The Global Maritime Distress and Safety System (GMDSS)

The Sailor RM2042 VHF DSC Modem fitted to most ALBs

DSC Distress Alerts:

If the Alert is sent to ‘All Stations’, (as in a Distress Alert), then all DSC receivers within range will sound an alarm and store the details of the Alert in the receiver’s memory. After hearing the alarm all radio operators in the immediate area are to tune to Channel 16 and be ready to log the voice Mayday message, which should immediately follow the Distress Alert


Another improvement in technology has lead to the use of Emergency Position Indicating Radio Beacons, or EPIRB's. These are radio beacons that are only used in a distress and once activated, will transmit a continuous distress signal. This signal will be detected by one of a number of specialised satellites and relayed back to a Rescue Co-ordination Centre. The signals can contain details of the vessel’s identity and position, greatly speeding up the process of organising a suitable rescue. Should a vessel sink, most EPIRB's are designed to automatically float free and self-activate.


Ships required to fit GMDSS equipment are also required to carry Search And Rescue Radar Transponders, (or SART's). These are radar transponders that are normally used in a life raft and provide Search And Rescue units with a homing signal, when they are ‘interrogated’ by a radar set at 9GHz.


Within the GMDSS, the oceans of the world have been divided into four distinct areas, known as Sea Areas. These Sea Areas have been defined by the type of radio coverage offered by the Coast Radio Stations, (or Coastguard Radio Stations) and available communication satellites. Under the GMDSS, the radio equipment which a vessel has to carry now depends on which of these Sea Areas it operates in.

*See side note for sea area explanations

Sea areas: *

Sea Area A1

The area within the radio telephone coverage range of at least one VHF coast station in which continuous DSC Alerting is possible.

Sea Area A2

The area, (excluding Sea Area A1), within the radio telephone coverage range of at least one MF coast station in which continuous DSC Alerting is available.

Sea Area A3

The area, (excluding Sea Areas A1 and A2), within the coverage of an INMARSAT geostationary satellite in which continuous alerting is available. This extends from latitudes 76 degrees North to 76 degrees South.

Sea Area A4

The area outside A1, A2 and A3. The Polar Regions, above 76 degrees North and below 76 degrees South.