External Communication

  • VHF
  • MF-HF
  • SART

Marine VHF radio is a worldwide system of two way radio transceivers on ships and watercraft used for bidirectional voice communication from ship-to-ship, ship-to-shore (for example with harbormasters), and in certain circumstances ship-to-aircraft. It uses FM channels in the very high frequency (VHF) radio band in the frequency range between 156 and 174 MHz, inclusive. In the official language of the International Telecommunication Union the band is called the VHF maritime mobile band. In some countries additional channels are used, such as[1] the L and F channels for leisure and fishing vessels in the Nordic countries (at 155.5–155.825 MHz). Transmitter power is limited to 25 watts, giving them a range of about 100 kilometres (62 mi; 54 nmi).

DSC equipment, a part of the Global Maritime Distress Safety System (GMDSS), provides all the functionality of voice-only equipment and, additionally, allows several other features:

The ability to call another vessel using a unique identifier known as a Maritime Mobile Service Identity (MMSI). This information is carried digitally and the receiving set will alert the operator of an incoming call once its own MMSI is detected. Calls are set up on the dedicated VHF channel 70 which DSC equipment must listen on continuously. The actual voice communication then takes place on a different channel specified by the caller.

A distress button, which automatically sends a digital distress signal identifying the calling vessel and the nature of the emergency
A built in GPS receiver or facility to connect an external GPS receiver so that the user’s location may be transmitted automatically along with a distress call.

High frequency (HF) is the ITU designation for the range of radio frequency electromagnetic waves (radio waves) between 3 and 30 megahertz (MHz). It is also known as the decameter band or decameter wave as its wavelengths range from one to ten decameters (ten to one hundred metres). Frequencies immediately below HF are denoted medium frequency (MF), while the next band of higher frequencies is known as the very high frequency (VHF) band. 

The HF band is a major part of the shortwave band of frequencies, so communication at these frequencies is often called shortwave radio. Because radio waves in this band can be reflected back to Earth by the ionosphere layer in the atmosphere – a method known as “skip” or “skywave” propagation – these frequencies are suitable for long-distance communication across intercontinental distances and for mountainous terrains which prevent line-of-sight communications. The band is used by international shortwave broadcasting stations (2.31–25.82 MHz), aviation communication, government time stations, weather stations, amateur radio and citizens band services, among other uses.









The radar-SART is used to locate a survival craft or distressed vessel by creating a series of dots on a rescuing ship’s radar display. A SART will only respond to a 9 GHz X-band (3 cm wavelength) radar. It will not be seen on S-band (10 cm) or other radar. Shipboard Global Maritime Distress Safety System (GMDSS) include one or more search and rescue locating devices. The radar-SART may be triggered by any X-band radar within a range of approximately 8 nautical miles (15 kilometers). 

Each radar pulse received causes the SART to transmit a response which is swept repetitively across the complete radar frequency band. When interrogated, it first sweeps rapidly (0.4 microsecond) through the band before beginning a relatively slow sweep (7.5 microseconds) through the band back to the starting frequency. 

This process is repeated for a total of twelve complete cycles. At some point in each sweep, the radar-SART frequency will match that of the interrogating radar and be within the pass band of the radar receiver. If the radar-SART is within range, the frequency match during each of the 12 slow sweeps will produce a response on the radar display, thus a line of 12 dots equally spaced by about 0.64 nautical mile (1.2 km) will be shown. When the range to the radar-SART is reduced to about 1 nautical mile (2 km), the radar display may show also the 12 responses generated during the fast sweeps. These additional dot responses, which also are equally spaced by 0.64 nautical mile (1.2 km), will be interspersed with the original line of 12 dots. They will appear stronger and larger the closer the interrogating radar gets, slowly becoming arcs at first until the SART is within 1NM, the arcs will become full circles indicating the active SART is in the general area.






An emergency position-indicating radiobeacon (EPIRB) is a type of emergency locator beacon, a portable battery powered radio transmitter used in emergencies to locate airplanes, vessels, and persons in distress and in need of immediate rescue. In the event of an emergency, such as the ship sinking or an airplane crash, the transmitter is activated and begins transmitting a continuous radio signal which is used by search and rescue teams to quickly locate the emergency and render aid. The signal is detected by satellites operated by an international consortium of rescue services, COSPAS-SARSAT.

The basic purpose of this system is to help rescuers find survivors within the so-called “golden day” (the first 24 hours following a traumatic event) during which the majority of survivors can usually be saved. The feature distinguishing modern EPIRBs, often called GPIRBs, from other types of emergency beacon is that it contains a GPS receiver and broadcasts its position, usually accurate within 100 metres (330 ft), to facilitate location.

The standard frequency of a modern EPIRB is 406 MHz. It is an internationally regulated mobile radiocommunication service that aids search and rescue operations to detect and locate distressed boats, aircraft, and people. It is distinct from a Satellite emergency position-indicating radiobeacon station.

A two-way radio is a radio that can both transmit and receive radio waves (a transceiver), unlike a broadcast receiver which only receives content. It is an audio (sound) transceiver, a transmitter and receiver in one unit, used for bidirectional person-to-person voice communication with other users with similar radios. Two-way radios are available in stationary (base station), mobile (installed in vehicles), and hand-held portable models. Hand-held two-way radios are often called walkie-talkies, handie-talkies or hand-helds. Two-way radios are used by groups of geographically separated people who need to keep in continuous voice communication, such as aircraft pilots and air traffic controllers, ship captains and harbormasters, emergency services personnel like firemen, policemen, and ambulance paramedics, taxi and delivery services, soldiers and military units, fast food and warehouse employees, and radio amateurs.

Two-way radio systems usually use a single radio channel and operate in a half-duplex mode: only one user on the channel can transmit at a time, so multiple users must take turns talking. The radio is normally in receive mode so the user can hear all other transmissions on the channel. When the user wants to talk he presses a “push-to-talk” button, which turns off the receiver and turns on the transmitter; when he releases the button the receiver is activated again. Other two-way radio systems operate in full-duplex mode, in which both parties can talk simultaneously. This requires either two separate radio channels or channel sharing methods such as time division duplex (TDD) to carry the two directions of the conversation simultaneously on a single radio frequency. A cell phone is an example of a full-duplex two-way radio. During a phone call, the phone communicates with the cell tower over two radio channels; an incoming one to carry the remote party’s voice to the user, and an outgoing one to carry the user’s voice to the remote party.


Inmarsat C is a two-way store and forward communication system that transmits messages in data packets in ship-to-shore, shore-to-ship and ship-to-ship direction.

Inmarsat C comprises of a small omnidirectional antenna, compact transceiver (transmitter and receiver), messaging unit and, if GMDSS-compliant or with a distress function, a Dedicated Distress Button to activate a Distress Alert.

Inmarsat Mini C terminals are the smallest models available, with some incorporating the antenna and transceiver in the same above deck unit and, depending on the model, supporting the same communication services as Inmarsat C terminals.

To protect crew and vessels, modern Inmarsat C and Mini C terminals include an integrated Global Navigational Satellite Services (GNSS) receiver, providing automatic terminal position updates and reporting when a distress alert is initiated. The vessel’s position data (position, course and speed) is shared with reporting applications.

FleetBroadband is a maritime global satellite internet, telephony, SMS texting and ISDN network for ocean-going vessels using portable domed terminal antennas.

These antennas, and corresponding indoor controllers, are used to connect phones and laptop computers from sailing vessels, on any ocean, with the rest of the world. All FleetBroadband antennas require line-of-sight to one of three geosynchronous orbit satellites, so the terminal can be used anywhere, including on land.

The FleetBroadband network was developed by Inmarsat and is composed of three geosynchronous orbiting satellites called I-4 that allow contiguous global coverage, except for the poles.[1] FleetBroadband systems installed on vessels may travel from ocean to ocean without human interaction. If there is line-of sight to one of the three I-4 satellites, then connectivity can be achieved, even in rough rolling seas. Since the FleetBroadband network uses the L band, rain fade is much less of an issue than the larger VSAT Ku band or C Band systems.

The FleetBroadband service was modeled after terrestrial Internet services where IP-based traffic Internet Protocol dominated over ISDN and other earlier communication protocols. Many corporations and IT departments are standardizing around IP traffic for data, and voice and text communication, so it is assumed Inmarsat is filling that long-term communications requirement.


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Supporting high-quality voice, IP data, prepaid services and global GMDSS, Iridium Certus will enable a complete range of maritime applications.

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