For the ocean sailor requiring long-range radio capability, the choice lies between HF-SSB radio (Marine or Amateur) and satellite communications. For reliable communications on a world-wide basis HF-SSB has its limitations.

These deficiencies provided the incentive for the development of satellite communications (SATCOM) for use at sea.

Satellite communication, the use of artificial satellites travelling in Earth orbits to provide communication links between various points on Earth, is man’s most important non-military exploitation of space technology.  Communication satellites permit the interchange of live television between nations and continents, and an international telephone service via Earth stations located in over 50 countries.  Basically, the technique involves transmitting the desired signals from an Earth station to an orbiting satellite.

 Equipment on board the satellite receives and amplifies these signals, then re-broadcasts them to another Earth station, thus completing the desired communication link.

To transmit TV signals and thousands of telephone signals simultaneously requires high-capacity circuits and in the case of communications satellites this is achieved via microwaves. These are very short radio waves of wavelength ranging from ten centimetres down to one centimetre corresponding to a frequency range between three gigahertz and 30 gigahertz (a gigahertz [gHz] is 1,000,000,000 [10′] cycles per second, or Hertz). Microwaves are launched or received by parabolic (bowl or dish-Shaped) reflectors, or from specially designed horn-Shaped antennae. The waves travel in straight lines in narrow beams so that transoceanic microwave systems were impossible before the advent of artificial satellites.

The present INMARSAT configuration of four operational and six spare satellites in geo-stationary orbits has been designed to give world-wide coverage between latitudes 75’N and 75’5 (see fig. 68-2), and provides extensive overlap coverage in many areas of the world. The International Maritime Satellite Organisation (Inmarsat) is a unique example of technical and commercial cooperation between nations.  Established in 1979 under an IM10 convention, its initial membership has grown from 26 to 60 countries, including all the major maritime nations and many developing nations.  Through their signatory organisation (usually telecommunication authorities such as British Telecom International for the UK) member countries own the organisation and contribute to the common system costs.

Inmarsat levies space segment utilisation charges on those Coast Earth Stations (CESS) accessing the system but end-user charges are not established by Inmarsat but by the telecommunications administrations offering satellite communications.

The Inmarsat space segment is based on four operational communications satellites, with backup satellites ready to be used if necessary.  As fig 68-2 shows, one is over the central Pacific Ocean, one over a position just off the Ivory Coast (Atlantic East). One over northern Brazil (Atlantic W) and one over the Indian Ocean, between them they cover almost the whole world with a high degree of overlap, and they operate in conjunction with 37 CES (Coast Earth Stations).

Each satellite draws electrical power from its own solar panels and is equipped with communication antennae and a supply of fuel for its small thruster motors which respond to signals from the Satellite Control Centre (SCC) (based in Inmarsat headquarters in London) which manoeuvre the satellite to maintain a geo-stationary Earth orbit approximately 36,000 km (22,300 miles) above the Earth’s equator.  The function of each satellite is to re-route messages between the Earth~ based segments of the Inmarsat system (NCSS, CES’s and SESS).

A Network Co-ordination Station (NCS), of which there are four. One in each ocean region, monitors and controls communication traffic within its region.  Using special inter-station signalling links, NCS’s exchange operational information on SES’s with the appropriate CES, which then knows how to respond to or from a particular SES (see fig. 68-2). 

A Coast Earth Station (CES) is a powerful land-based receiving and transmitting station which acts as a gateway between the Inmarsat space segment and the national or international fixed telecommunications network.  Generally, each CES in the Inmarsat system is owned and operated by an Inmarsat signatory (BT in the UK).

A Ship Earth Station (SES) is the name given to an Inmarsat maritime mobile terminal.  At present (1994) there are three different systems to which an SES may subscribe.  Inmarsat-A provides a wide range of services such as telephone, Telex, Fax, data, slow-scan and compressed video TV and a host of other features, but in practice this system is confined to merchant ships and millionaire’s motor-yachts, partly because of its enormous expense but also because the high space, weight and power requirements make it impractical for small yachts, particularly the onboard steerable antenna (aerial dish) 1 metre in diameter which must be monitorised and gyro stabilised to keep it pointing at the satellite and which does not have a response rate fast enough to cope with the movement of small yachts.  Small craft therefore prefer to use Inmarsat-C or Inmarsat-M.


The Inmarsat-C messaging system is based on digital technology, which means that anything that can be coded into digital data (text or numerical information, for example) can be sent and received as messages over the system, which does not support voice communication.

An Inmarsat-C SES omni-directional antenna (fig. 68-3) is about the size of a quart vacuum flask, has no moving parts and can transmit and receive signals from Inmarsat satellites anywhere within its horizon, even when the yacht is pitching and tossing in the heaviest seas.  It can do this at a data signalling rate of 600 bits per second.

An Inmarsat-C SES transceiver comprises the two parts shown in fig. 68-4, data terminal equipment (DTE) and data circuit, terminating equipment (DCE).  Depending on the manufacturer’s model the DTE and DCE may or may not be contained within one case.

The number of input/output devices are a matter for the operator to decide but the minimum would include a built-in keyboard, display and printer. Extras might include a personal computer with full-size keyboard, VDU and printer plus disk storage.  It is also possible to connect other input/output devices to the terminal such as navigational equipment (e.g. Decca, Loran-C, Navstar GPS or other systems) or monitoring sensors such as weather or course and speed sensors).

The DTE comprises electronics which (a) interface the SES to the operator and to the input/output devices (b) provide the text editor for formatting a message for transmission (c) store the prepare message until the DCE indicates that it is ready to transmit the message (d) display on the monitor and/or printer the messages received from the DCE and (e) update the SES status display.

The DCE comprises the omni-directional antenna, receiver and transmitter, and associated electronics which (a) provide the interface between the OTE and the satellite system (b) control the signalling and message access to the satellite and (c) assemble the complete received message and transfer it to the DTE for display. 

The two principal classes of Inmarsat-C SES are illustrated in fig. 68-5. A Class-1 SES may be used only for ship-to-shore and shore-to-ship messages and distress alerting, but is not able to receive EGC messages.  A Class-2 SES which is similar to Class 1 but also capable of receiving EGC messages when not engaged in other traffic.

Enhanced Group Call (EGC) receive facility enables an SES to receive maritime safety information from governments and authorised maritime authorities.  These information providers must be authorised by the IMO under the GMDSS to distribute Maritime Safety Information (MSI) from shore to ship. The Inmarsat system enables a land-based centre to broadcast an EGC message to selected groups of SES’s (e.g. within a specified geographical area, or to a specified type or class of vessel).

Authorised information providers include (i) Hydrographic Offices for navigational warnings and electronic chart correction data (ii) National Services for meteorological warnings and forecasts (iii) RCC’s (Rescue Co-ordination Centres) for shore-to-ship distress alerts and other urgent information and (iv) International Ice Patrol for North Atlantic ice hazards.  Fig 68-5 shows how an EGC message is broadcast.

To send an EGC message, the information provider prepares the message including its priority and sends it by telex to the CES specifying the group of SES’s or the area that is to receive the message.  The CES then processes the message and forwards it over an inter-station link to the NCS for the ocean region.  As shown in fig. 68-6, the NCS broadcasts the message on the NCS common channel throughout the ocean region.

Although all EGC Receive-equipped SES’s in the ocean region synchronised to the NCS common channel can receive the EGC message, the message is accepted only by those EGC receivers which have been pre-programmed for the area or group conditions contained in the message.  All other EGC receivers will reject the message.


As has been shown above, the Inmarsat-C communication system provides data massaging communications via satellite to and from virtually anywhere on the globe.  User terminals are small, simple and robust, with fixed omni-directional antennae weighing only a few kilos.  Because of the flexibility of the Inmarsat-C system, it can support a broad range of receive-only and transmit-only services as well as two-way massaging on a store and forward basis.

The Inmarsat-C system supports a wide range of applications including automatic position reporting by interfacing the terminal with a GPS receiver and (optionally) a compass and log as shown in fig 68-7.

The NMEA 0183 interface is a multipoint interface which allows the simultaneous connection of more than two pieces of equipment, e.g., a compass, speed log and GPS receiver so that the terminal can acquire course, speed and position every ten seconds.  The Inmarsat-C terminal can be programmed to transmit this information at fixed intervals to a pre-selected destination (e.g., chartered yachts can transmit their position on a regular basis so that the owner knows where they are).  It can also be programmed to respond to an incoming message, containing a special password, by transmitting its position.  The owner of a vessel can at any time send this ‘polling command’ with the appropriate PIN number and retrieve the vessel’s position.

Inmarsat-C can also be used to receive Electronic Chart Corrections, news bulletins, weather forecasts, electronic mail, etc.


In a major technological breakthrough, a new mobile satellite service Inmarsat-M was introduced in May 1993.  Using very efficient digital transmission techniques, the Inmarsat-M service can be accessed by smaller and lighter maritime terminals than previous satellite telephone systems (Inmarsat-A).  Maritime terminals are now available which can be fitted on craft as small as 10 metres (33 ft), bringing mobile satellite telephone communications to owners of leisure craft (yachts, motor cruisers, etc) in addition to many of the smaller commercial vessels.  It means the beginning of a new era of maritime communications for leisure craft.

The Inmarsat-M system uses advanced digital voice-coding techniques to enable secure quality transmissions with minimal power.  Initial outlay is relatively small.  The lightweight M-Sat Radome (fig. 68-8) contains the above deck stabilised antenna which is connected to a compact below-deck terminal via a single co-axial cable.  Total weight is around 9 kg.  These terminals cost less to buy than all earlier satellite systems, and call charges are significantly lower.

Inmarsat-M originally offered voice and (slow speed) fax communications but during 1994 has evolved to provide a voice band data facility (i.e. 2.4k/bits, using the public telephone network).

The service is accessed through BT’s CES at Goonhilly, Cornwall (fig 68-9), using two of the four Inmarsat sate sites which cover Europe, Africa, the Middle East and the Americas.  Goonhilly completes the interconnection to the UK national and International telephone networks, providing automatic direct dialling to over 200 countries world-wide.  With voice, fax and data facilities at your fingertips, your yacht can become an extension of your office and you can spend more time at sea.

A single-channel M-Sat unit can provide telephone/fax/data communications one at a time.  If you require simultaneous telephone calls, or simultaneous telephone calls and fax, a multi-channel installation will be necessary.


The long-term successor to Inmarsat-A is INMARSAT-B (or B-SAT).  The system is being phased in gradually during the mid 1990s and is planned as the ultimate replacement for Inmarsat-A.  In the meantime, both services will be run in parallel well beyond the year 2000.

Using very efficient digital transmission techniques, the B-SAT service is set to revolutionise mobile satellite communications by offering the same range of services as the analogue Inmarsat-A service, but at considerably reduced call charges.

B-SAT terminals will be much the same size, weight and cost as Inmarsat-A terminals but lower satellite power consumption and capacity will result in lower call charges.  B-SAT offers both greater flexibility and even higher quality calls.  Soon, live face-to-face videoconferencing will be possible with HSD (High Speed Data) links over the Integrated Services Digital Network (ISDN).

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