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Communication Equipment

The Global Maritime Distress and Safety System (GMDSS) is built on a communication platform which must full fill two primary requirements:

  • fast, accurate and reliable distress alerting including the related distress, urgency and safety communication;
  • the reliable dissemination of Maritime Safety Information.

The different Sea Areas, require different ranges of availability which implies the application of different technologies. Maritime radio telephony is available in different frequency bands offering very effective short-range communication on VHF up to world-wide communication with HF-SSB radio. This system based on terrestrial radio stations is complemented with communication over satellites offering data transmission and telephone communication in the Sea Areas 1,2 and 3.

Radio Equipment

VHF radio equipment with DSC controller is required for ships cruising in Sea Area 1 as well as for ship-to-ship communication (on-scene communication in distress situations). In Sea Area 2 DSC capable MF-SSB equipment is required for GMDSS compliance. Finally, in the polar regions of Sea Area 4 HF-SSB equipment with DSC capability is required. For short-range communication also (non-DSC) hand-held VHF radio-phones are available. These can also be used on survival rafts in distress situations to establish communication to ships in close vicinity.

More information on radio equipment and communication (regulations and technical details) is available in the section "Marine Radio Communication".

Satellite Equipment

SOLAS compliance for Area 3 requires satellite communication equipment. Although today, many satellite systems are available for communication purposes, only the INMARSAT-B and INMARSAT-C systems are compliant to the GMDSS standard (requiring a general alerting capability in Sea Area 3). On smaller vessels only the INMARSAT-C system, which can be operated with a small omni-directional antenna, is feasible.

The International Maritime Satellite Organization (INMARSAT) is an international consortium comprising over 75 international partners who provide maritime safety communications for ships at sea. In accordance with its convention, INMARSAT provides the space segment necessary for improved distress communications as well as maritime correspondence services.


This system provides distress alerting and distress priority messaging by telex. Every INMARSAT-C Land Earth Station (LES) is connected by a reliable telecommunications link to a Rescue-Coordination Center (RCC) with facilities to organize Search-and-Rescue activities in response to a distress alert or distress priority message received from a Mobile Earth Station (MES). Distress calls will be automatically routed through the INMARSAT-C network to an RCC, which will establish communications with the vessel in distress to organize the required rescue operations. This system does not provide voice communication, but it can be used to send and receive text or data messages (telex, e-mail, ...) at low data rates.

The INMARSAT-C system is a satellite modem, which provides a data interface between the Mobile Earth Station (MES) and the Land Earth Station (LES) using a transmitter-receiver link over a geo-synchronous satellite. The antenna must be able to maintain a Line-of-Sight to the selected satellite. Maritime INMARSAT-C uses a 10-centimetre omni-directional antenna and is able to transmit and receive messages even when the vessel is pitching and rolling in heavy seas.
The technique used for sending messages is known as store-and-forward messaging. To allow for error correction while sending the data packets from the mobile station (MES) to the land station (LES), the received message is temporally stored at the LES before it is forwarded over the public telecommunication networks to the intended destination.

The INMARSAT-C system supports Enhanced Group Calls (EGC), a technique that can be used to disseminate information to all ships or a group of ships. This technique is for example used by the INMARSAT SafetyNET service to broadcast weather reports and safety related information to all INMARSAT receivers that are logged in at the time of transmission. The SafetyNET service is part of the (GMDSS) and there are no extra user fees associated with receiving these maritime safety informations. The ability to receive SafetyNET service information is required for all ships that sail beyond coverage of NAVTEX.

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