Maritime Communication Essentials: GMDSS, VHF, and Safety Protocols

Posted on Aug 5, 2025 in Electronics

Maritime Communication Fundamentals

Understanding Shipboard Communication

  • What is inter-ship communication? It is communication between vessels and coast stations.
  • What is intra-ship communication? It is internal communication within the ship.

Message Priorities in Maritime Communication

  • What does the “Priority” of a message indicate? It indicates the importance of the message.
  • When is the Priority “Distress” used? When there is serious and immediate danger for the vessel, crew, and passengers.
  • When is the Priority “Urgency” used? When there is serious danger for the vessel, crew, and passengers.
  • When is the Priority “Safety” used? When there is imminent risk to navigation.
  • When is the Priority “Routine” used? When it is transmitted to ensure safe navigation.

Global Maritime Distress and Safety System (GMDSS)

  • What is the purpose of GMDSS? The main purpose is to prevent accidents by providing Marine Safety Information and to minimize the consequences of marine accidents through effective communication.
  • What types of vessels are classified as “Non-GMDSS”? Small vessels such as yachts, non-self-propelled ships, men-of-war, and troopships.
  • What does the terrestrial system of GMDSS consist of?
    • Radio-Telephony (RTF)
    • Digital Selective Calling (DSC)
    • Direct Printing Telegraphy (DPT)
    • Navigational Telex (NAVTEX)
    • Search and Rescue Radar Transponder (SART)
  • What does the satellite system of GMDSS consist of?
    • Inmarsat
    • COSPAS/SARSAT
    • Emergency Position Indicating Radio Beacon (EPIRB)
    • Status Recording System (STAREC)
  • “Vessel in Distress” – indicate the transmission frequency. DSC announcement → DSC acknowledgment → VHF message.
  • How is a VHF message announced by a Non-GMDSS vessel? On VHF, by addressing the vessel being called three times and then identifying oneself three times.
  • What does a NAVTEX message generally contain? It generally contains meteorological warnings, Search and Rescue (SAR) operations, and other important and urgent data.

Satellite Systems and Distress Beacons

  • What is the main difference between Inmarsat satellites and COSPAS-SARSAT satellites? Unlike Inmarsat satellites, COSPAS/SARSAT satellites are not geostationary. They orbit the Earth on a North/South and South/North path, passing closely over both poles. COSPAS/SARSAT is the only system that offers a means to contact distant stations.
  • Explain the working of an EPIRB and a SART.
    • The EPIRB (Emergency Position Indicating Radio Beacon) transmits UHF distress signals. These signals are received by satellites and relayed to Rescue Coordination Centers, providing the vessel’s position and identification.
    • The SART (Search and Rescue Radar Transponder) of a vessel in distress is activated by the radar of a passing vessel. When activated, it transmits an alert that creates a series of dots on the Plan Position Indicator (PPI) of the passing vessel, enabling it to locate the vessel in distress.

VHF Marine Radio Operations

VHF Channels and Usage

  • What marine VHF channel is used for distress calling and messages? Channel 16.
  • What is the supplementary channel for Channel 16? Channel 67.
  • What marine VHF channel should a listening watch be maintained on whilst at sea? An automatic watch on VHF DSC distress, safety, and calling Channel 70.
  • What marine VHF channel may be used for ship-to-aircraft coordinating Search and Rescue? Channel 06 is for ship-to-aircraft coordinating Search and Rescue.
  • What marine VHF channel is reserved for ship-to-ship general communications? Channel 77, for general/routine ship-to-ship communications.
  • What is a working channel? A channel used to exchange messages by radio-telephone.

Radio Log Book and Communication Protocols

  • What information is required to be entered into the radio log book? The required information includes date and time, ship position, call signs or MMSI, content of distress, urgency, or safety messages, and any incidents related to radio communication.
  • Generally speaking, what type of station controls the communication process? The station being called.

Advanced VHF Channel Usage

  • What marine VHF channel is dedicated for VHF DSC? Channel 70. This channel is used for Digital Selective Calling (DSC) distress, urgency, safety, and routine alerting with all stations.
  • What marine VHF channel is reserved for the exchange of ship-to-ship Marine Safety Information? Channel 13. This channel is used for inter-ship Maritime Safety Information (bridge-to-bridge communication) and may also be used by port authorities for vessel harbor movement communications.

Radio Communication Regulations and Priorities

  • How are radio transmissions identified? They are identified by the ship’s name and/or call sign and MMSI in the DSC unit.
  • Whose authority is the vessel’s radio station placed under? The authority of the ship’s captain.
  • What is the order of priority of marine communications traffic? Distress, Urgency, Safety, and Routine/General.
  • What do you understand about ‘Confidentiality or Secrecy of transmission’? It means that radio transmissions must be used for official purposes according to international regulations.
  • What is a calling channel? Calling channels are used to establish initial contact with other stations.
  • What type of channel would a Maritime Safety Information message be broadcast on? A safety message will always be transmitted on a working channel.

Standard Marine Communication Phrases (SMCP)

Common SMCP Terms and Meanings

  • “I can transmit on Channel 77”: Indicates VHF channels available.
  • “OVER”: My transmission is ended, and I expect a response from you.
  • “GO/CHANGE to Channel 13”: Switch to VHF Channel 13.
  • “Read to me the information that I have just given”: Please read back.
  • “Indicate that you have received what I have just said”: Understood.
  • “Do not terminate this conversation or change the subject because I have more to say”: Stay ON.
  • “How well are you receiving me?”: How do you read me?
  • “Proceed with your message”: Go ahead.
  • “Yes, or permission granted”: Affirmative.
  • “Remain on VHF channels”: Stand by on VHF channels.
  • “I repeat”: Say Again.
  • “Use SMCP during this conversation”: Please Use SMCP.

Key Concepts in Maritime Communication

General Maritime Communication Terms

  • What does maritime communication comprise? Communication between vessels and coast stations, including inter-ship communication and intra-ship communication.
  • How is a distress alert referred to? Mayday.
  • How is an urgency message referred to? Pan Pan.
  • What is VHF used for? To bridge short distances.
  • What do you say before changing from speaking to listening? Over.
  • What does MMSI stand for? Maritime Mobile Service Identity.

GMDSS Components and Systems

  • How many sea areas does GMDSS distinguish? Four.
  • Which are the four Inmarsat regions? Atlantic Ocean Region West (AOR-W), Atlantic Ocean Region East (AOR-E), Indian Ocean Region (IOR), and Pacific Ocean Region (POR).
  • What is an EPIRB? It is a small radio transmitter capable of transmitting UHF distress signals.
  • Where must a NAVTEX be installed? On GMDSS vessels.
  • What is COSPAS-SARSAT? It is an international satellite-based Search and Rescue system, established by Canada, France, USA, and Russia.

Standard Marine Communication Phrases and GMDSS Sea Areas

Additional SMCP Phrases

  • “This is”: My name (call sign) is…
  • “On VHF”: On Channel…
  • “Say Again”: I say again.
  • “Thank you”: Some statement of gratitude.
  • “Nothing more”: I have finished my message.
  • “OUT”: I am terminating my transmission. The conversation is ended, and no response is expected.
  • “OVER”: My transmission is ended, and I expect a response from you.
  • “Agree VHF channel / Switch to VHF Channel”: I suggest that you switch/change/go to Channel… / Agree VHF channel… / Switching to Channel….

GMDSS Sea Areas Explained

  • A1 Sea Area:
    • VHF DSC-alerting (Channel 70) and radio-telephony.
    • Extends approximately 30 miles offshore.
  • A2 Sea Area:
    • DSC-equipped MF radio-telephone in addition to equipment required for Sea Area A1.
    • Extends approximately 200 miles offshore.
  • A3 Sea Area:
    • Extends between 70 degrees latitude North and 70 degrees latitude South.
    • Requires Inmarsat A, B, or C ship/earth station or a DSC-equipped HF radio-telephone/telex, in addition to equipment for A1 and A2 areas.
  • A4 Sea Area:
    • Areas outside A1, A2, and A3. These are the Polar Regions.
    • Must carry DSC-equipped HF radio-telephone/telex, in addition to equipment required for areas A1 and A2.
    • This area does not have Inmarsat coverage but has COSPAS/SARSAT coverage.