According to IMO (1999) the GMDSS represents a worldwide network of automated emergency communications for ships at sea. SOLAS convention Chapter IV states that all ocean-going passenger ships and cargo ships of 300 gross tonnage and upwards are required to carry radio equipment that conforms to international standards.

The main purpose of GMDSS is to prevent unanswered distress calls and delay in Search and Rescue actions when distress situations occur. 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).

The SOLAS convention Chapter IV Radio communication, 
Regulation 7 Radio equipment: General, lists all radio equipment required onboard the vessels.

safety systems onboard vessels


  • INMARSAT – a global mobile satellite communication system providing two-way data and messaging.
  • NAVTEX - an international automated service for delivery of navigational and meteorological warnings, forecasts and urgent maritime safety information to ships.
  • Emergency position indicator radio beacon (EPIRB) – automated identification and locator device for Search and Rescue operations.
  • Search and Rescue locating equipment – automatically leading Search and Rescue units to the position of distress by signaling search and rescue radar transponders.
  • Digital selective calling (DSC) - a standard for sending pre-defined digital messages from ship to ship, ship to shore and shore to ship.

DSC is considered the automated watch on distress channels running on VHF, MF/HF radios. The satellite communication is based on satellite network that reaches up to sea area A3. Emergency Position Indicating Radio beacon’s (EPIRB) and Search and Research transponders (SART) are sending out distress and locating signals under emergencies.

EPIRB’s are small portable devices using the global COSPAS-SARSAT Satellite System and sending signals on the 406MHz frequency, whereas the SART devices are portable radar transponders operating on the radar ”X” and ”S” frequency bands. The Search and Rescue transponders can stay afloat, when a ship has sunk and are also used on life boats. EPIRB automatically send distress signals, when coming in contact with water.  


Danphones GMDSS system

The GMDSS operates over 4 sea areas of coverage from shore to ship. The 4 sea areas are classified as: A1, A2, A3 and A4. 

Sea area A1: Covered by VHF radio enabling Digital Selective Calling (DSC) and Radio Telephone (RT). Determined range of approximately 20-30 nautical miles.

Sea area A2: Covered by MF radio also enabling DSC and RT. Determined range up to 150 nautical miles.

Sea area A3: Covered by HF radio and Inmarsat geostationary satellite enabling DSC and satellite communication 70°N and below 70°S.

Sea area A4: Categorized as the polar regions above 70°N and below 70°S. This area requires HF radio with DSC.


The 4 sea areas in the GMDSS framework use different equipment in the respective area.

They are as follows:

AREA   RANGE               EQUIPMENT
A1        20 to 50 M               VHF DSC
A2        50 to 400 M             VHF + MF
A3        70° N to 70° S          VHF + MF + Satelite
A4        Above 70° N or S     HF + MF + VHF

The different types of equipment operate in different frequencies in a specific band:

  • MF -  Medium Frequencies: 300 KHz to 3 MHz
  • HF -  High Frequencies: 3 MHz to 30 MHz
  • VHF - Very High Frequencies: 30 MHz to 300 MHz


Very High Frequencies (VHF)

For the purposes of maritime communication, the range of 156 MHz to 174 MHz is allocated.

Channel 16, which is set at 156.800 MHz, is for Distress, Urgency and Safety communication.

Channel 70, which is set at 156.525 MHz, is for routine VHF DSC (Digital Selective Calling) watch.

Communication channels are set put above and below Channel 16 to avoid any interference on Channel 16.


GMDSS Compliance, Adherence and Co-operation


With IMO SOLAS GMDSS being a global standard and committed by more than 170 countries compliance, adherence and co-operation is of vital importance. 

One instrument is Ship Radio Inspection, which can be by shipping companies, maritime authorities, coast guards and naturally classification companies, like DNV-GL, Lloyds, Bureau veritas, ABS, Rina etc.




Certified radio service suppliers have undergone training in the conduction of radio inspections.

Danphone co-operate with MARTEC the maritime & polytechnic college, who have developed the AIS and GMDSS Radio Surveyor Course for Radio Service Suppliers engaged in the inspection and testing of radio communication equipment and Automatic Identification System (AIS) on Vessels. This course is recognized by all classification societies and all participants receive a radio inspectors certificate approved by DNV after successful completion of the course.
Find more information on DNV’s approval of service supplier here.  



All communication and safety equipment on board vessels are required to undergo annual inspections such as GMDSS radio survey and VDR/SVDR annual performance testing (APT). These inspections should comply with SOLAS chapter IV, Functional Requirements of all communication and safety equipment onboard vessels.



Danphone’s Futronic test box for radio inspections is based on SOLAS Chapter IV requirements for radio equipment and capable of testing all regulated radio and safety equipment related to the GMDSS other than INMARSAT with approved self-test functionalities.

Futronic meets SOLAS Chapter IV, Functional Requirements, to the communications equipment onboard vessels by providing efficient and accurate testing on:

VHF radios

DSC data, ATIS, frequency, power, VSWR and deviation.
Measuring frequency and power simultaneously.

MF/HF radios

Measuring DSC data and frequency simultaneously.

NAVTEX receivers

Reception of transmitted NAVTEX messages.



AIS transponders

Measuring AIS data, frequency, power and VSWR simultaneously on:

  • Class A & B transponders
  • AIS base stations
  • Aircraft Search and Rescue transponders
  • Aids to Navigation devices (AtoN)

Search and rescue transponders 

Measuring Data & frequency on: 

  • Man Over Board (MoB) devices 
  • AIS SART beacon data & frequency

Furthermore, Futronic MKll complies with DNV’s regulations for test equipment (Service Suppliers Engaged in the Inspection and Testing of Radio Communication Equipment and Automatic Identification System on Vessels, March 2014; 2.9 Equipment and 2.11 Minimum required instruments).