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GMDSS Operational zones

Updated 11 Oct 2019

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For the purpose of GMDSS, four operational zones have been established, loosely based on distance from shore and in range of different communication systems.

  • Sea Area A1: the area within the radiotelephone coverage of at least one VHF coast station in which continuous DSC (Digital Selective Calling) alerting is available;
  • Sea Area A2: the area, excluding Sea Area A1, within the radiotelephone coverage of at least one MF coast station in which continuous DSC alerting is available;
  • Sea Area A3: the area, excluding Sea Areas A1 and A2, within the coverage of an approved satellite constellation in which continuous alerting is available; and
  • Sea Area A4: an area outside sea areas A1, A2 and A3.

In practical terms, this means that ships operating exclusively within about 35 n-miles from the shore may need to carry only equipment for VHF-DSC communications; those which go beyond this distance, up to about 150 to 400 nautical miles from shore, should carry both VHF-DSC and MF-DSC equipment; while those operating further from the shore but within the footprints of an approved satellite service should additionally carry approved satellite terminal(s). Previously, and until approved Iridium terminals are available, this has meant that only Inmarsat Connected vessels met this requirement.

In the early days of GMDSS, Inmarsat C was the preferred option and minimum requirement where satellite services were mandated. The larger Inmarsat A and B systems were also approved but these were quite expensive and considered as ‘overkill’ by many shipowners.

Current compliant services include Inmarsat B, Inmarsat C, Mini C and Fleet 77. Inmarsat’s L-Band satellite network is available in areas A1 to A3 but does not extend to area A4 which is effectively waters in Polar regions.

In these areas HF communications are required although vessels equipped with Iridium communication systems can communicate with shore and ship-to-ship providing both vessels have the equipment.

In 2018, after more than four years of lobbying, Iridium was finally given the green light as an authorised GMDSS supplier at MSC 99 and ending Inmarsat’s monopoly on safety service provision. Before Iridium can commence providing services it must first enter into a Public Services Agreement with IMSO and begin production of suitable equipment, either directly or in conjunction with equipment suppliers. Iridium is expected to be in a position to begin services in 2020. At the same MSC meeting in May 2018, Inmarsat’s Fleet Broadband service was also given approval for use in GMDSS. The IMO document, Resolution MSC.451(99) – Statement of recognition of maritime mobile satellite services provided by Iridium satellite LLC officially ended the Inmarsat monopoly under GMDSS.

Radio rules in coastal waters

Only ships operating in areas A3 and A4 are obliged to carry satellite communications, which means that radios (operating on VHF, HF and MF) are still considered the primary means of communication in emergency situations. In addition, search and rescue transponders (SARTs) and NAVTEX (Navigational Telex) are also required for GMDSS compliance.

SARTs are devices that are used to locate survival craft or distressed vessels by creating a series of dots on a rescuing ship’s X-band radar display. The detection range between these devices and ships, dependent upon the height of the ship’s radar mast and the height of the SART, is normally less than about ten miles. Initially only radar SARTS were allowed but since the advent of AIS, a hybrid AIS-SART has been permitted as an alternative. Most SARTs are cylindrical and in safety-orange colour.

NAVTEX is an international automated MF direct-printing service for delivery of navigational and meteorological warnings and forecasts, as well as urgent marine safety information to ships. It was developed to provide a low-cost, simple, and automated means of receiving information aboard ships at sea within approximately 200 nautical miles of shore. A NAVTEX is usually in a bracket-mounted cabinet with a small LCD screen displaying broadcast messages with an optional printout. Inmarsat’s SafetyNET service is an alternative to NAVTEX for ships that are equipped with satellite GMDSS equipment and it provides similar information.

Ensuring GMDSS availability

GMDSS regulations define three methods of ensuring availability of GMDSS equipment at sea:

  • At-sea electronic maintenance, requiring the carriage of a qualified radio/electronic officer (holding a GMDSS First- or Second-class Radio-Electronics Certificate) and adequate spares and manuals;
  • Duplication of certain equipment; or
  • Shore-based maintenance

Ships engaged on voyages in sea areas A1 and A2 are required to use at least one of the three maintenance methods outlined above, or a combination as may be approved by their administration. Ships engaged on voyages in sea areas A3 and A4 are required to use at least two of the methods outlined above. The lower requirement for A1 and A2 areas recognises that, being closer to shore, ships will have more opportunity to rectify problems.

The vast majority of ships do not opt for at-sea maintenance, preferring instead to duplicate the equipment and use shore-based maintenance (for A3 ships), or use shore-based maintenance only (A1 and A2 ships).

GMDSS equipment is required to be powered from three sources of supply:

  • ship’s normal alternators/generators;
  • ship’s emergency alternator/generator (if fitted); and
  • a dedicated radio battery supply. (The batteries are required to have a capacity to power the equipment for 1 hour on ships with an emergency generator and 6 hours on ships not fitted with an emergency generator).

Safety beyond GMDSS

Safety communications are not confined to the GMDSS requirements alone as there are other areas of SOLAS where communications that relate to safety are mandated and beyond that there is voluntary equipment that can be used to enhance safety, some of which will interact with GMDSS.

Included among the various equipment and systems outside of GMDSS are personal locator beacons (PLBs), which transmit alerts on emergency frequencies, and mandatory radar equipment that makes use of the communications spectrum. Technically, ships’ radar systems operate using radio transmissions, although they are considered collision avoidance systems rather than communication devices because they transmit information intended to be used by other vessels and shore stations.

The Automatic Identification System (AIS) was initially developed purely as a response to the issue of collision avoidance and as a corollary to aid shore-based VTS operators as well as navigators on ships to properly identify radar targets. AIS consists of a transponder system in which ships continually transmit their ID, position, course, speed and other data over VHF. The data transmitted is derived from ship’s equipment as regards position, course and speed, from initial input for the ID, which comprises ship’s name and call sign, and from direct manual input for other details such as port of destination and type of cargo.

Updated information is transmitted at regular intervals of very short duration. When received by other ships, the data is decoded and displayed for the officer of the watch, who can view AIS reports from all other AIS-equipped ships within range in graphic and text format. The AIS data may optionally be fed to the ship’s integrated navigation systems and radar plotting systems to provide AIS ‘tags’ for radar targets. It can also be logged to the ship’s Voyage Data Recorder (VDR) for playback and future analysis.

In 2000, IMO adopted a new requirement as part of a revised new SOLAS Chapter V for all ships to carry AIS capable of providing information about the ship to other ships and to coastal authorities automatically. The regulation requires AIS to be fitted aboard all ships of 300gt and upwards engaged on international voyages, cargo ships of 500gt and upwards not engaged on international voyages and all passenger ships irrespective of size. The requirement became effective for all ships by 31 December 2004.

The IMO regulation requires ships fitted with AIS to maintain AIS in operation at all times except where international agreements, rules or standards provide for the protection of navigational information. The regulation requires that the AIS must provide information – including the ship’s identity, type, position, course, speed, navigational status and other safety related information – automatically to appropriately-equipped shore stations, other ships and aircraft and to receive automatically such information from similarly-fitted ships.

AIS transmitters can also be attached to navigational marks or to hazards and transmit information that will complement the sight/ sound signals that may be present. These fixed AIS transmitters can also be used to give other information such as current strength and direction.

Although initially intended only for navigation use by ships and shore authorities, AIS data is now regularly disseminated by commercial operations either to subscribers or on a gratis basis allowing almost anyone to determine any specific ship’s current whereabouts and operational status. The IMO does not condone this use but appears powerless to prevent it.

Because AIS operates on VHF radio, there is a natural limit to the distance over which it can be transmitted. However, there is a small but growing number of service providers using satellites that can receive AIS signals when ships are out of the range of shore stations. These services are generally referred to as satellite AIS or S-AIS. Most of the service providers say that their services are targeted purely at national security organisations but others make no secret of the fact that their customers are often commercial organisations including commodity traders and analysts.

It is fair to say that AIS has not been universally welcomed by navigators or ship operators. Many believe that its introduction was rushed and insufficient thought given to its use under operational circumstances. Officially AIS is an aid to navigation and not a collision avoidance method in its own right. Some seafarers believe that AIS overrides COLREGs but official advice from some flag states makes clear this is not so.

Just as with radar in its early days, AIS has been blamed for causing rather than helping to avoid collisions. One of the factors that many say was ill-considered was the fact that, by being obliged to transmit its identity, cargo and destination, a ship can easily be identified by those with criminal intent or even terrorists. Some operators address this by either transmitting false information at certain stages of a voyage or by switching the device off except when in very close traffic situations.

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