Shipping Emergency

The basic concept of the GMDSS is that search and rescue authorities ashore, as well as shipping in the immediate vicinity of the ship in distress, will be rapidly alerted to a distress incident so they can assist in a co‑ordinated search and rescue operation with the minimum of delay.

The system also provides for urgency and safety communications and the dissemination of maritime safety information, including navigational and meteorological warnings. This means that every ship will be able, irrespective of the area in which it operates, to perform those communication functions considered essential for the safety of the ship itself and of other ships operating in the same area.

Although satellites play an important part in the GMDSS, terrestrial radio is still important. The GMDSS combines various subsystems ‑ which all have different limitations with respect to coverage ‑ into one overall system, and the oceans are divided into four sea areas:

Area A1		Within range of VHF coast stations (about 20‑30 miles)
Area A2		Beyond area Al, but within range of MF coastal stations (about l00 miles)
Area A3		Beyond the first two areas, but within coverage of geostationary maritime communication satellites (in practice this means Inmarsat). This covers the area between roughly 70 deg N and 70 deg S.
Area A4		The remaining sea areas. The most important of these is the sea around the North Pole (the area around the South Pole is mostly land). Geostationary satellites, which are positioned above the equator, cannot reach this far.

Equipment requirements vary according to the area (or areas) in which the ship operates. Coastal vessels, for example, only have to carry minimal equipment if they do not operate beyond the range of shore‑based VHF radio stations. Ships which do go further from land have to carry MF equipment as well as VHF. Ships which operate beyond MF range have to carry HF or Inmarsat equipment in addition to VHF and MF. Ships which operate in area A4 have to carry HF, MF and VHF equipment.

In addition to radio equipment, the GMDSS introduced requirements for other equipment designed to improve the chances of survival, including float-free emergency position‑indicating radio beacons (EPIRBs) and radar transponders for location of the ship or survival craft in distress.

The GMDSS enables a ship which is in distress to send a message in various ways and be virtually certain that it will be heard and acted upon. The distress or safety message is picked up by ships in the area and by shore stations within range if sent on MF and VHF or by shore stations if transmitted using HF, Inmarsat or COSPAS‑SARSAT.

While ships can transmit an alert using any or all of the distress equipment available on board, ships transmit a ship‑to‑ship alert on MF or VHF and a ship‑to‑shore alert in areas A3 or A4, as appropriate, by a ship earth station, HF digital selective calling (DSC) communications or a satellite EPIRB as appropriate. Ships sailing in area A2 should at least be capable of transmitting a ship‑to‑shore alert on 2,187.5 kHz using DSC and satellite EPIRB and ships sailing in area A1 should at least be capable of transmitting a ship‑to‑shore alert on 156.525 MHz (Channel 70) using DSC. The system means that distress messages are received much more reliably than before and with the SAR Convention of l979, search and rescue operations are better co‑ordinated.

In the event of an incident, the whole operation is co‑ordinated by a designated rescue co‑ordination centre (RCC) which is informed of the alert either through the Inmarsat or COSPAS‑SARSAT systems or from coast radio stations participating in the GMDSS.

For ships equipped with an Inmarsat ship earth station (SES) sending a distress alert is both simple and certain. It only involves pressing a special distress button or using an abbreviated dialling code. This automatically gives priority access to the system and establishes contact with a coast earth station (CES) via the Inmarsat satellite. The message automatically gives the ship's name and position. The CES, being part of the international search and rescue system, will for its part immediately inform the rescue co‑ordination centre (RCC) located nearest to the ship in distress and thereby initiate a search and rescue operation.

Although satellite communications provide a primary means of sending distress messages for ships fitted with an Inmarsat SES, the latter can also use other features of the system if desired ‑ for example they can establish direct contact with an RCC by telex or telephone.

Back to top

The GMDSS and the implementation of the SAR Convention have both been made possible by advances in technology. But technology is continuing to evolve. Much of the work carried out by IMO to develop a global radio-communications system for shipping is based on a resolution adopted in 1979, which in turn took into account technology as it then existed. Since then a great deal has changed. The invention of the cellular telephone means that

However, larger vessels must have the radio equipment specified in the GMDSS regulations. For smaller vessels, not covered by the GMDSS, most coastal authorities do not recommend cellular telephones as a substitute for the marine radio distress and safety systems in the VHF maritime radio band.

• Inmarsat‑A:		The Inmarsat‑A ship earth station consists of a parabolic antenna (or dish) which is usually mounted on the ship's superstructure, while below deck there is telex and telephone equipment and associated hardware. Some ships may add computers and visual display units (VDUs) to the system.
• Inmarsat‑C:		Inmarsat‑C does not provide for voice communications, but it does enable telex operations and distress messages to be relayed in the same way. Its omnidirectional antenna has the advantage of not having to be stabilized and its small size and low cost also makes Inmarsat‑C ideal for small craft, such as yachts and fishing vessels. It has been accepted as an alternative to Inmarsat‑A or an HF radio installation for all SOLAS Convention ships operating in GMDSS sea area A3.
• Inmarsat-E:		This was announced in January 1997 and is also fully compliant with the GMDSS. It combines the position determination of the Global Positioning System (GPS) with Inmarsat's own technology, thereby greatly increasing the speed at which a distress alert can be delivered, since no time is lost waiting for a satellite to appear over the horizon. The system includes float-free and hand-held EPIRBs.

The ability of EGC to be selective has considerable advantages as far as safety is concerned. For example, it enables messages to be sent to ships in the area nearest to a ship in distress or can enable the RCC to select the fastest (or nearest) ships to respond to a distress call. No other ships would have to be inconvenienced.

Inmarsat have developed two EGC services. The first, called FleetNET, is a commercial service and enables shipowners and others to send information to specific ships. This is done by preceding the message by a special calling code and for additional commercial security the message itself can be encoded.

The second service, called SafetyNET, is the one that is of interest to the GMDSS. It enables ships on the high seas in areas not covered by NAVTEX (which has a maximum range of about 500 nautical miles) to receive NAVTEX‑type information. It can also be used selectively, so that safety information can be sent to ships in a particular area, rather than all ships. SafetyNET EGC broadcasts are only used by authorized services such as NAVAREA co‑ordinators, meteorological offices, rescue co‑ordination centres and so on.

Trials of the EGC system in 1987 proved highly successful, with an error rate of close to zero. The trials also showed that receivers and antennas can be produced that are cheap and small enough to be used on small craft of below 300 gross tonnage, such as fishing boats. EGC is attractive as a means of distributing maritime safety information. NAVTEX may never be provided in some coastal areas where broadcasts of MSI are too few to justify the cost and is not be able to serve ocean areas which are out of range. SafetyNET fills in these "blanks" in the NAVTEX system.

These devices have been available for many years. They are designed to provide an alert in the event of a sudden disaster: they can be fully automatic so that if a ship sinks, the EPIRB will float free and automatically transmit a distress message. Others are designed to be activated manually and, except when they are used as a second means of alerting, are usually located in or close to survival craft. They will continue to transmit a signal for at least 48 hours after the accident to enable search and rescue units to home in on the signal.

Under the GMDSS, satellite EPIRBs operate either on l.6 GHz (the Inmarsat frequency) or the 406 MHz frequency used by the COSPAS ‑SARSAT system, which was established in l982 and consists of a number of polar‑orbiting satellites which provide world‑wide coverage. The system, whose Secretariat is based at Inmarsat's London headquarters, enables distress messages transmitted by EPIRBs carried by ships or aircraft to be positioned with a degree of accuracy which enables SAR units to find the persons in distress as quickly as possible.

The satellite system involves a minimum of four polar orbiting satellites in low altitude polar orbit and there are currently 33 LUTs (local user terminals) or monitoring stations and 19 MCCs (mission control centres) which ensure processing of distress alerts from 406 MHz beacons anywhere in the world. Because the system's satellites are in a low polar orbit, there may be a delay in receiving the distress message, unless the footprint^[2] of the satellite is simultaneously in view of a monitoring station (LUT). World-wide coverage is only possible with satellite EPIRBs operating on 406 MHz as those operating on the aeronautical emergency frequency 121.5 MHz or 243 MHz can only be received when an LUT is within the satellite's footprint. Signals on 406 MHz are recorded on the satellite and transmitted to an LUT as it comes within the satellite footprint*.

Not all ships operating in area A3 are equipped with Inmarsat ship earth stations. Those that are not can use HF radiocommunications as an alternative, and even Inmarsat equipped ships need HF radio when out of Inmarsat range (COSPAS‑SARSAT is designed purely for EPIRB distress alerts, not two‑way communications).

Distress alerting and safety calling on terrestrial frequencies (HF, MF and VHF) is carried out by means of digital selective calling (DSC). For distress and safety purposes a number of frequencies have been assigned. They are 2,187.5 kHz in the MF band: 4,207.5 kHz, 6,312 kHz, 8,414.5 kHz, 12,577 kHz and 16,804.5 kHz in the HF bands; and 156.525 MHz (Channel 70) in the VHF band. The reason for the large number of frequencies in the HF band is that propagation characteristics vary according to the geographical position and time of day and the choice of a frequency will therefore depend on where and when the incident occurs. HF ships keep watch on at least 8,414.5 kHz and also on the HF frequency most suited to the area in which they are sailing. Watch may also be kept on all frequencies by means of a scanning receiver.

DSC distress messages include such information as the identity of the caller (automatically), the nature of the distress (it provides for nine different indicators ranging from fire or explosion to abandoning ship); and the position of the ship and the time, both of which may be automatically included in the message if the ship has position‑fixing equipment and a navigation interface. After the initial distress alert and acknowledgement, subsequent communications are made by radiotelephony or narrow‑band direct printing (NBDP) as indicated in the distress message.

A medium‑range service is provided using 2,187.5 kHz for DSC and 2,182 kHz for radiotelephony (the current radiotelephone distress and calling frequency). This is used for SAR co‑ordinating functions and on‑scene communications, while 2,174.5 kHz is used for distress and safety traffic by NBDP. The 5l8 kHz frequency is used for NAVTEX messages.

The GMDSS is designed to ensure that help arrives with the minimum of delay. But in some cases it will be impossible to reach the scene of an accident before the ship involved sinks. In such cases, the distress alert will have been sent by the ship or automatically by the satellite EPIRB, and the survivors will have embarked into survival craft. The main difficulty facing rescuers is simply finding them.

One device which assists rescue units to locate ships and survival craft by night or day in all weather conditions is the radar transponder, which is activated by receiving a radar pulse. It then automatically sends out a series of pulses which are displayed on the radar screen of the interrogating ship or aircraft. This clearly identifies the transponder's position, making it much easier for SAR units to reach the spot quickly.

All ships of 300 gross tonnage and above are required to carry a receiver, capable of receiving international NAVTEX broadcasts in areas where these are provided. NAVTEX became mandatory for all cargo ships of 300 gross tonnage and above and all passenger ships under the GMDSS, on 1 August 1993.

NAVTEX, which operates on 518 kHz, is the chief means of transmitting short range maritime safety information (MSI) including navigational warnings, meteorological forecasts and warnings, ice reports, search and rescue information, pilot messages and details of changes to navigational aids.

NAVTEX messages are normally sent only in English using narrow‑band direct printing (NBDP) and are received on board the ship on a special printer ‑ a great improvement on information circulated in printed form, which can take days or even longer to reach ships. Many small ships do not carry radiotelegraph equipment and a dedicated radio officer, so at present, receive MSI by radiotelephone broadcasts on different frequencies at scheduled times. Broadcasts can often be missed when other duties have to take priority.

It was recognized at the start that NBDP offered an excellent means of sending out this information. The fact that the messages are in written form and can be studied at leisure is important, especially when the recipients are not fluent in English. Another advantage is that information which is required can be selected by the operator on the equipment, so that which is not needed, will not be printed. However, important information which should be received by all ships will always be printed.

Although the receiving ship can be selective to some extent, and the receiver is unattended, a ship cannot reject navigational and meteorological warnings and search and rescue information. However, NAVTEX is not regarded primarily as a means of transmitting distress information ‑ under the GMDSS this is done on the distress and safety frequencies.

The early experiments with NAVTEX proved so successful that in 1979, countries bordering the Baltic Sea established the first NAVTEX network. This was then extended to NAVAREA I, which covers the sea areas off north western Europe. A resolution adopted by the IMO Assembly in the same year recommended Administrations to introduce NBDP broadcasts for promulgating navigational and meteorological warnings to shipping as part of the WWNWS (World-Wide Navigational Warning System).

Since then, NAVTEX has spread to many other NAVAREAS and in 1987, it was formally adopted as a component of the GMDSS by means of the IMO Assembly resolution A.617(15). This invites Governments to encourage the use of NAVTEX and provides information on how to commence NAVTEX services.

International NAVTEX messages are broadcast at fixed times on 518 kHz in English. But in many areas there is interest in transmitting similar information in a second language (for the benefit of local shipping, fishermen and so on). Messages with a high degree of urgency affecting safety in specific sea areas may also be transmitted in national languages on 518 kHz and in some areas messages are also sent out on the 4 MHz frequencies.

Although IMO's chief interest is the safety of ocean‑going merchant shipping, NAVTEX can also be of great value to smaller craft, such as private yachts. The equipment required to receive NAVTEX messages is comparatively cheap from about £500 upwards ‑ and it does not have to be permanently attended.

One important issue before IMO and ITU while developing GMDSS, concerned the role of the radio officer under the GMDSS. Before GMDSS, ships required to be fitted with radio-telegraphy equipment had to carry a radio officer trained in the use of Morse Code. The introduction of the GMDSS, however, meant the gradual phasing‑out of Morse radiotelegraphy in favour of direct‑printing telegraphy ("TELEX" by radio).

As a result, many Governments maintained that there was no need to have a radio officer on board, since it requires no special skills to use a radiotelephone or operate the other emergency equipment that is required by the system. Other Governments insisted that although a radio officer may not be required, a radio specialist would still be needed to carry out on‑board maintenance and emergency repairs.

A compromise between these two positions was successfully achieved and the 1988 SOLAS amendments state that ships operating in areas A1 and A2 must ensure the availability of equipment "by using such methods as duplication of equipment, shore‑based maintenance or at‑sea electronic maintenance capability, or a combination of these, as may be approved by the Administration". In areas A3 and A4 a combination of at least two of these methods must be used.

Regulation 16 in Chapter IV of SOLAS states that "every ship shall carry personnel qualified for distress and safety radiocommunication purposes to the satisfaction of the Administration. The personnel shall be holders of certificates specified in the radio regulations as appropriate, any one of whom shall be designated to have primary responsibility for radio communications during distress incidents".

While the rapid transmission and reception of distress messages is the most important task of radio at sea, it is essential that warnings be given to ships on matters which can affect their safety. These include the establishment and malfunction of lights, sound signals, buoys and other aids to navigation; the location of wrecks and other hazards and the establishment of offshore structures.

To ensure such information is received by all ships likely to be affected by it, IMO and the International Hydrographic Organization (IHO) established a World‑Wide Navigational Warning Service (WWNWS). This service was adopted by the IMO Assembly in 1977 and a revised system was adopted by the Assembly in 1979.

^[1]Using High Frequency (HF) equipment solves the problem of range, but HF equipment was not a mandatory requirement before the GMDSS. With HF, the ionospheres of the earth act as a "mirror", reflecting radio waves back to earth.

^[2]Footprint means the line of sight from the LUT to the satellite as it rises above the horizon at the time when it has received sufficient information to enable the position of the EPIRB to be calculated until it sets.

Back to top

The GMDSS

How GMDSS Works

Evolving technology

The components of the GMDSS

Enhanced group calling (EGC)

Emergency position‑indicating radio beacons (EPIRBs)

High frequency (HF) service

Digital selective calling (DSC)

Short‑range service

Survival craft radio equipment ‑ Radar transponders

NAVTEX

Radio Personnel

The World‑Wide Navigational Warning Service (WWNWS)