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Global Maritime Distress and Safety System-GMDSS

iMarinex communication system mainly refers to the GMDSS system, which is the English abbreviation of the Global Maritime Distress and Safety System. GMDSS is a distress and safety communication system established on the basis of modern radio communication technology to meet the needs of maritime search and rescue and safety communications, and the system also meets the conventional communication services of ships


GMDSS is a shore-based ship communication system. The basic concept of GMDSS is that the search and rescue authorities on shore, as well as other ships in the vicinity of the ship in distress and the person in distress, can quickly receive the distress alarm and quickly carry out search and rescue coordination. GMDSS can also provide emergency and safety communications and broadcast maritime safety information (navigational warnings, weather warnings, weather forecasts, and other emergency safety information, etc.). In other words, no matter what sea area the ship is sailing in, it will be able to carry out all communication tasks that are important to the safety of the ship and other ships sailing in the same sea area.


Functions of GMDSS GMDSS requires that all vessels at sea, regardless of the sea zone in which they sail, must have the following 9 functions:

(1) To send a ship-to-shore distress alarm, at least two separate devices should be used, each of which should use a different wireless telecommunications service;

(2) receive shore-to-ship distress alerts;

(3) sending and receiving ship-to-ship distress alerts;

(4) Sending and receiving search and rescue coordination communication messages;

(5) Sending and receiving on-site communication information;

(6) Sending and receiving positioning signals;

(7) sending and receiving maritime security information (MSI);

(8) sending and receiving conventional radiocommunication messages between ships and shore radiocommunication systems or networks;

(9) Send and receive bridge-to-bridge communication information.


GMDSS provides an alarm method that enables a ship in distress to emit an alarm signal indicating that it needs immediate assistance. The International Convention on Search and Rescue at Sea establishes an internationally harmonized SAR Plan. The world is divided into 13 search and rescue districts and cities, and the standards for the organization, cooperation and search and rescue procedures of search and rescue are defined. The International Convention for the Safety of Life at Sea (SOLAS) requires signatory States to provide rescue services and coast stations to be on duty along their coastlines, providing coordinated search and rescue and assistance to ships and persons in distress at sea, in accordance with the Search and Rescue Convention, established by the International Maritime Organization (IMO) in 1985.

Maritime demarcation of GMDSS According to the 1974 SOLAS Convention, a ship’s radio equipment is equipped according to the tonnage of its ship. In the GMDSS, the deployment of the ship’s radio equipment is determined according to the sea area in which the ship is sailing, so in the GMDSS IMO clearly defines four sea areas.

The marine area division of the GMDSS (see Figure) is as follows:


Schematic diagram of the division of the GMDSS sea area

A1 Sea Area – within the wireless telephone coverage of at least one VHF coast station, where VHF DSC alarms can be implemented. This sea area can be extended to about 30~50 nautical miles from the VHF coast station position to the sea as an alarm area.

A2 Sea Area – Ship to shore MF DSC alarms can be implemented in this sea area within the wireless telephone coverage of at least one MF coast station. This area is set to be approximately 150 nautical miles offshore, but does not include any designated A1 area. In fact, the coverage of the A2 zone has reached 250 nautical miles from the MF coast station.

A3 Sea Zone – Within the INMARSAT geostationary satellite coverage, i.e., within 70° of the Earth’s north and south latitudes, but excluding the designated A1 and A2 sea zones. This sea area can be continuously alarmed by ships and shores.

Sea area A4 – the area other than the A1, A2 and A3 sea areas, which are basically the sea areas near the north and south poles outside the 70° north and south latitudes. This area can only be alarmed using HF radio communication equipment.

The communication system in GMDSS can be summarized into four major subsystems, namely the ground communication system, the maritime satellite communication (INMARSAT) system, the positioning and positioning system, and the maritime safety information broadcasting system. Each subsystem further comprises a plurality of communication equipment , and the communication equipment mainly comprises:


(1) Terrestrial communication equipment, including MF/HF combination radio, radio with DSC, NBDP radio terminal equipment, portable VHF wireless walkie-talkie, VHF¬DSC wireless telephone equipment, etc.;

(2) Satellite communication equipment, including station A (discontinued in 2007), station B, station C, station M, station D, station P, station F and station E, etc.;

(3) Positioning and positioning equipment, including emergency radio position indicator (EPIRB) and search and rescue radar transponder (SART-SAR Radar Transponder);

(4) Maritime safety information broadcasting and receiving equipment, including navigation warning receiver (NAVTEX), enhanced group call (EGC) equipment or satellite communication equipment with EGC receiving function, etc.

  1. Terrestrial Communications
  2. Long-distance service In ship-to-shore and shore-to-ship direction communications, high-frequency (HF) can be used for long-distance communication. In the coverage area of the INMARSAT system ship communication system 4 1 chapter, both high-frequency and satellite communications can be used. Outside the coverage area of INMARSAT, generally referred to as the A4 sea zone, HF is the only means of long-range communication, and in the 4, 6, 8, 12 and 16 MHz bands, frequencies for use by the long-range communications service are specified. General diagram of ship communication equipment


  1. Medium-range service The medium-range service is communication on frequencies in the 2MHz band. In ship-to-shore, ship-to-ship and shore-to-ship directional communications, distress alerts and safety calls can be made using DSC at 2187.5kHz; Use radiophone over 2182kHz for distress and safety communications, including search and rescue coordination communications and field communications. 2174.5kHz will be used for distress and security communications for narrowband direct-print telegraphy (NBDP).
  2. Proximity service The proximity service is communication in the frequency band of very high frequency (VHF) radiotelephony. Capable of proximity distress alerts and distress communications at frequencies of:

(1) 156.525MHz (70 channels) for distress alerts and safety calls using DSC;

(2) 156.8MHz of the VHF16 channel for distress and safety communications, including search and rescue coordination communications and on-site communications, using a radiophone.

(3) VHF operating frequency used for daily communication.


GMDSS terminal


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