GMDSS Complete Guide: Sea Areas, DSC, EPIRB, SART & Distress Procedures

GMDSS Architecture and Sea Areas

GMDSS defines four sea areas (A1–A4) based on the radio coverage available in each region. The sea area determines which equipment a vessel must carry under SOLAS Chapter IV Regulation 7. A vessel’s flag state administers certification, and the sea areas in which a vessel operates are stated on its Safety Radio Certificate.

Sea Area A1 is defined as the area within range of at least one VHF DSC coast radio station. In practice, this means within approximately 20 to 50 nautical miles of the coastline, though exact coverage varies by station. Vessels operating in A1 must carry a VHF DSC radio, a Search and Rescue Transponder (SART) or AIS-SART, a means to receive Maritime Safety Information (NAVTEX or equivalent HF reception), and a 406 MHz EPIRB.

Sea Area A2 is within the coverage range of at least one MF (medium frequency) DSC coast station, generally up to approximately 150–400 nautical miles from shore. Vessels operating in A2 require all A1 equipment plus an MF DSC radio capable of operating on 2187.5 kHz (the dedicated MF DSC distress frequency).

Sea Area A3 is the area covered by geostationary Inmarsat satellites, which broadly covers latitudes between 70°N and 70°S. This encompasses most of the world’s major shipping lanes. Vessels in A3 must carry either an Inmarsat ship earth station (SES) or an HF DSC radio in addition to the A1 and A2 equipment requirements.

Sea Area A4 covers the polar regions not included in A3 — above 70°N and below 70°S. These are the most challenging waters for communication because geostationary Inmarsat satellites have very low elevation angles or no coverage at all. Vessels operating in A4 must carry HF DSC radio equipment. Low earth orbit (LEO) satellite systems such as Iridium are increasingly used for polar communication but are not yet a formal SOLAS substitute for all HF requirements.

SOLAS Chapter IV Regulation 7 also requires two independent means of sending a distress alert and the ability to receive MSI appropriate to the trading area. Officers should be familiar with the equipment carriage table, which lists minimum requirements for each class of vessel operating in each sea area.

Digital Selective Calling (DSC)

DSC is the primary distress alerting mechanism within GMDSS. Every GMDSS-equipped vessel is assigned a unique Maritime Mobile Service Identity (MMSI) — a nine-digit number that identifies the vessel to all DSC receivers and to rescue coordination systems worldwide. The MMSI must be programmed into every DSC controller on board and, critically, should be linked to the vessel’s GPS so that position data is automatically included in any distress transmission.

DSC transmissions are categorised by priority:

• Distress: Initiated by holding the protected DISTRESS button for approximately 5 seconds. The controller broadcasts the vessel’s MMSI, nature of distress (if pre-selected), position, and UTC time on Channel 70 (VHF) or 2187.5 kHz (MF). All receiving stations immediately acknowledge the alert.
• Urgency: Used for communications with very urgent content that is not a life-threatening emergency. Transmitted on Channel 70 / 2187.5 kHz, followed by a voice PAN PAN message on Channel 16 or 2182 kHz.
• Safety: Used for important navigational or meteorological warnings. Transmitted as SECURITE on Channel 16 after a DSC alert on Channel 70.
• Routine: Used for ship-to-ship or ship-to-shore routine calls, proposing a working channel for voice communication.

VHF Channel 70 is the dedicated international DSC calling channel. Voice traffic on Channel 70 is strictly prohibited — it is used exclusively for digital DSC messages. Similarly, MF 2187.5 kHz is the dedicated MF DSC distress and calling frequency, with 2182 kHz used for the subsequent voice distress call.

When a vessel receives a DSC distress alert, it must acknowledge the alert using its own DSC controller. This acknowledgment is critical — it informs the distressed vessel that the alert has been received and prevents continuous re-transmissions from flooding the DSC channel. After acknowledgment, the receiving vessel switches to Channel 16 to establish voice communication and render assistance or relay to the coast station.

EPIRB — Emergency Position-Indicating Radio Beacon

The 406 MHz EPIRB (Emergency Position-Indicating Radio Beacon) is the cornerstone of the GMDSS satellite distress alerting capability. When activated, the EPIRB transmits a coded signal to the COSPAS-SARSAT satellite constellation, a cooperative international system involving both geostationary (GEOSAR) and low earth orbit (LEOSAR) satellites. The GEOSAR component provides near-instantaneous alerting in most sea areas, while LEOSAR provides Doppler-based position accuracy of approximately 3–5 km.

Each EPIRB is coded with a unique identifier — either the vessel’s MMSI number or a manufacturer serial number — linked to a national database. When the signal reaches a ground station (Local User Terminal, LUT), the position and identification data are forwarded to a Mission Control Centre (MCC), which relays the alert to the appropriate RCC.

There are two operational categories under SOLAS:

• Category I: Fitted with an automatic hydrostatic release unit (HRU) that releases the EPIRB from its bracket and activates it when submerged to a depth of 1.5–4 metres. This allows the EPIRB to float free and activate automatically if a vessel sinks rapidly. It can also be manually activated.
• Category II: Manual release and activation only. The crew must manually deploy and activate the EPIRB, making it unsuitable as the sole EPIRB on SOLAS vessels.

Registration is mandatory and critical. An unregistered EPIRB, or one registered with incorrect vessel details, will generate a distress alert that cannot be immediately matched to a ship name, vessel description, or next-of-kin contacts. This delays the entire SAR response. Registration must be updated whenever the vessel changes name, owner, flag state, or MMSI.

In addition to the 406 MHz satellite signal, the EPIRB also transmits a 121.5 MHz homing signal. This allows SAR aircraft and vessels to home in on the EPIRB’s position with a radio direction finder once they arrive in the vicinity. The battery must have sufficient capacity for a minimum of 48 hours of continuous transmission at -20°C. EPIRBs must be serviced annually and the battery replaced every 5 years (or in accordance with the expiry date marked on the unit).

The self-test function — accessible via a dedicated button — briefly activates the unit in a test mode without transmitting a real distress signal to the satellite system. Officers must never activate an EPIRB in any mode other than self-test unless there is genuine distress. False alerts are a criminal offence in most jurisdictions and consume limited SAR resources.

SART — Search and Rescue Transponder

The Search and Rescue Transponder (SART) is a portable radar transponder designed to assist rescuers in locating a survival craft or casualty in the water. When interrogated by a vessel’s 9 GHz X-band radar, the SART responds with a sweep of signals that appears on the rescuing vessel’s radar display as a distinctive pattern of 12 equally spaced dots, forming a line that points toward the SART. As the rescue vessel closes in, the dots elongate into arcs and eventually become concentric circles around the SART position. This distinctive display allows the OOW to identify a SART response immediately and differentiate it from ordinary radar clutter.

Typical ship-to-ship range for a SART is 5–10 nautical miles; from an SAR aircraft flying at 1,000 feet, detection range can reach approximately 30–40 nm. The SART must be held or mounted as high as practicable above sea level, because X-band radar is limited by line-of-sight propagation.

To activate a SART, extend the antenna to its operating position and switch the unit from standby to transmit. A visual or audible indicator confirms activation. When a radar pulse is detected, the SART’s indicator will flash or beep more rapidly, alerting survivors that a rescue vessel is nearby. The self-test verifies battery and circuitry without generating a radar response.

AIS-SART is an increasingly common alternative. Rather than responding to X-band radar, the AIS-SART broadcasts AIS position messages using an MMSI number beginning with 970, which allows it to appear as a vessel target on ECDIS displays, chartplotters, and AIS receivers. This offers a significant advantage over radar-only SARTs because the position is displayed with a label on the chart, allowing an accurate course to be steered from the outset. However, AIS-SARTs require vessels with AIS receivers in the vicinity — they cannot be detected by SAR aircraft without AIS capability.

Under SOLAS, most vessels are required to carry two SARTs (or one SART and one AIS-SART), one for each side of the vessel. They must be stowed in an accessible location for immediate deployment into survival craft.

NAVTEX

NAVTEX (Navigational Telex) is the GMDSS system for the automatic reception of navigational warnings, meteorological forecasts, ice reports, and SAR information. It operates on two primary frequencies:

• 518 kHz: The international NAVTEX frequency, broadcasting in English. Coverage is typically within 400 nautical miles of the transmitter station.
• 490 kHz: The national NAVTEX frequency, used for broadcasts in the local language of the coastal state.

Each NAVTEX message carries a four-character header in the format B1B2B3B4: B1 is a letter identifying the transmitter station; B2 is a letter identifying the message category; B3B4 is a two-digit serial number. The receiver uses this header to avoid printing duplicate messages and to filter messages from stations the user has de-selected.

Key message categories include: A — navigational warnings; B — meteorological warnings; D — SAR and piracy information; L — NAVTEX transmitter station information. Category D is mandatory — it must never be disabled on the NAVTEX receiver, as SAR messages may contain information directly relevant to assisting a vessel in distress in the vicinity.

NAVTEX is linked to the IMO’s NAVAREA warning system, which divides the world’s oceans into 21 NAVAREAs, each coordinated by a designated national hydrographic office. NAVTEX transmitters broadcast NAVAREA sub-regional warnings, while high seas warnings (NAVAREA I–XXI) are delivered by SafetyNET via Inmarsat-C EGC broadcast to vessels in Sea Area A3.

Inmarsat-C and Fleet Broadband

Inmarsat-C is a store-and-forward messaging system that operates through the Inmarsat geostationary satellite constellation. Unlike voice systems, Inmarsat-C sends and receives short data messages — typically used for position reporting, voyage updates, crew communications, and weather downloads. For GMDSS purposes, Inmarsat-C supports distress alerting directly to the Inmarsat Land Earth Station (LES), which forwards the alert to the relevant RCC.

The Enhanced Group Call (EGC) SafetyNET service is the mechanism by which Inmarsat delivers Maritime Safety Information (MSI) — including NAVAREA high-seas warnings, meteorological forecasts, and SAR messages — directly to vessels in Sea Area A3. The Inmarsat-C terminal must have EGC enabled and the correct NAVAREA(s) programmed for the vessel’s operating region. SafetyNET is the high-seas equivalent of NAVTEX for A3 vessels.

Fleet Broadband (FB) — available in FB150, FB250, and FB500 variants — provides IP-based voice and data communications via Inmarsat. While not a core GMDSS component, Fleet Broadband supports crew welfare, operational communications, and serves as an additional means of distress communication. Inmarsat B and the older Fleet 33 provide voice and medium-speed data.

For Sea Area A4 operations in polar waters, Iridium LEO satellites offer global coverage including the poles. Iridium-based systems can be used as a supplementary communication tool and are increasingly being integrated into GMDSS-compliant equipment, though flag state approval should be confirmed. Ships operating in ice or polar waters should also carry additional HF equipment as specified by their administration.

Distress, Urgency and Safety Procedures

GMDSS defines three categories of maritime radio communication by priority:

A MAYDAY RELAY must be transmitted by any vessel that hears a distress call but has reason to believe it has not been received and acknowledged by a coast station or another vessel. The MAYDAY RELAY is prefixed: MAYDAY RELAY MAYDAY RELAY MAYDAY RELAY, all stations, this is [relaying vessel name], followed by the original distress message. Officers must not hesitate to relay a distress — waiting for confirmation at the cost of delay is the wrong decision.

GMDSS Exam Focus Points

Oral examiners for STCW certifications consistently probe officers on the following GMDSS points. Ensure you can answer each of these clearly and without hesitation:

• MMSI programming: Know how to programme MMSI into all DSC controllers, the consequences of incorrect MMSI, and the procedure for obtaining a new MMSI from the flag state authority.
• EPIRB registration consequences: An unregistered or incorrectly registered EPIRB means SAR authorities cannot confirm vessel name, call sign, or emergency contacts — delaying response time by hours. Registration is a statutory requirement under SOLAS.
• DSC acknowledgment timing: Coast stations typically have priority to acknowledge DSC distress alerts. Ships within the distress area should wait approximately 5 minutes before acknowledging if no coast station has responded, to avoid multiple acknowledgments simultaneously overwhelming Channel 70.
• Switching from SART to voice: Once visual contact is established with the rescue vessel, the SART may be switched off to avoid interfering with the rescuing vessel’s radar. Communication switches to VHF Channel 16.
• Category I vs. Category II EPIRB: Category I is mandatory for SOLAS vessels — it floats free automatically on sinking. Category II is manual-only and may be used in addition to, but generally not as the primary EPIRB for a SOLAS vessel.
• Receiving a DSC distress alert: Plot the position, note the vessel’s own position and CPA, acknowledge the DSC alert on Channel 70, switch to Channel 16 for voice communication, inform the master, enter the distress in the radio log, and maintain a continuous watch until released by the RCC.