The Global Maritime Distress and Safety System (GMDSS)

On the 1st February 1999, the International Maritime Organisation, (IMO), implemented a new, worldwide network of emergency communications for vessels at sea. This new system is known as the Global Maritime Distress and Safety System, (or GMDSS).

The basic concept of the GMDSS is that the search and rescue authorities, as well as shipping in the immediate area, are rapidly alerted to a distress incident so that they can assist in a coordinated search and rescue operation with the minimum of delay. The new system also provides for Urgency and Safety communications and the broadcasting of Maritime Safety Information, (NAVTEX).

The introduction of the GMDSS began in February 1992 and was fully in place by 1999. The GMDSS requirements now apply to all vessels over 300 Gross tonnage, passenger vessels carrying more than twelve passengers and fishing vessels over 12 meters, these are known as compliant vessels.

Although Lifeboats are not legally required to fit GMDSS equipment, it has been installed for operational reasons. In order for crew members to operate this they must hold a minumum of a Long Range Certificate, (LRC), which entitles the holder to operate both MF/HF and VHF equipment on any vessel not subject to compulsory-fit under the SOLAS convention.

Digital Selective Calling

The cornerstone of the GMDSS is Digital Selective Calling, (usually referred to as DSC).

Before the introduction of Digital Selective Calling, a radio operator would have to call another radio station using an RT Calling frequency, (typically Channel 16 VHF or 2182 kHz MF). This method of contacting another station relied on the station being called maintaining a listening watch on the relevant calling frequency.

But by using DSC, a radio operator can now send a digital signal, (known as an Alert), to a selected station prior to the voice transmission. The DSC's digital signal takes only seconds to transmit, but contains the transmitting station's identity and can include position. This Alert will act in a similar way as a telephone pager, automatically sounding an alarm on the called station's DSC receiver.

The Sailor RM2150 MF/HF DSC modem fitted to most ALBs

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 an alarm, radio operators will carefully monitor the radiotelephone knowing that a voice message is about to be transmitted that directly affects them.

An Alert can also be directed at a single, selected station. This is achieved by giving every radio station a unique code number, known as its Maritime Mobile Service Identity number, (or MMSI). An MMSI number works in the same way as a telephone number. By including a station’s MMSI number in the Alert, only that station’s receiver will sound an alarm. All other DSC receivers within range will remain silent.


These are radio beacons that are used only in a distress situation 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 EPIRBs are designed to automatically float free and self-activate.

Before abandoning ship, casualties would have already activated an EPIRB and equipped the liferaft with a SART and a handheld VHF radio.


Compliant vessels are also required to carry Search And Rescue Radar Transponders, (or SARTs). 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. There is also a SART that operates on VHF utilising the Automated Identification System (AIS) VHF channels. This SART has a GPS in built and can give accurate updated positional information.

The GMDSS Communication System

Under the new SOLAS Agreement, GMDSS compulsory-fit ships must be able to:

  • Transmit Ship-to-Shore Alerts by two separate and independent systems, each using a different radio-communication service.
  • Receive Shore-to-Ship Distress Alerts.
  • Transmit and receive Ship-to-Ship Distress Alerts and Bridge-to-Bridge communications.
  • Transmit and receive SAR Co-ordinating Communications and On-Scene Communications.
  • Transmit and receive homing or locating signals.
  • Transmit and receive Maritime Safety Information.
  • Transmit and receive general shore-based radio communications.