SOLAS Chapter IV — Radio Communications (GMDSS)

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 station range.
• A2: 100–150 NM — MF-DSC coast station range.
• A3: 70°N to 70°S — satellite coverage zone.
• A4: polar — HF skywave coverage.
• ≥ 300 GT — the SOLAS Chapter IV application threshold.

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 — the statutory SOLAS IV mandate.
• 2 separate paths — independent distress alerting.
• ≤ 50 m — GPS distress position precision.
• 518 kHz — the international NAVTEX MSI frequency.
• Zero delay — dedicated bridge distress panel.

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.

• Channel 70 — the VHF-DSC watch at 156.525 MHz.
• 2187.5 kHz — the MF-DSC distress watch.
• 8414.5 kHz — mandatory HF-DSC distress frequency.
• 518 / 490 kHz — NAVTEX international and local channels.
• ≥ 2 SARTs — carriage on ships of 500 GT and above.

Mandatory Equipment Carriage by GMDSS Sea Area

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 and without the emergency diesel generator running.
• 24V DC — radio reserve battery voltage.
• ≤ 10 hours — battery recharge time limit.
• 3 methods — the statutory maintenance options.
• Daily / monthly — DSC test and battery capacity check.

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

1. 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.

2. 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.

3. 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.

4. 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.