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SOLAS Chapter IV — Radio Communications (GMDSS)

The Global Maritime Distress and Safety System: sea areas A1–A4, functional requirements, radio equipment and reserve power.

10 min read
Intermediate
Safety
Key Principles at a Glance 6 points
  • SOLAS Chapter IV mandates the Global Maritime Distress and Safety System (GMDSS) on all cargo ships ≥ 300 GT and all passenger vessels on international voyages.
  • GMDSS establishes four maritime operational zones based strictly on communication reach: Sea Area A1 (VHF-DSC, 20–30 NM), A2 (MF-DSC, 100–150 NM), A3 (Recognized Mobile Satellite, 70°N to 70°S), and A4 (Polar HF-DSC).
  • Vessels must fulfill nine statutory functional requirements, guaranteeing at least two independent, redundant pathways for ship-to-shore distress alerting.
  • Carriage requirements scale by sea area: A1 requires VHF/DSC Ch 70; A2 adds MF/DSC 2187.5 kHz; A3 adds Inmarsat-C/Iridium or HF/DSC; A4 mandates full HF scanning transceiver arrays.
  • The reserve electrical energy source (24V DC battery bank) must power essential radio installations for 1 hour if an emergency generator is provided, or 6 hours if no emergency generator supplies the radio bus.
  • Ships operating in Sea Areas A3 and A4 must satisfy equipment availability through at least two of three statutory options: duplication of equipment, shore-based maintenance contracts, or qualified at-sea electronic repair.

1. Chapter IV & GMDSS: Overview & Sea Areas

SOLAS Chapter IV governs the Global Maritime Distress and Safety System (GMDSS), an automated global communications network combining terrestrial digital radio and satellite space segments. Unlike historical telegraphy that relied on manual Morse listening watches on 500 kHz, GMDSS links automated terrestrial Digital Selective Calling (DSC) and satellite Ship Earth Stations (SES) directly to rescue coordination centers ashore.

A1: 20–30 NM VHF-DSC Coast Range
A2: 100–150 NM MF-DSC Coast Range
A3: 70°N to 70°S Satellite Coverage Zone
A4: Polar HF Skywave Coverage
≥ 300 GT SOLAS Ch. IV Application
GMDSS SEA AREAS (A1, A2, A3, A4) & PROPAGATION BOUNDARIES SOLAS IV Reg 2 • Terrestrial Propagation vs Geostationary Satellites • Polar HF Coverage EARTH SURFACE CURVATURE COAST RADIO STATION A1: 20–30 NM A2: 100–150 NM SAT A3: 70°N to 70°S SEA AREA A1: VHF COAST COVERAGE Within radius of ≥1 VHF coast station with continuous DSC alerting (Ch 70). Typical line-of-sight range: 20 to 30 nautical miles. SEA AREA A2: MF COAST COVERAGE Within coverage of ≥1 MF station with continuous DSC watch (2187.5 kHz), excluding A1. Typical ground-wave range: 100 to 150 nautical miles. SEA AREA A3: RECOGNIZED SATELLITE Within geostationary Inmarsat (or Iridium) satellite coverage (~70°N to 70°S), excluding A1/A2. Worldwide oceanic shipping lanes between polar limits. SEA AREA A4: POLAR REGIONS Waters outside A1, A2, and A3 (primarily above 70°N and below 70°S). Geostationary look-angle lost; HF ionospheric sky-wave radio mandatory. REGULATORY DEFINITION: Sea areas are defined by communication infrastructure availability rather than geographical coordinates. If a coast state ceases MF DSC service, that coastline automatically transitions from Sea Area A2 to Sea Area A3.
Figure 1: GMDSS Sea Areas & Radio Propagation Architecture. Sea Area A1 relies on line-of-sight VHF-DSC (20–30 NM). Sea Area A2 relies on MF ground-wave propagation (100–150 NM). Sea Area A3 covers global oceanic waters between 70°N and 70°S under geostationary satellite constellations. Sea Area A4 encompasses polar waters where high-frequency (HF) sky-wave radio is mandatory.

The Four GMDSS Sea Areas Explained

Sea Area A1 (VHF Coverage)
An area within the radiotelephone coverage of at least one VHF coast station providing continuous Digital Selective Calling (DSC) alerting on Channel 70 (156.525 MHz). Nominal range: 20 to 30 nautical miles.
Sea Area A2 (MF Coverage)
An area, excluding Sea Area A1, within the radiotelephone coverage of at least one MF coast station providing continuous DSC alerting on 2187.5 kHz. Nominal ground-wave range: approximately 100 to 150 nautical miles.
Sea Area A3 (Satellite Footprint)
An area, excluding Sea Areas A1 and A2, within the coverage of an IMO-recognized mobile satellite service (Inmarsat geostationary satellites between ~70°N and 70°S, or Iridium global constellation).
Sea Area A4 (Polar Regions)
An area outside Sea Areas A1, A2, and A3, representing the extreme polar caps north of 70°N and south of 70°S. Geostationary satellite elevation angles fall below 5°, requiring dedicated High Frequency (HF) terrestrial transceiver arrays.

2. Functional Requirements of GMDSS

SOLAS Chapter IV Regulation 4 establishes that every ship, regardless of size or trade route, must be capable of executing nine specific radio communication functions while at sea. Central to this standard is the principle of redundancy: distress alerting from ship to shore must be possible via at least two completely separate and independent communication systems.

9 Functions Statutory SOLAS IV Mandate
2 Separate Paths Independent Distress Alerting
≤ 50 m GPS Distress Position Precision
518 kHz International NAVTEX MSI Freq
Zero Delay Dedicated Bridge Distress Panel
GMDSS COMMUNICATIONS ARCHITECTURE & THE 9 STATUTORY FUNCTIONS SOLAS IV Reg 4 • Two Independent Distress Alerting Paths • Satellite & Terrestrial Network Integration SOS VESSEL IN DISTRESS MMSI / GPS Active PATH 1: TERRESTRIAL DSC PATH 2: SATELLITE SES COAST RADIO STATION (CRS) DSC Receiver LAND EARTH STATION (LES) Satellite Gateway MRCC / JRCC SAR COORDINATION Dispatches Aircraft, Cutters & Navtex MSI SAR CUTTER On-Scene Comms SAR AIRCRAFT Homing 121.5 MHz NINE STATUTORY FUNCTIONS: 1. Ship-to-Shore Alert • 2. Shore-to-Ship Alert • 3. Ship-to-Ship Alert • 4. SAR Coordination • 5. On-Scene Comms • 6. Locating (SART) • 7. MSI Broadcasts • 8. General Comms • 9. Bridge-to-Bridge (Ch 13/16).
Figure 2: GMDSS Dual-Path Distress Alerting & SAR Operations Architecture. When a vessel activates a distress alert, the distress call is routed simultaneously across independent terrestrial DSC channels and satellite SES uplinks. Maritime Rescue Coordination Centres (MRCC) process the alert, broadcast urgent relays, and direct surface cutters and aircraft using dedicated tactical frequencies and homing beacons.

The Nine Mandatory Functional Requirements (SOLAS IV Reg 4)

1. Ship-to-Shore Distress Alerting
Transmitting distress alerts to shore authorities by at least two separate and independent means, each using a different radiocommunication service (e.g. satellite SES plus terrestrial DSC).
2. Shore-to-Ship Distress Alerting
Receiving distress alerts, distress relays, and urgent SAR operational instructions transmitted from shore-based MRCC stations to ships in the vicinity of an incident.
3. Ship-to-Ship Distress Alerting
Transmitting and receiving ship-to-ship distress alerts over VHF Channel 70 DSC and MF 2187.5 kHz DSC to alert vessels immediately adjacent to the casualty.
4. SAR Coordination Communications
Transmitting and receiving operational search and rescue traffic between the On-Scene Coordinator (OSC), craft participating in the search, and the responsible MRCC.
5. On-Scene Communications
Tactical radiocommunications between the casualty vessel, lifeboats, survival craft, rescue boats, and attending SAR helicopters using VHF Channel 16 and Channel 06.
6. Locating Signals (Homing)
Transmitting and receiving locating signals to guide search craft directly to the survival craft, executed via 9 GHz Radar SARTs, AIS-SARTs, and 121.5 MHz EPIRB homing transmitters.
7. Maritime Safety Information (MSI)
Receiving continuous navigational warnings, meteorological forecasts, meteorological warnings, and urgent safety alerts via NAVTEX (518 kHz) and satellite Enhanced Group Call (EGC).
8. General Radiocommunications
Transmitting and receiving routine operational and administrative messages between the ship and shore-based management networks via radiotelephony, telex, or satellite email/voice.

3. Radio Equipment by Sea Area

Radio carriage requirements under SOLAS Chapter IV scale progressively based on the vessel's certified Sea Area of operation. All installations must be capable of transmitting and receiving Distress, Urgency, and Safety traffic without delay, backed by an uninterrupted electrical reserve source.

Ch 70 VHF-DSC Watch (156.525 MHz)
2187.5 kHz MF-DSC Distress Watch
8414.5 kHz Mandatory HF-DSC Distress
518 / 490 kHz NAVTEX International / Local
≥ 2 SARTs Carriage on Ships ≥ 500 GT
BRIDGE GMDSS OPERATING CONSOLE FRONT ELEVATION SOLAS IV Reg 6–11 • Centralised Bridge Console • Dedicated Distress Push-Buttons • Continuous Watch Receivers CH: 16 / 70 VHF / DSC TRANSCEIVER DISTRESS 3s Hold 2187.5 kHz DSC MF/HF SCANNING TRANSCEIVER DISTRESS Multi-Band INM-C LOGGED ON SATELLITE SES DISTRESS 518 kHz ACTIVE NAVAREA WARNING RECVD NAVTEX RECEIVER 24.6V Battery Volts +4.2A Charge Current AC MAINS ON POWER CHANGEOVER PANEL DISTRESS ALARM PANEL Visual & Audible Distress Beep CENTRAL ALARM DISPLAY WATCHKEEPING MANDATE: Continuous automated watch is kept on VHF Channel 70 DSC (156.525 MHz), MF 2187.5 kHz DSC, and the primary HF DSC distress channel (8414.5 kHz) plus one associated HF channel.
Figure 3: Navigating Bridge GMDSS Radio Console Layout. Centralized workstation integrating VHF/DSC, MF/HF scanning transceiver, satellite Ship Earth Station (SES), NAVTEX receiver, reserve battery instrumentation, and dedicated distress alert push-buttons protected by spring-loaded safety covers.

Mandatory Equipment Carriage by GMDSS Sea Area

Appliance / Subsystem Sea Area A1 Sea Area A2 Sea Area A3 Sea Area A4
VHF Transceiver with DSC Mandatory (Ch 16 / 70) Mandatory Mandatory Mandatory
VHF DSC Watch Receiver Continuous Ch 70 watch Continuous watch Continuous watch Continuous watch
MF Transceiver with DSC Not required Mandatory (2187.5 kHz) Mandatory Mandatory
Inmarsat-C / Iridium SES Not required Not required Mandatory (Option 1) Inoperable in polar A4
MF/HF Transceiver with DSC Not required Not required Mandatory (Option 2) MANDATORY (4, 6, 8, 12, 16 MHz)
NAVTEX Receiver (518 kHz) Mandatory (where provided) Mandatory Mandatory Mandatory
Satellite EPIRB (406 MHz) Mandatory (Float-free HRU) Mandatory Mandatory Mandatory
Radar SART (9 GHz) 1 (<500 GT) / 2 (≥500 GT) Min 2 on board Min 2 on board Min 2 on board
Two-Way VHF Walkie-Talkies 2 (<500 GT) / 3 (≥500 GT) Min 3 on board Min 3 on board Min 3 on board

4. Power Supplies, Maintenance & Radio Log

SOLAS Chapter IV Regulation 13 mandates that every radio installation be supplied with electrical power from three distinct sources: the ship's main electrical source, the emergency source (Emergency Diesel Generator), and a dedicated Reserve Source of Energy (battery bank) configured for instant, uninterruptible changeover.

1 Hour / 6 Hours Reserve Battery Duration (With/Without EDG)
24V DC Radio Reserve Battery Voltage
≤ 10 Hours Battery Recharge Time Limit
3 Methods Statutory Maintenance Options
Daily / Monthly DSC Test & Battery Capacity Check
GMDSS ELECTRICAL POWER DISTRIBUTION & RESERVE BATTERY CIRCUIT SOLAS IV Reg 13 • Main 220V AC • Emergency Generator Feed • 24V DC Reserve Source (1h / 6h Capacity) MAIN 220V AC (MSB) Normal Navigation Power EMERGENCY 220V (ESB) From Emergency Generator AC CHANGEOVER Zero-Break Relay FLOAT CHARGER Auto Boost / Float 24V DC BANK Lead-Acid / Gel 1h / 6h DURATION VITAL 24V CONSUMERS VHF / DSC Transceiver MF/HF DSC Transceiver Satellite SES (Inmarsat-C) GPS Receiver Input Emergency Console Light I = Irx + 0.5(Itx) + Ilight BATTERY SIZING RULE: If the emergency generator is capable of powering the GMDSS console, reserve batteries must supply 1 hour of operation. If the emergency generator does NOT feed the radio, batteries must provide 6 hours of continuous duty.
Figure 4: GMDSS Electrical Power & Reserve Energy Distribution Circuit. Normal power is fed from the 220V MSB, automatically backed by the Emergency Diesel Generator (ESB). In the event of total blackout, the dedicated 24V DC accumulator battery bank takes over instantaneously without interruption to power VHF, MF/HF, satellite terminals, and emergency console lighting for 1 hour (or 6 hours without EDG).

The Three Statutory GMDSS Maintenance Options (Sea Areas A3 & A4)

Option 1: Duplication of Equipment
Carrying a complete duplicate set of radio equipment (e.g. secondary VHF/DSC and secondary Inmarsat-C or MF/HF system) connected to independent antennas and power feeds.
Option 2: Shore-Based Maintenance
Maintaining a written agreement with a recognized service company guaranteeing worldwide port attendance, immediate spare parts supply, and radio survey support.
Option 3: At-Sea Maintenance
Carrying a dedicated radio electronics officer holding an electronic repair qualification together with full onboard spare module cards, test benches, and calibration instruments.
Statutory Requirement
Ships in A1 or A2 must apply at least one maintenance method. Ships sailing in Sea Areas A3 and A4 must employ a combination of at least two methods (typically Duplication + Shore-Based Maintenance).

Statutory GMDSS Routine Testing & Radio Log Duties

1
Daily Operational Checks: Test VHF-DSC internal loop back diagnostics (without transmitting on-air), verify GPS input coordinates and UTC clock on all units, confirm NAVTEX paper/printer status, and inspect battery voltage on float charge.
2
Weekly Live DSC Call Test: Transmit an operational test DSC call to a coast radio station (CRS) on MF (2187.5 kHz) or HF where permitted, verify receipt of automatic acknowledgment, and test two-way portable survival craft VHF transceivers on non-distress channels.
3
Monthly Battery & SART/EPIRB Checks: Inspect battery terminals for corrosion and electrolyte specific gravity; run off-charge battery discharge test; verify EPIRB and SART self-test lights without generating live distress signals; check hydrostatic release expiry dates.
4
Radio Log Book Entries: All distress and safety traffic, daily/weekly test results, battery maintenance records, and designated qualified radio personnel must be logged in the official GMDSS Radio Log, retained on board for at least 3 years.

Oral Examination Viva Bank: SOLAS Chapter IV

What are the four GMDSS Sea Areas and their defining communication technologies?
Sea Area A1: within VHF coast station range with continuous DSC (20–30 NM). Sea Area A2: within MF coast station range with continuous DSC (100–150 NM, 2187.5 kHz), excluding A1. Sea Area A3: within coverage of IMO-recognized geostationary mobile satellites (Inmarsat/Iridium between 70°N and 70°S), excluding A1 and A2. Sea Area A4: polar waters outside A1, A2, and A3, requiring High Frequency (HF) radio with DSC across 4, 6, 8, 12, and 16 MHz.
How long must the GMDSS reserve source of electrical energy supply the radio installation?
Under SOLAS IV Regulation 13, the reserve battery bank must supply power to all required radio equipment simultaneously for 1 hour if the ship is equipped with an emergency electrical source (Emergency Diesel Generator) that meets all radio supply requirements. If the emergency generator does not power the radio installation, the reserve battery bank must provide at least 6 hours of continuous operation.
What maintenance methods are required for ships trading in Sea Area A3 or A4?
Ships operating in Sea Areas A3 and A4 must use a combination of at least two of the three statutory methods: (1) Duplication of equipment (carrying spare VHF/DSC and satellite/HF sets), (2) Shore-based maintenance (service contract with accredited depot), or (3) At-sea electronic maintenance capability (qualified personnel and spares). In practice, commercial ships almost universally choose Duplication + Shore-Based Maintenance.
What should you do if an accidental DSC distress alert is transmitted from your vessel?
Never switch off the equipment. Switch the transmitter to the corresponding radiotelephony distress channel immediately (Channel 16 for VHF, 2182 kHz for MF, 8291 kHz for HF) and broadcast an "All Stations" cancellation message stating the ship's name, call sign, MMSI, position, and explicitly declaring: "Please cancel my accidental distress alert of [date, UTC time]." Record the event and cancellation in the GMDSS Radio Log.