GMDSS Communication functions

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GMDSS Communication functions

  • Distress alerting is the rapid and successful reporting of a distress incident to a unit which can provide or co-ordinate assistance, as prescribed in RR N3112* this would be a rescue co-ordination center (RCC) or another ship in the vicinity. When an alert is received by an RCC, normally via a coast station or coast earth station, the RCC will relay the alert to SAR units and to ships in the vicinity of the distress incident.


  • The communication arrangements under the GMDSS are designed to enable distress alerting to be performed in all three directions – ship-to-shore, ship-to-ship and shore-to-ship – in all sea areas the alerting function is based on both satellite and terrestrial means and the initial distress alert is primarily transmitted in the ship-to-shore direction. 
  • When the distress alert is transmitted by DSC on VHF, MF or HF, ships within DSC range of the ship in distress will also be alerted (ship-to-ship alerting) 
  • A distress alert is normally initiated manually and all distress alerts are acknowledged manually. When a ship sinks, the (EPIRB) is automatically activated. 
  • Ships operating exclusively in sea area A1 may, in lieu of satellite EPIRBs, use VHF EPIRBs on channel 70. 
  • The relaying of a distress alert from an RCC to ships in the region of a distress incident is made by satellite communication or by terrestrial communication, using appropriate frequencies. In either case, to avoid all ships in a large sea area being alerted, an ‘‘area call’’ is normally transmitted so that only those ships in the region of the distress incident are alerted. On receipt of a relayed distress alert, ships in the area addressed are required to establish communication with the RCC concerned to enable the assistance to be co-ordinated.


SAR co-ordinating communications

In general, these are the communications necessary for the co-ordination of ships and aircraft participating in a search and rescue operation following a distress alert and include communications between RCCs and any on-scene commander (OSC) or co-ordinator surface search (CSS) in the area of the ship in distress

For SAR operations, messages are transmitted in both directions, as distinct from ‘‘alerting’’, which is generally the transmission of a specific message in one direction only, and distress and safety traffic by RadioTelephone (R/T) and direct-printing telegraphy will normally be used for passing such messages On-scene communications normally take place in the VHF and MF bands on frequencies designated for distress and safety traffic by RadioTelephone CH16 (156.8MHZ) or direct-printing telegraphy (2182 KHZ).

These communications between the ship in distress and assisting units relate to the provision of assistance to the ship or the rescue of survivors. When aircraft are involved in on-scene communications they are normally able to use 3023, 4125 and 5680 kHz. In addition, SAR aircraft can be provided with equipment to communicate on 2182 kHz or 156.8 MHz or both, as well as on other maritime mobile frequencies. Locating is the finding of a ship in distress or its survival craft or survivors, as defined by regulation In the GMDSS this function is performed by means of 9 GHz SAR radar transponders (SARTs) carried by the ship in distress or its survivors, whose position is indicated when the SART is interrogated by the searching unit’s 9 GHz radar. Use of the frequency 121.5 MHZ (used by aircraft) in most satellite EPIRBs is provided for homing by aeronautical SAR units.

Bridge-to-bridge communications are inter-ship safety communications conducted from the position from which the ship is normally navigated normally performed by VHF RadioTelephone 


maritime safety information (MSI)

Ships need to be provided with up-to-date navigational warnings and meteorological warnings and forecasts and other urgent maritime safety information (MSI). MSI is made available by narrow-band direct-printing (NBDP) telegraphe broadcasts, using forward error correction (FEC), on the frequency 518 kHz by NAVTEX service (International NAVTEX service ) and 490 KHZ (national NAVTEX service) and, for ships which navigate beyond the NAVTEX coverage, by broadcasts via the Inmarsat enhanced group call (EGC) system (known as the International SafetyNET system). broadcast system by HF direct-printing telegraphy NBDP IN Area 4