Satellite communications are possible through several systems. But only GMDSS compliant systems can guarantee 100% reliability, which is particularly important for SAR communications. Less expensive systems are available, but those tempted to use them should be mindful that they are not always reliable.


The International Maritime Satellite Organization (Inmarsat) is integral to the GMDSS through the provision of space-based means for relaying safety and distress messages via four geostationary satellites on positions above the equator. Inmarsat is a private operated company. The footprints of these satellites we call Ocean Regions:
  • Atlantic Ocean Region East (AOR-E)
  • Pacific Ocean Region (POR)
  • Indian Ocean Region (IOR)
  • Atlantic Ocean Region West (AOR-W)

Satellite coverage is not convenient for ships in the Polar Regions. Line-of-sight, a pre-requisite for the operation of the system, is possible above approximately 70 degrees north and south too infrequently (the satellite should be at least 5 degrees over the horizon).

Three systems are configured to provide most of the necessary GMDSS functions: Inmarsat-B, and Inmarsat-C and Inmarsat-Fleet (F 77).

The Inmarsat system is primarily useful for Sea Area 3. Distress alerts from Inmarsat ship stations are given absolute priority, automatically routed through the Inmarsat system to Rescue Co-ordination Centres (RCCs).

So the Inmarsat system allows distress alerts from ships at sea over long distances. The system also allows distress alerts from shore-to-ship (through EGC [enhanced group call], part of the Inmarsat-C complex, or by basic telex group calls to Inmarsat-Fleet or Inmarsat-B terminals)

A third function of the satellite system is to facilitate communications during SAR (search and rescue) operations.

A fourth function of the satellite system is to disseminate Maritime Safety Information (MSI). This process may work two ways, with the shore station alerting ships as well as ships alerting shore stations (regarding navigational or meteorological dangers).

Finally, various instruments of the satellite system may be used for general radio communications, allowing ships full communication with the shore—through telephone, FAX machines, telex, e-mail, etc.—so that potential crisis situations may at times be averted using expert advice or essential information transmitted from shore stations.

Different systems are used for different purposes. Inmarsat-C can only be used for shorter text messages, such as telex, e-mail, etc. On the other hand the cost for the user is relatively low. This resource is suitable for all vessels.
The Inmarsat-B system is more expansive and expensive, allowing for all types of communication, including voice and extensive data transmission. Of course these capabilities require greater resources - the antenna, for example, are quite large and heavy - and thus the system is not suitable for small vessels.
The Inmarsat-Fleet system has several advantages of its own. The capability depends on which of the three antenna sizes (33[cm], 55, and 77) are in use. All three allow voice communication, but the amount of data transmission (via e-mail) is dependant on antenna size. Fleet 33, naturally, is quite suitable for small craft. A spot beam, in telecommunications parlance, is a satellite signal that is specially concentrated in power (i.e. sent by a high-gain antenna) so that it will cover only a limited geographic area on Earth. From the aspect of GMDSS compliance, there will be reduced spot beam coverage for the Fleet 55 and Fleet 33.

Inmarsat-M and Inmarsat-miniM provide a lighter and thus more mobile means of using a Ship Earth Station (SES). These systems are not GMDSS compliant because they haven't world-wide coverage. They have the advantage of small equipment sizes - important especially regarding the antenna - and can be used for voice communication as well as some text. Though the Inmarsat-M includes a two-way global telephone, FAX, and computer data communications, it has no means for the direct printing of messages. Therefore, despite its similarity and usefulness, especially for smaller vessels, it does not meet GMDSS requirements, most importantly for receiving distress alerts, and is not part of the GMDSS system.

Inmarsat Ground Segment

Land Earth Stations (LES), also called Coast Earth Stations (CES), are stations owned and operated by telecommunications service providers in various countries around the world. Several LES's are situated in each Ocean region.

The LES operates as an international communication exchange, connecting communication from: ship to shore, shore to ship and ship to ship. The connections that have been made from the ship, through a LES could go to anywhere in the world.

In addition to providing normal communication, each LES has a direct connection with a Maritime Rescue Co-ordination centre (MRCC) for emergency communications. Usually the LES can provide a connection to a medical centre, when 'Medical Advice' is required.

For a list of MRCC's Associated with Inmarsat LES see appendix.

In each Ocean region there is one LES in a special role, this is the Network Coordination Station (NCS). The role of the NCS is to allocate a free (unoccupied) working channel to ships and to the LES's. The NCS retains control of all working channels on a particular satellite. No LES can hold a channel when there is no connection to be made. In this way, the satellite service is used most efficiently.

Two other LES station have special role:The Network Operations Centre (NOC) looks after the functioning of the complete communication network process. The Satellite Control Centre (SCC) is responsible for the functioning and positioning of the satellites in the four Inmarsat regions.

The SCC, NOC and the NCS's are all connected to each other to ensure a smooth and reliable communication network/process within the Inmarsat system. The Satellite Control Centre (SCC) and the Network Operations Centre (NOC) are located at the INMARSAT headquarters in London.

The communication unit on board is known as the Ship Earth Station (SES). Within the Inmarsat system also land mobile users or airplanes may use the system. in which case they are known as Mobile Earth Stations (MES). When a connection is made from a SES/MES, through a particular satellite, the call request includes the LES identity through which the ship operator wishes to be connected. The NCS allocates a working channel and directs the SES/MES to 'meet' on that channel.
Last modified: Saturday, 25 April 2020, 7:50 PM