Digital Selective Calling (DSC) Test Calls To Coast Stations

The International Telecommunications Union Sector for Radiocommunications has indicated that excessive test calls on MF/HF DSC distress and safety frequencies are overloading the system to the point where interference to distress and safety calls has become a cause for concern.. To minimize possible interference, live testing on DSC distress and safety frequencies with coast stations should be limited to once a week as recommended by the International Maritime Organization

US Coast Guard Sea Implementation of Areas A1 (VHF)

Currently, the USCG is implementing GMDSS  in Sea Area A1.One element of the USCG National Distress and Response System Modernization Project, is called "Rescue 21", which updates the USCG VHF distress system to include DSC capability & direction finding capabilities.

**The Coast Guard can't reliably receive VHF DSC distress calls where Rescue 21 has not been installed.

US Coast Guard Sea Implementation of Areas A2 (MF)

Many USCG Sectors operate MF DSC on a limited basis. The US currently does not have a declared Sea Area A2.

US Coast Guard Sea Implementation of Areas A3 &A4 (HF)

US Coast Guard DSC (HF) Equipped Shore Stations Areas A3 &A4

*The stations have limited MF capability.

Interconnection to a GPS Receiver

All DSC-equipped radios, and most GPS receivers, have an NMEA 0183 two-wire data protocol.  That NMEA protocol allows any model of GPS to be successfully interconnected to any model of radio, regardless of manufacture.  Although NMEA has no standard for the type of  cable or connector used, many if not most DSC and GPS receiver manufactures generally use ribbon cable with no connectors. These wires are simply connected between the radio and the GPS by twisting the wires (some people solder) and tape (some people use waterproof heat shrink tubing).  Note also that NMEA 0183 and IEC 61162-1 data interfaces are identical.

**The Coast Guard urges, in the strongest terms possible, that you take the time to interconnect your GPS and DSC-equipped radio.  Doing so may save your life in a distress situation! Before interconnecting your radio & GPS consult the owner's manuals.

The single largest operational problem of the U.S. Coast Guard concerning DSC had been responding to the large number of MF/HF DSC distress relays being sent by ships.  ITU regulations require each relay to be individually acknowledged.  The Coast Guard treats each distress alert relay as if it were a separate distress.  Worse, certain radios insert the identity of a ship sending a relay, rather than relaying the identity of the ship in distress. The USCG requested that vessels not relay any DSC distress message which has already been acknowledged.  If you do relay a distress message, make sure the identity of the vessel in distress is correct, and send the relay to a USCG radio station using an identity such as 003669999, rather than sending it to all ships.

Since this problem was identified, radio operators have cooperated to reduce the number of relays transmitted.  Consequently, this is far less of a problem now.

Continuing DSC problems include:

• The biggest problem is the lack of follow-up voice comms after transmission of a DSC call, particularly a distress call
• Unnecessary and frequent alarms
• Distress alerts without accurate location information
• Distress alerts with unregistered MMSI identification
• Limited use of DSC for routine communications
• Inconsistent and illogical software menu defaults
• Alarms disrupting ongoing radiocommunications

In 2001 the International Telecommunications Union, in addressing these problems, began a major update to their DSC standard Rec ITU-R M.493 to address these problems. Changes were adopted in and published in early 2004. The Safety of Life at Sea Convention now requires radios be interconnected to electronic position fixing devices (e.g. GPS receivers).  Radios meeting these new requirements should show significant improvement over earlier models, and many problems listed above should no longer occur.  Of course, new radios will be designed to be fully interoperable with older radios.

The International Maritime Organization Communications and Search & Rescue Subcommittee released COMSAR Circular 25 of 15 March 2001, which includes simplified flowcharts on the actions a person on a ship should perform on receipt of a distress alert using DSC-equipped radios.  The documents is in Acrobat PDF format.

DSC Forum

You may sign up for the free GMDSS email list and participate in the discussion forum sponsored by Densham and Associates, Australia. The Navigation Center nor the U.S. Coast Guard endorses this site; it is mentioned for the reference purposes only.

The DSC protocol is defined by ITU-R Recommendation M.493 (series), available from the International Telecommunications Union in Geneva, Switzerland.  DSC operation is defined by ITU-R Recommendation M.541 (series).

Class A:

• Distress call
• All-ships call
• Individual station call
• Semi-automatic/automatic service call
• Use of distress, urgency, safety and routine priorities
• Nature of distress
• Distress coordinates
• Time for last (distress) position update
• Type of subsequent communications
• Distress relay
• Distress acknowledgment
• Test call (for MF/HF only)
• Radio frequency or channel
• Display
• Receive geographical area calls
• Alarm
• Optional means for canceling a distress alert
• Polling
• Position acknowledgement
• Test call
• Test acknowledgement
• Data
• Data acknowledgement

All DSC options provided. Required on MF/HF and VHF radios used by SOLAS-regulated ships. Class A includes polling and vessel tracking, data, and numerous other functions in addition to voice.

Class B:

Required on VHF and MF radios used by SOLAS-regulated ships, though most such radios in fact meet Class A. Class B required capabilities include:

• Distress call
• All-ships call
• Individual station call
• Semi-automatic/automatic service call
• Use of distress, urgency, safety and routine priorities
• Nature of distress
• Distress coordinates
• Time for last (distress) position update
• Type of subsequent communications
• Distress relay
• Distress acknowledgment
• Test call (for MF/HF only)
• Radio frequency or channel
• Display
• Receive geographical area calls
• Alarm
• Optional means for canceling a distress alert

Class D:

Minimum DSC capability for VHF marine radios carried by recreational boaters, commercial fishing vessels, and other non-SOLAS regulated vessels. Class D required capabilities include:

• Distress call
• All-ships call
• Individual station call
• Use of distress, urgency, safety and routine priorities
• Nature of distress
• Distress coordinates
• Time for last (distress) position update
• Type of subsequent communications
• Radio VHF channel
• Display
• Receive distress relay and distress acknowledgment calls
• Alarm
• Distress acknowledgement (receive)
• Geographical area call (receive)
• Test call
• Test acknowledgement

Class E:

Minimum DSC capability for HF marine radios carried by recreational boaters, commercial fishing vessels, and other non-SOLAS regulated vessels. Class E required capabilities include:

• Distress call
• Individual station call
• Use of distress, urgency, safety and routine priorities
• Nature of distress
• Distress coordinates
• Time for last (distress) position update
• Type of subsequent communications
• Radio channel or frequency
• Display
• Receive distress relay and distress acknowledgment calls
• Test call
• Test acknowledgement

Back to Classes

RTCM SC101

RTCM Recommended Minimum Standard for DSC, Version 1.0, 10 Aug 1995, RTCM Paper 56-95/SC101-STD. Applies to VHF and to MF/HF. This standard is not generally recognized outside of the U.S. and should be replaced soon by the ITU Class D and E.  SC101 required capabilities include:

• Distress call
• All-ships call
• Individual station call
• Use of distress and routine priorities
• Use of safety priority (MF/HF only)
• Distress coordinates
• Time for last (distress) position update
• Acknowledgment or unable to comply response
• Receive distress relay and distress acknowledgment calls
• Receive Geographical area calls
• Test call (MF/HF only)
• Alarm

Back to Classes

ITU-R Rec. M.825-3:

DSC transponder system for Vessel Traffic Services or ship- to-ship interrogation and identification. Rec. M.825 is superseded by the Universal Shipborne Automatic Identification System.

Back to Classes

ITU-R Rec. M.821:

Optional expansion to DSC protocol.  Perhaps the most important M.821 expansion, now incorporated in most new DSC-equipped radios, improves the accuracy of distress position from 1 mile to the accuracy of your interconnected GPS receiver (about 13 meters).

More information concerning DSC

• USCG Notice to Mariners MF/HF DSC Guidelines (pdf)
• DSC Operational Procedures for Ships
• How Digital Selective Calling Works
• Maritime Mobile Service Identities Explained
• Why Coast Guard Supports DSC on Marine Radios