Start hereRules checked 20 October 2026
The Global Maritime Distress and Safety System (GMDSS) is built on one promise: wherever a ship is, she can raise the alarm ashore by at least two separate and independent means, and the rescue coordination centre can talk back. Since 1 January 2024 that promise is delivered under a completely new text of SOLAS Chapter IV, adopted by resolution MSC.496(105). “Inmarsat” has disappeared from the convention, Iridium can now provide GMDSS services where the flag accepts it, and HF radio telex is no longer a GMDSS requirement.
Radio equipment that sits quietly on the bridge for years is exactly the equipment that fails when it is finally needed. In 2024 the Tokyo MOU recorded 1,244 radio communication deficiencies, and “absence or failure of the proper operation of the radio equipment for distress and safety communication” is on the IMO list of grounds for detention. This lesson explains the sea areas, what each requires, and what happens during the radio survey.
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In this lesson
- Where does each requirement first appear in SOLAS Chapter IV?
- In plain words
- In detail
- What the rules say
- Flag and class notes
- Key numbers
- What inspectors find
- Quiz: 5 questions
Try it
Where does each requirement first appear in SOLAS Chapter IV?
In plain words
The GMDSS divides the oceans into four sea areas according to what shore radio stations can hear:
- A1: within range of a VHF coast station with continuous digital selective calling (DSC) alerting. Its extent depends on the coast stations and is declared by each coastal State.
- A2: outside A1 but within range of a medium frequency (MF) coast station with continuous DSC alerting, which reaches much further offshore than VHF.
- A3: outside A1 and A2 but within the coverage of a recognized mobile satellite service supported by the ship’s own satellite station.
- A4: everywhere else, which in practice means the polar regions.
Every ship carries a basic set: VHF radio with DSC, a float-free satellite EPIRB, a radar SART or AIS-SART, portable VHF radios for the survival craft and a receiver for safety messages. She then adds equipment for the furthest sea area she trades in: MF for A2, a satellite station for A3, MF/HF for A4. Her Cargo Ship Safety Radio Certificate states those sea areas. A qualified radio surveyor checks it all every year; in between the crew test the equipment, keep the radio log and look after the reserve batteries.
In detail
What changed on 1 January 2024
Resolution MSC.496(105), adopted on 28 April 2022, replaced the whole text of Chapter IV with effect from 1 January 2024. The main changes for ships and surveyors:
- Recognized mobile satellite service (RMSS). “Inmarsat” was replaced throughout, and sea area A3 is defined by the coverage of the RMSS supported by the ship’s own station. Iridium is recognised alongside Inmarsat, but only usable where the flag accepts it.
- HF NBDP removed. HF direct-printing telegraphy (radio telex) is no longer required for distress and safety communications.
- MSI receivers by trading pattern. Regulation 7.1.4 simply requires receivers able to receive MSI and SAR information throughout the voyage; the guidance in MSC.1/Circ.1645 helps decide which.
- Survival craft radios moved. The two-way VHF apparatus and radar SARTs or AIS-SARTs moved from Chapter III regulation 6 into Chapter IV regulation 7.
- New performance standards. The footnote to regulation 14 lists the revised standards adopted at MSC 105, such as MSC.511(105) for VHF and MSC.512(105) for MF/HF.
Keeping the equipment available
Under regulation 15, ships in A1 or A2 ensure availability by methods such as duplication of equipment, shore-based maintenance or at-sea electronic maintenance, or a combination; ships in A3 or A4 use a combination of at least two, as approved by the Administration. The choice is shown on the Record of Equipment (Form R) and decides which spares, manuals and duplicate units the surveyor expects to see.
The reserve source of energy (regulation 13) must run the VHF and the MF, MF/HF or ship earth station for at least 1 hour on ships with a compliant emergency generator, or 6 hours without one. If it is a battery bank, it must be recharged to its minimum capacity within 10 hours, and its capacity must be checked at intervals not exceeding 12 months when the ship is not at sea. A common method is a full discharge and recharge at normal operating current.
Radio equipment that can include the ship’s position in a distress alert must get it automatically from a navigation receiver; if that fails, the position and time must be updated by hand at intervals not exceeding four hours while under way (regulation 18).
The radio survey
A qualified radio surveyor, using suitable test equipment, surveys the installation for the Cargo Ship Safety Radio Certificate: a periodical survey within three months either side of each anniversary date, and a renewal survey at the end of the certificate’s validity. The HSSC Survey Guidelines, 2025 list the checks; for a full survey they include:
| Equipment | Typical checks (A.1207(34), section 4 of the radio part) |
|---|---|
| Whole installation | Sea areas declared, equipment fitted for them, method of ensuring availability, plans of antennas and energy sources |
| Reserve source of energy | Capacity, siting, specific gravity or voltage of batteries, charger able to recharge in 10 hours |
| VHF | Channels 6, 13 and 16, frequency tolerance, controls, operation on all power sources, on-air contact |
| VHF DSC | Off-air check of the MMSI, test call to and from a coast station or another ship, alarm audible |
| MF/HF | Antenna tuning, frequency tolerance, contact with a coast station, DSC test call, DSC watch receiver keeping continuous watch |
| Ship earth station | Operation from all power sources, distress function by an approved test procedure |
| MSI receivers | Correct operation by recent messages or self-test |
| EPIRB | Float-free mounting, lanyard, self-test including GNSS, coded identity and MMSI match the ship, battery and hydrostatic release expiry, 406 MHz emission checked without alerting the satellites, annual test and five-yearly shore-based maintenance done |
| Radar SART or AIS-SART | Stowage, response on the ship’s 9 GHz radar or AIS, battery expiry |
| Two-way VHF | Operation on channel 16 and one other, charging or primary battery expiry |
| Documents | Radio licence, operators’ certificates, radio log, ITU publications, operating manuals (and service manuals if at-sea maintenance is the declared option) |
What the rules say
- SOLAS regulation IV/4.1.1.1 (2024 text) requires every ship at sea to be capable of “transmitting ship-to-shore distress alerts by at least two separate and independent means, each using a different radiocommunication service”.1
- SOLAS regulation IV/2.1.17 defines sea area A3 as the area outside A1 and A2 “within the coverage of a recognized mobile satellite service supported by the ship earth station carried on board”.1
- SOLAS regulation IV/7.1 requires every ship to carry VHF with DSC on channel 70 and radiotelephony on channels 6, 13 and 16, a radar SART or AIS-SART, an MSI and SAR information receiver and a float-free EPIRB; IV/7.3 adds a SART on each side and three two-way VHF sets on passenger ships and cargo ships of 500 GT and upwards.1
- SOLAS regulation IV/13 requires a reserve source of energy for at least 1 hour (6 hours without a compliant emergency source), and reserve batteries recharged within 10 hours and capacity-checked at intervals not exceeding 12 months, not at sea.1
- SOLAS regulation IV/15.7 requires ships in sea areas A3 or A4 to ensure availability by a combination of at least two methods, such as duplication, shore-based maintenance or at-sea maintenance.1
- SOLAS regulation IV/16.1 requires every ship to carry personnel qualified for distress and safety communications, holding the certificates in the Radio Regulations, with one person designated with primary responsibility during distress incidents.1
- The HSSC Survey Guidelines, 2025, paragraph 5.11, require the radio survey to be done by a qualified radio surveyor using suitable test equipment, and paragraph 4.4.1 places the radio periodical survey within three months before or after each anniversary date.2
Flag and class notes
- Iridium and the certificate. Indian Register’s technical circular on the 2024 amendments notes that Iridium is acceptable for GMDSS only where the flag recognises it, and that ships certified for A3 show the RMSS used (for example Inmarsat or Iridium) on the Cargo Ship Safety Radio Certificate. Check your flag’s circular before choosing a satellite service.3
- Who may survey. Class approves radio survey firms under IACS UR Z17, which expects the radio inspector to have the supplier’s internal training and at least a year as an assistant, preferably with a GMDSS General Operator’s Certificate (GOC) or Radio Electronic Certificate (REC), and lists minimum test instruments.4
- Flag lists. Some flags authorise radio surveyors directly rather than through class. Ask your flag or recognized organization which radio inspection firms they accept in the port where the survey will be held.2
Key numbers
| What | Value |
|---|---|
| New SOLAS Chapter IV in force | 1 January 20241 |
| VHF DSC distress channel | 156.525 MHz (channel 70)1 |
| MF DSC distress frequency | 2 187.5 kHz1 |
| Reserve source of energy | 1 hour (with compliant emergency generator) or 6 hours (without)1 |
| Reserve battery recharge / capacity check | within 10 hours / at least every 12 months1 |
| Manual position update if GNSS input fails | at intervals not exceeding 4 hours1 |
| EPIRB shore-based maintenance | at intervals not exceeding 5 years1 |
| Radio periodical survey window | 3 months either side of each anniversary2 |
| Tokyo MOU radio communication deficiencies, 2024 | 1,2445 |
What inspectors find
- Radio deficiencies. The Tokyo MOU recorded 964, 1,353 and 1,244 radio communication deficiencies in 2022, 2023 and 2024, and failure of distress and safety radio equipment is a ground for detention in the IMO Procedures for port State control, 2025.56
- Crew who cannot use it. Deck officers unfamiliar with operating and testing radio equipment, or with how maritime safety information reaches the ship, is listed as an operational failing that may justify detention.6
- Paperwork. The Cargo Ship Safety Radio Certificate and its Record of Equipment are among the first documents checked; equipment on board must match Form R.6
- EPIRB details. Expired batteries or hydrostatic releases and an MMSI or beacon identity that does not match the ship are specific items the radio surveyor checks.2
FAQ
Our ship has a working Inmarsat-C. Do we need to change it because of the 2024 amendments?
Not because of the amendments alone. IMO guidance on the validity of installed radio equipment is in MSC.1/Circ.1460/Rev.5 (January 2025). Replacement equipment should meet the current performance standards.
Is NAVTEX still required?
The 2024 text requires a receiver able to receive maritime safety information and SAR information for the whole voyage, without naming NAVTEX. Check MSC.1/Circ.1645 and your flag’s guidance for what covers your trading area.
Does the radio survey replace the EPIRB annual test?
The radio surveyor checks that the EPIRB annual test was done, and the two are often done in the same visit. Legally they are separate requirements: the survey is under SOLAS Chapter I and the EPIRB test under regulation IV/15.9.
Quiz5 questions
Sources
- 1
IMO Resolution MSC.496(105), Amendments to the International Convention for the Safety of Life at Sea, 1974 (adopted 28 April 2022, in force 1 January 2024), annex, new text of chapter IV, regulations 2, 4, 7 to 13, 15, 16 and 18. https://www.amsa.gov.au/sites/default/files/2023-11/resolutions_mscresolutions_res.msc_.496105_230713_050220-1.pdf
- 2
IMO Resolution A.1207(34), Survey Guidelines under the Harmonized System of Survey and Certification (HSSC), 2025, annex, general part paragraphs 4.4.1 and 5.11, and part 4 (Cargo Ship Safety Radio Certificate), paragraphs (RI) 4.1.1 to 4.1.4 and (RP) 4.2. https://wwwcdn.imo.org/localresources/en/OurWork/IIIS/Documents/A%2034-Res.1207%20-%20SURVEY%20GUIDELINES%20UNDER%20THE%20HARMONIZED%20SYSTEMOF%20SURVEY%20AND%20CERTIFICATION%20(HSSC),%202025%20(Secretariat).pdf
- 3
Indian Register of Shipping, Technical Circular No. 030/2023, Amendments to SOLAS Chapter IV (Radiocommunications). https://www.irclass.org/technical-circulars/amendments-to-solas-chapter-iv-radiocommunications/
- 4
IACS Unified Requirement Z17, Procedural requirements for service suppliers, annex 1, section on firms engaged in surveys, inspections and testing of radio communication equipment (text read: Rev.11, 2015; check the current revision on the IACS website). https://iacs.org.uk/
- 5
Tokyo MOU, Annual Report on Port State Control in the Asia-Pacific Region 2024, Tables 7 and 14. https://www.tokyo-mou.org/wp/wp-content/uploads/ANN24-1.pdf
- 6
IMO Resolution A.1206(34), Procedures for port State control, 2025 (adopted 3 December 2025), appendix 2, section 5, item 9; appendix 7, paragraph 5.1.4.5; appendix 12, item 6. https://wwwcdn.imo.org/localresources/en/KnowledgeCentre/IndexofIMOResolutions/AssemblyDocuments/A.1206(34).pdf
Rules checked 20 October 2026. VSL Safety is not an approved training provider and doesn't issue certificates. These lessons don't replace STCW training, or the manufacturer's training and certification that service technicians need.
