Global Maritime Distress and Safety System (GMDSS) Explained

GMDSS (Global Maritime Distress and Safety System) definition, working, equipment, sea areas, regulations, requirements and more have been explained in utmost details in this article.

Global Maritime Distress and Safety System (GMDSS) Explained

Definition of GMDSS

Global Maritime Distress and Safety System (GMDSS) is the internationally accepted set of safety procedures, types of equipment and communication protocols used to increase safety at the seas and make it easier to rescue all distressed ships, boats and aircrafts. As per the rules of IMO (1999) the GMDSS represents a worldwide network of automated emergency communications for ships on sail. SOLAS Convention Chapter IV states that all ocean-going passenger ships and cargo ships of 300 gross tonnage and above are required to carry radio equipment that confirms to international standards. The main purpose of GMDSS is preventing unanswered distress calls and delay in Search and Rescue actions when there are situations of distress. GMDSS assures “that any emergency at sea will result in a distress call and the response to that call will be immediate and effective” – (IMO, Shipping Emergencies – Search and Rescue and the GMDSS, March 1999).

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Important Elements and Equipment of GMDSS

The important elements and equipment of GMDSS are listed below:

  • INMARSAT: INMARSAT is a Satellite operated system which incorporates ship earth station terminals – Inmarsat B, C and F77. It provides telex, telephone and data transfer services between shore-to-ship, ship-to-shore, and ship-to-ship along with a priority telex and telephone service connected to shore rescue centers.
  • NAVTEX: It is an internationally adopted automated system which is used to distribute marine safety information (MSI), and includes weather forecasts and warnings, navigational warnings, search and rescue notices and other similar safety information.
  • Emergency Position Indicating Radio Beacon (EPIRB): EPIRB is an equipment used to help in determining the position of survivors during a SAR (Search and Rescue) operation. It is a secondary means of distress alerting.  
  • Search and Rescue Locating Equipment: Search and Rescue Radar Transponder are the primary equipment. It used to home search and rescue units to the position of distress which transmits upon interrogation.  
  • Digital Selective Calling (DSC): DSC is a calling service between ship-to-ship, ship-to-shore or shore-to-ship, for safety and distress information mainly on high or medium frequency and VHF maritime radio.

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Uses of GMDSS

Fully implemented GMDSS was first introduced on 1st February, 1999. Since then, GMDSS was set as a standard for usage of communication protocol, procedures and safety equipment to be used at the time of any distress situation by the vessel. All passenger and cargo vessels, which are above 300GT and are involved in voyages in international waters are compelled to carry equipment as per the GMDSS.

When a ship uses the Global Maritime Distress and Safety System (GMDSS), what basically happens, is that it sends a distress signal through satellites or radio communication equipment. GMDSS is also used as a medium to send and receive important maritime safety information and also as a general communication channel.

Requirements (Documents To Be Carried Onboard)

Following are the documents which must be carried onboard ship, with regard to the GMDSS:

  • The Ship’s Radio License
  • Radio Operators Certificate
  • Safety Radio Certificate
  • GMDSS Radio Log Book
  • Antenna Rigging Plan
  • A Valid Shore Based Maintenance Certificate
  • ITU list of all Cell Signs and Numerical Identities of  Stations used by Maritime Mobile and Maritime Mobile Satellite Services
  • ITU list of Coast Stations
  • ITU list of Ship Stations
  • ITU list of Radio determination and Special Service Stations

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GMDSS Sea Areas

Global Maritime Distress and Safety System (GMDSS) Explained

The GMDSS sea areas serve two purposes:

  1. Describing areas where the GMDSS services are available.
  2. Defining what radio equipment GMDSS ships must carry.

The GMDSS sea areas are classified in following 4 areas:

  • Sea Area A1: The area  within coverage of VHF coast stations where digital selective calling alert (DSC) is available (CH. 70/156.525 MHz) so one must use VHF capable transceivers with DSC capabilities. Typically, this area could extend from 30 to 40 nautical miles (56 to 74 km) from a coastal radio station.
  • Sea Area A2: This excludes Sea Area A1 and provides coverage of a minimum of one MF coast station and continuous DSC (2187.5kHz) alerting is also out there, hence a VHF and MF radio station set up is required. Sea Area A2 typically extends up to 180 nautical miles (330 km) offshore during daylight hours and 150 nautical miles (280 km) offshore during night time hours.
  • Sea Area A3: Excluding Sea Areas A1 & A2, Sea Area A3 lies within the coverage of INMARSAT geostationary satellites. Here a complete VHF radio and either a MF/HF radio or an INMARSAT station is required. Sea Area A3 covers 70-degrees North Latitude and 70-degrees South Latitude.
  • Sea Area A4: This area excludes Sea Area’s A1, A2 & A3 and is mostly the polar regions. A complete VHF and MF/HF radio station must be used in this area. Sea Area A4 covers 71-degrees North Latitude and above 71-degrees South Latitude.

In order to understand the Sea Areas in a better way, we need to take a look at the ranges, with regard to the frequencies in a specific band.

  1. Medium Frequencies: 300KHz to 3MHz
  2. High Frequencies: 3MHz to 30MHz
  3. Very High Frequencies: 30MHz to 300MHz

For the purpose of maritime communication, the range of frequency allocated is 156MHz to 174MHz. This range comes under the VFH or the Very High Frequencies. Channel 16 is set at 156.800MHz, is used for Distress, Urgency and Safety communication. Channel 70 set at 156.525MHz is used for routine VFH Digital Service Calling (DSC) watch.

GUARD channels are set above and below Channel 16 to avoid any interference on Channel 16. One cannot have seamless traffic on Channel 16 with interference with regard to other communication aside from safety, distress and urgency. So, the Guard channel frequencies are 156.775MHz and 156.825MHz.

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SOLAS Requirements for GMDSS

All the vessels voyaging in the international waters must comply with SOLAS Chapter IV for smooth and clear operation of distress system all over the world. The following requirements are laid down:

  • Transmission of distress signal from vessel to shore by at least two separate and independent methods.

Every vessel under GMDSS must have at least two different communication methods for vessel to shore distress transmission from the following- EPRIB, Digital Selective Calling (DSC), Inmarsat C. Each ship should able to receive shore to ship distress alerts and warnings by either of the two means- DSC or NAVTEX.

  • Transmission and receiving of distress alerts and warnings in between two ships.

Each and every vessel under GMDSS must be able to transmit and receive distress signal between ship to ship by two methods- VHF Channel 13 and DSC.

  • Transmission and receiving of search and rescue coordinating communications.

Every vessel listed under GMDSS must be capable of receiving and transmitting search and rescue coordinating communications by any of the following means- NAVTEX, HF/MF/VHF or Inmarsat.

  • On scene communication transmission and receiving.

All vessels under GMDSS must meet the requirements to co-ordinate search and rescue and other distress communication in between ships at the scene of incident. Usually MF/HF/VHF are used.

  • Transmitting and receiving signals for locating.

Vessels under GMDSS must be fitted with proper equipment for maritime distress and operation and as described in SOLAS chapter V like radars, are more.

  • General radio communication that are to be transmitted and received from shore-based networks.

Vessels under GMDSS consist of general communication system for official, business and personal and private crew communications and can be done using DSC and INMARSAT.

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