GMDSS: Sea Areas, Equipment and Routine Checks

What GMDSS does: functions, calls and distress frequencies

In short: Read first: distress signals and the OOW’s duties (a proper radio watch is one of them). The GMDSS (Global Maritime Distress and Safety System) is the worldwide radio system in SOLAS Chapter IV. It lets a ship in distress raise the alarm quickly, be found, and talk to rescuers, and it sends safety information to every ship.

• Six functions to remember:
  1. Alerting — including the position of the unit in distress
  2. Search and rescue coordination
  3. Locating (homing)
  4. Maritime safety information (MSI) broadcasts
  5. General communications
  6. Bridge-to-bridge communications
• What a ship must carry depends on the sea area it trades in (next section), not only on its size.
• Who it applies to: SOLAS Chapter IV covers cargo ships of 300 GT and over and all passenger ships on international voyages.

DSC calls: categories and addresses

• DSC (Digital Selective Calling) sends a short digital alert or call. Then you talk by voice on the paired frequency.
• Priority, highest first: Distress (Mayday) · Urgency (Pan Pan) · Safety (Sécurité) · Routine.
• Call types: distress · distress relay · urgency · safety · individual/routine · group · geographic area · telephone (to connect a phone call through a coast station).
• A limited Class D controller (VHF DSC fitted voluntarily, typical on small craft) offers only distress, distress relay, urgency, safety and individual/routine calls.

Distress frequencies

• First the DSC alert, then the voice message on the paired frequency.
• Two more VHF channels every GMDSS ship must have: Ch 13 (bridge-to-bridge, safety of navigation) and Ch 6 (on-scene SAR, including with aircraft) (SOLAS IV/7).

Sea areas A1, A2, A3 and A4

In short: Sea areas are set by radio coverage, not by a fixed distance. A1 and A2 are declared by each coastal government. A3 depends on the satellite service the ship carries. A4 is everything else (SOLAS IV/2, revised by IMO MSC.496(105)).

• The mile figures only give you a feel. The real limits are the ones each coastal state declares, listed in ALRS Vol. 5 (NP285).
• Each area has its own regulation: A1 = SOLAS IV/8, A2 = IV/9, A3 = IV/10, A4 = IV/11.

What changed on 1 January 2024

• SOLAS Chapter IV was rewritten by IMO MSC.496(105), in force 1 January 2024. The word “Inmarsat” was replaced by Recognised Mobile Satellite Service (RMSS).
• Iridium was recognised as a second GMDSS satellite provider in 2020. Inmarsat is no longer the only one.
• A3 now depends on the ship earth station on board:
  • Inmarsat → A3 stays as before (about 70° N/S).
  • Iridium → A3 is global and swallows A4. An Iridium ship has no A4.
  • A regional satellite system → A3 is only that system’s footprint.
• Two different ship earth stations on board? The one with the smaller coverage sets the ship’s A3.
• Older references list Inmarsat Fleet 77 for A3. That service has been phased out and replaced by Inmarsat’s newer service (Fleet Safety). Check the current position before the exam.
• Older sources say “A3 = Inmarsat, up to 70° N/S”. That is still true for an Inmarsat ship, but it is no longer the definition.

Minimum equipment: the basic set and what each area adds

In short: Every GMDSS ship carries a basic set (SOLAS IV/7), then adds equipment for each sea area it trades in (SOLAS IV/8–11).

The basic set (every GMDSS ship)

• VHF radio installation: DSC on Ch 70, voice on Ch 16, 13 and 6, and a continuous DSC watch on Ch 70.
• SART (radar SART or AIS-SART): 1 if under 500 GT, 2 if 500 GT and over (one each side). Older texts give the same limit in GRT.
• Two-way portable VHF for survival craft: 2 if under 500 GT, 3 if 500 GT and over.
• Since 2024 the SARTs and the portable VHFs are in SOLAS IV/7. Before, they were in SOLAS III/6.
• MSI receiver(s) able to receive MSI and SAR information for the whole voyage (SOLAS IV/7 as amended in 2024). In practice (IMO MSC.1/Circ.1645):
  • a NAVTEX receiver where an international NAVTEX service exists;
  • outside NAVTEX areas, an EGC receiver on the satellite service (for example Inmarsat SafetyNET) or an HF NBDP receiver.
  • The pre-2024 wording was: NAVTEX if the ship trades where NAVTEX is provided; an Inmarsat EGC receiver where Inmarsat covers but MSI is not given by NAVTEX or HF.
• Satellite EPIRB on 406 MHz. Since 2024 a VHF EPIRB is no longer accepted in A1.

What each sea area adds

• A ship trading in several areas meets the rules of every area it enters.
• Since 2024, MF/HF is primary equipment only for A4. For A3 the primary system is the satellite station.
• Under COMSAR.1/Circ.32/Rev.3, two VHF, one MF/HF and one RMSS station can be enough for an A3 ship.
• HF NBDP (radio telex) is no longer needed for distress traffic in A3 and A4.

Locating: EPIRB and SART

• EPIRB: transmits on 406 MHz. Satellites (Cospas-Sarsat) pass the alert to an MRCC with the ship’s position. The message carries the ship’s identity, so the EPIRB must be registered.
• EPIRB battery: at least 48 h of operation (IMO A.810(19)).
• Satellites stopped processing the old 121.5/243 MHz beacons on 1 February 2009 (Cospas-Sarsat). Only 406 MHz beacons are detected by satellite.
• SART: answers a ship’s or aircraft’s X-band (9 GHz) radar and shows as a line of 12 dots on the screen. Battery: 96 h on standby plus 8 h transmitting (IMO A.802(19)). More in radar in search and rescue.

Keeping it working: maintenance and power supply

In short: GMDSS must be available at sea. SOLAS IV/15 gives three ways to make sure of it; SOLAS IV/13 sets three power sources and the battery time.

Three maintenance methods (SOLAS IV/15)

1. At-sea electronic maintenance: a qualified radio-electronic officer on board (holding a GMDSS first or second-class Radio-Electronic Certificate), with enough spares and manuals.
2. Duplication of equipment.
3. Shore-based maintenance.

• A1 and A2: at least one method.
• A3 and A4: at least two methods.
• Duplication means each extra radio has its own antenna and is installed and ready for immediate use. Equipment with equal or wider coverage counts as a duplicate.

Power supply (SOLAS IV/13)

• Three sources:
  1. Ship’s main generators
  2. Ship’s emergency generator (if fitted)
  3. A dedicated radio battery (the reserve source of energy)
• Battery capacity: 1 hour on ships with an emergency generator; 6 hours on ships without one.
• The batteries are charged by an automatic charger, itself fed from the main and the emergency supply.
• Changeover from AC to battery is automatic and “no break”, so no data in the equipment is lost (for example the position in the DSC controller).

NAVTEX and satellite MSI

In short: NAVTEX prints maritime safety information for coastal waters on 518 kHz, in English. Further offshore, MSI comes by satellite (EGC, for example Inmarsat SafetyNET) or by HF. Station lists and schedules are in ALRS Vol. 5 (NP285).

• 518 kHz = international service, in English. 490 kHz = national services, often in the local language.
• Range: designed for about 250 NM from the station.
• Each message header has a station letter, then a message category letter, then a serial number (for example “FA56”: station F, category A, number 56).
• Choose the stations you want on the receiver. Pick the ones for your passage.

Categories you cannot switch off

• A — navigational warnings
• B — meteorological warnings
• D — search and rescue information and piracy warnings
• L — additional navigational warnings (IMO recommends it is not rejected)
• A, B, D and L are message categories, not stations. The receiver will not let you reject them.
• Useful optional category: E — weather forecasts (see weather information at sea).

Inmarsat C

• A satellite text terminal: store-and-forward messages, a pre-formatted distress alert and the SafetyNET EGC service for MSI.
• Low power, small omnidirectional antenna; it can run from the ship’s 24 V emergency battery.
• It must be logged in to the right ocean region to receive MSI and send alerts (checked daily).

Daily, weekly and monthly checks

In short: Routine tests prove the equipment works without sending a real alert. Record every test and result in the GMDSS radio log. On a yacht this is in effect a compulsory bridge book, because the bridge is the radio room. Follow the manufacturer’s instructions for each item; the usual schedule (in the MCA GMDSS Radio Log and ALRS Vol. 5) is below.

Daily

• DSC controllers (VHF and MF/HF): run the built-in test, without radiating a signal.
• Radio batteries: check the charge (voltage) and the charger.
• Printers (NAVTEX, Inmarsat C, MF/HF): paper loaded.
• Inmarsat C: logged in to the right ocean region.

Weekly

• MF/HF DSC: a live test call to a DSC coast station (2187.5 kHz or an HF channel), and check you get the acknowledgement. Not more than once a week: more test calls overload the system.
• If the ship has been out of range for more than a week, make the test at the first chance.
• VHF DSC: built-in test only. Never test with a distress alert.

Monthly

• EPIRB: self-test without using the satellite system. Check mounting, damage, battery and hydrostatic release expiry dates, lanyard. Each self-test uses some battery: do not overdo it.
• SART: check its stowage and mounting, damage and battery expiry date. Test it sparingly: it shows on every radar nearby. In port, warn the port authority first.
• Radio batteries: full examination — security, terminals, and electrolyte level on wet cells.
• Portable VHF for survival craft: test on a channel other than Ch 16.
• Aerials and insulators: check them.

Yearly and longer

• Annual radio survey of the whole installation by a radio surveyor.
• EPIRB: tested every year and shore-based maintenance at least every 5 years (SOLAS IV/15).