A guide to GMDSS sea areas and radio equipment

29 December 2025

Ian Lockyer, Marketing Manager, Icom UK Ltd

Ian Lockyer, Marketing Manager, Icom UK Ltd

The Global Maritime Distress and Safety System (GMDSS) represents the world's most essential framework for ensuring life-at-sea safety. Established by the International Maritime Organization (IMO), GMDSS is a globally mandated, integrated radio safety system and procedural protocol designed to guarantee that any vessel in distress can be located and assisted anywhere on the planet.

For leaders in the maritime industry, understanding and correctly implementing GMDSS is not merely a regulatory obligation, it is a core responsibility that directly impacts lives.

A key pillar of GMDSS is the division of the world's oceans into four distinct Sea Areas. This segregated approach ensures that vessels are equipped with the appropriate communication technology for the specific environments they operate in. Matching the right equipment to the right area is essential for maintaining constant and reliable communication.

The four sea areas of GMDSS

Each sea area is defined by the type of communications coverage available, dictating the minimum required radio equipment a vessel must carry.

Sea Area A1: This area covers coastal waters within range of a shore-based VHF radio station with Digital Selective Calling (DSC) capabilities. Typically this extends 30-40 nautical miles from the coast. To operate safely here a vessel must have a VHF transceiver with DSC functionality. This is the most basic level of GMDSS readiness, suitable for vessels that primarily operate close to land.

Sea Area A2: Excluding Area A1, this zone extends into waters within range of at least one shore-based MF (Medium Frequency) station that provides continuous DSC alerting. The range of an MF signal can extend up to approximately 180 nautical miles from the coast during the day and 150 nautical miles at night. A vessel in this area must be equipped with both a VHF radio station and an MF radio station. The combination of VHF for short-range communication and MF for medium-range alerting provides a robust safety net as a vessel moves further offshore.

Sea Area A3: This expansive area, which excludes A1 and A2, is defined by its coverage by INMARSAT or Iridium geostationary satellites. The satellite network covers the globe between 70° North and 70° South latitude. This vast area requires a more sophisticated communications setup. Vessels here must be equipped with a full VHF radio, as well as either an MF/HF (Medium/High Frequency) radio or an INMARSAT or Iridium satellite station. The use of satellites ensures reliable communication over long distances, addressing the unique challenges of trans-oceanic voyages.

Sea Area A4: This is essentially the polar regions, including areas above 70° North and below 70° South latitude. Given the extreme and remote nature of these environments, communication is entirely dependent on land-based radio signals. Vessels operating these areas must be equipped with a full VHF radio and an MF/HF radio station. The reliability of MF/HF communication is paramount in these high-latitude regions where satellite connectivity may be difficult.

Other essential equipment

Beyond the primary radio stations, GMDSS regulations mandate that all SOLAS (Safety of Life at Sea) vessels carry a suite of additional safety equipment to ensure comprehensive distress and rescue capabilities.

  • Search and Rescue Transponders (SARTs): These devices respond to radar signals from search and rescue vessels, pinpointing the location of a distressed vessel or survival craft.
  • Navigational Telex (NAVTEX) receiver: A dedicated receiver for a global service that provides automated broadcasts of navigational and meteorological warnings.
  • Emergency Position Indicating Radio Beacon (EPIRB): An essential device that, when activated, transmits a distress signal to a satellite network, alerting rescue coordination centres worldwide.
  • Portable radios: These are vital for communication in survival craft. GMDSS regulations specify the number of portable VHF radios required based on the vessel's size.

For maritime professionals and network engineers, the GMDSS framework is a testament to the power of a standardised, globally integrated communication network. It is a system built not just on technology but on the principle that no ship should ever be beyond the reach of help.

Effective communication at sea is the bedrock of safety. By understanding the specific requirements of each GMDSS Sea Area and the purpose of every piece of safety equipment, maritime leaders can make informed decisions that safeguard their crew, their cargo and their reputation. In a world where every nautical mile holds a new possibility, being prepared is the best possible strategy.