Lesson 1 Brief Introduction to GMDSS

In November 1988 a decade of work reached a successful conclusion when a conference held at IMO headquarters adopted amendments to the SOLAS Convention and its 1978 Protocol. The amendments were intended to introduce GMDSS into the convention.The date chosen for entry into force，under the SOLAS"tacit acceptance"provisions，was 1st February 1992.

The basic concept of the system 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 coordinated search and rescue operation with the minimum delay.

The system will also provide for urgency and safety communications and the dissemination of maritime safety information（MSI），including navigational and meteorological warnings.In other words，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 will play an important part in GMDSS，they will not completely replace existing terrestrial radio. GMDSS combines various sub-systems which all have different limitations with respect to coverage into one overall system and the oceans are divided into four areas:

Area A1:within range of shore-based VHF coast stations（20～30 miles）;

Area A2:within range of shore-based MF coast stations（excluding A1 areas，in the order of 100 miles）;

Area A3: within the coverage area of geostationary maritime communication satellites（excluding A1 and A2 areas，approximately between 70°N and 70°S）;

Area A4:the remaining sea areas outside areas Al，A2 and A3.

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.

In all areas of operation the continuous availability of alerting should be provided.

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

In addition to radio equipment，GMDSS will introduce requirements for other equipment designed to improve the chances of survival.This includes EPIRBs and radar transponders for location of the ship or survival craft in distress.

Basically，GMDSS has seven functions，namely:1.alerting（from ship-to-ship，ship-to-shore，and shore-to-ship directions）; 2. search and rescue coordinating communications; 3. on-scene communications;4.locating;5.promulgation of maritime safety information;6.general radiocommunications;7.bridge-to-bridge communications.

The GMDSS consists of satellite communications and terrestrial communications. The use of satellite communications to improve maritime safety is particularly important for the introduction of global system and for establishing a reliable communication network.

Satellite communications will be used in both ship-to-shore and shore-to-ship directions. The INMARSAT satellite system，which employs geostationary satellites and operates in 1.5 and 1.6 GHz frequency bands，will provide a means of alerting from ships by using ship earth stations（SES）or satellite EPIRBs and a capability for two-way communications using radiotelex and optionally radiotelephone.Broadcast of maritime safety information to ships using radiotelex will also be provided through the INMARSAT system using either a standard ship station and associated equipment or dedicated facilities.A near polar-orbiting satellite EPIRB service（COSPAS-SARSAT system），operating in 406～406.1MHz frequency band，will provide a main means of distress alerting and determining the location of float-free EPIRBs operating through the system. Two types of shipborne equipment will be used for satellite communications:

（1）ship earth stations，approved by INMARSAT;

（2）satellite EPIRBs capable of being activated manually and of being activated automatically on floating free from a sinking ship.

The terrestrial communication comprises long-range service，medium-range service and shore-range service. HF will provide long-range service for use in the ship-to-shore and shore-to-ship directions.In areas covered by INMARSAT it can be used as an alternative to satellite communications and outside these areas it will provide the only long-range communications capability.Frequencies have been designated in 4，6，8，12 and 16MHz bands to provide means for transmitting and receiving distress alerts and safety calls and for passing distress and safety traffic.

Digital selective calling（DSC）will form the basis for distress alerting and safety calling.Coast stations participating in the HF distress and safety watch-keeping network will need to choose from five available frequency bands relaying an alert.The choice will depend on the position of the ship in distress，the geographical area to be alerted and the current propagation characteristics.It is envisaged that ships equipped with the HF option will maintain a watch on 8MHz alerting frequency and one of the other dedicated HF frequencies.The latter will be the frequency most appropriate to the area in which the ship is sailing.

Distress and safety communications following DSC calling will be performed by radiotelephony or narrow-band direct-printing（NBDP）or both.

A medium-range service will be provided on frequencies in 2MHz band.In the ship-to-shore，ship-to-ship and shore-to-ship directions，2187.5kHz will be used for distress alerts and safety calls by DSC，and 2182kHz will be used for distress and safety traffic by radiotelephony，including SAR coordinating functions and on-scene communications.2174.5kHz will be used for distress and safety traffic by radiotelex（NBDP）.

Frequencies near 500kHz will be used in the shore-to-ship direction. The frequency 518kHz will be used to transmit navigational and meteorological warnings in NAVTEX system. VHF will provide a short-range service on following frequencies:

（1）156.525MHz（channel 70）for distress alerts and safety calls by DSC;

（2）156.8MHz（channel 16）for distress and safety traffic by radiotelephony，including SAR coordinating functions and on_scene communications.