• Communication equipment include VHF radios, Digital Selective Calling (DSC), Satellite systems (IMARSAT, COSPAS – SARSAT) Global Maritime Safety System (GMDSS), etc.


For standard marine radio communication principally three kinds of equipment are used:

VHF transceivers for short-range (up to 50 Km) communications
HF-SSB transceivers for long-range (up to 5000 Km) communications
Satellite transceivers for world-wide communications

A transceiver is a combined transmitter-receiver communication unit, in which some of the hardware (e.g. the user controls) is combined to reduce cost and complexity. VHF equipment is relative cheap and most user friendly. HF-SSB equipment is more expensive and technically more difficult to manage.

The distance over which communication can be conducted, strongly depends on the radiation performance of the antenna used. This includes high-quality matched cabling between transmitter and antenna as well as a careful grounding installation.

Additionally, to the HF-SSB bands that are reserved for marine communication there are also the HF-SSB bands that are used by radio amateurs (HAM radio). If a valid HAM licence and associated call sign are available, also these HF-bands can be used to establish radio connections with other HAM stations on ships or on land. This offers an additional communication channel to obtain weather forecasts or technical (marine) information.

VHF Transceiver

Marine radio sets for use on VHF are very user friendly with only three basic controls:

1. a control to select the frequency for transmitting and receiving, using simple channel numbers (Ch.),
2. a control to set the audio volume level for the received signal (VOL),
3. and a third control to set the squelch level (SQL), which silences the normal background noise when no signal is being received.

Other controls that may be found on the VHF set allow to quickly shift to the distress and calling channel (16) or to scan a variety of channels.

The IMO regulations require that every VHF set be capable of operating on Channels 16 and a working channel, but today, all sets, even the small hand-helds, can tune to all marine channels (about 58 channels in the range of 156.0 MHz to 164.0 MHz). Many channels are appointed to specific types of communication such as ship-to-ship communication or distress communication.

Maritime VHF communication with Digital Selective Calling (DSC) is an integrated part of the Global Distress and Safety System (GMDSS) introduced by the IMO in 1991 in order to improve the safety of live at sea (primary for commercial shipping). In order to participate in the GMDSS a VHF tranceiver will have to be equipped with a DSC controller enabling Digital Selective Calling.

This is important also for pleasure yachts, since after the introduction of GMDSS, the IMO has discontinued the permanent watch obligation on the distress channels (for both commercial shipping and the coast stations). almost all newly build maritime VHF sets have DSC capability. But in order to use the DSC feature, the equipment must be registered with the national telecommunications authorities. Through registration, the DSC radio station will obtain a unique calling number and call sign. In most countries also an operator's certificate is required to use DSC featured VHF radio transceivers.

HF - SSB Transceiver

Transceivers for AM-SSB communications are larger and more complicated to handle than those for VHF. SSB equipment is mainly used for long-range communication. The maximum RF power is usually 150W. As on VHF transceivers all AM-SSB transceivers have an instant 2182kHz selection capability. This is the distress and calling frequency in the MF radio band. The range of SSB communications is dependent on sky waves, so it is extremely sensitive to atmospheric and ionospheric interference. Transmission conditions can vary strongly on a seasonally, daily and even hourly basis. With additional hardware including a radio modem, a text decoder and a printer, SSB equipment can also be used to pull down weather faxes around the world and gain an up-to-date and accurate picture of the weather systems and forecast for the region.

Some SSB coast stations with internet connections also offer a simple data communication service for mariners enabling the on-board transmission and reception of E-mail. This requires a radio modem and a PC-based software coder-decoder. Due to the limited audio bandwidth, the data rate (characters per second) is limited to about 1200 baud allowing for only text-based E-mail without binary attachments. For more information on this service and on the required hard- and software, please link to “SailMail.com“. For operating an AM-SSB radio station, a stations license and an operating permit are required. Also regard that in some countries (e.g. USA) it is not allowed to use SSB transceivers for short-range communication.

Satellite Transceiver

The four geo-stationary INMARSAT satellites form the space segment of the GMDSS system. In this scope, two different standards can be used for maritime communication: INMARSAT-B for direct-calling telephone, data and telex communication, and INMARSAT-C for telex communication.

Since the INMARSAT-B standard requires a large directional satellite antenna, which must be continuously kept aligned to the communication satellite, this communication standard is not feasible on small pleasure yachts. The communication standard INMARSAT-C is based on narrow-band telex communication and requires only a simple omni-directional antenna. This communication type will allow world-wide telex communication but no voice communication.

The required equipment for INMARSAT-C telex communication consists of a small satellite transceiver and a PC-based telex station using the PC-screen and keyboard for printing and writing text-based telex messages. Since recently, also Email service has become feasible with this equipment.

GMDSS Stations

Depending on the sea areas (A1-A4) a ship operates in, the GMDSS requirements will dictate what communications systems are carried. It is possible to assemble a GMDSS compliant set-up from individual components but most ships are fitted with an integrated station supplied by one of the many specialist communications and navigation equipment providers.

An integrated station has several benefits over a custom assembled setup. Firstly, compatibility and connectivity are guaranteed and secondly only a single power supply connection is needed. As most integrated systems are supplied by major equipment suppliers, the issue of spare parts and repairs is likely to be much less of a problem with access to an established network of agents and repair centers.

Even with an integrated station there are some peripheral items that are installed elsewhere, the bridge fixed VHF being a good example. The systems are mostly quite compact and with the screens only needed to display text messages, there is no need for the large displays used elsewhere on the bridge for ECDIS and radar use.

Vessels are obliged to carry trained GMDSS operators who must supervise the use of the communications but the automated emergency DSC aspect of the equipment means that in a distress situation, all necessary information can be sent automatically at the touch of a single button by any crew member.

Marine communications satellite systems

Satellite systems comprise two main components, the antenna which is installed above the bridge and the electronics and displays below. With most L-Band systems, the cost of the system and antenna will fall to the ship operator and the equipment will be owned outright.

The under-deck components of a satellite system are normally nothing other than a less than imposing box of electronics to which multiple components can be attached. If the system has been installed solely for GMDSS purposes, the only connected devices will be the GMDSS station and any remote displays.

Where the satellite system has been installed for reasons other than GMDSS, the attached devices can be many and various. In many ships the satellite communication unit will be connected to a local area network (LAN) to which will also be connected several PCs, communication devices such as telephones, faxes and possible wireless hubs allowing use of mobile phones, PDAs and tablets. Updating of electronic navigation charts is already common on many ships and as the rollout of mandatory ECDIS accelerates it will become even more so.

Another use that is growing is the monitoring of engines and other equipment on board. Sensors on engines recording temperature, pressure and multiple other parameters using a proprietary control unit can have the data they recorded compiled and sent via the satellite to the machinery supplier for constant diagnostics and to satisfy computer-based maintenance programmes. Remote monitoring and reporting need not be confined to machinery, it is possible to link an output from a ship’s VDR to the communication system and so supply the shore office with information for incident investigation or even real-time monitoring in emergencies.

On certain research and seismic vessels, the data from instruments can also be compiled and dispatched automatically. Despite satellite equipment having now been installed on ships for around four decades, it has to be said that the opportunities and benefits that it offers are only just beginning to be explored. However, with the world fleet growing rapidly in numbers and data usage expanding even faster, the limits of even the increased bandwidth allowed by expansion of VSAT into the Ku and Ka bands could be reached in the not too distant future.

Besides, the highly compact cylindrical Iridium antenna, the smallest and least powerful satellite antenna for use on board vessels is the usually conical shaped antenna of the Inmarsat C system. Both are omnidirectional and therefore require no moving parts inside the protective cover. The low power of the Inmarsat C antenna is one of the reasons why the system has to operate on a store and forward basis.

The next step up to Inmarsat Fleet requires a dish antenna that can move on its horizontal and vertical axes to stay aligned with the satellite. Inmarsat Fleet has three sizes of antenna with diameters of 33cm, 55cm and 77cm the particular size corresponding to the system installed. These antennae like all other marine versions are protected by a radome that can either be dome shaped or spherical.

VSAT antennae are more sophisticated still and also are generally much larger than the Inmarsat Fleet versions. They are 3-axis stabilized systems and the dish which can be 3m or more in diameters (although the trend is to smaller 1m size dishes) can move rapidly in any direction to maintain connectivity with the satellite. Most antennae are designed for use with a single band only but as ships are beginning to subscribe to more than one service and more antennae are needed, some manufacturers are looking to combine bands in a single hybrid antenna. As far as hybrid antennas are concerned, there are several major manufacturers which have developed antennas that are capable of instantly switching between Ku-band and Ka-band networks.