A1 is government-declared VHF radiotelephone coverage with continuous DSC alerting. A2 is corresponding MF coverage outside A1. Since 2024, A3 excludes A1/A2 and depends on continuous alerting coverage of the recognized satellite service supported by the ship earth station carried. A4 is outside A1, A2 and that A3. Polar waters can be A3 with a suitable approved global service. Missing MF equipment does not turn an A2 coverage area into A4.
VHF propagation is mainly line of sight; MF ground-wave and HF ionospheric propagation depend on conditions. No fixed distance guarantees coverage. Choose an available distress path that can reach assistance and verify current coast-station and satellite service information before sailing.
Read a sea area as a service definition
A sea area describes a combination of coverage and continuous distress alerting. It is not a circle that a student can draw at a fixed distance from every coast. Published coverage, the services actually provided and the ship's approved installation have to be considered together.
The most important modern distinction is that A3 is service-dependent. An old diagram using fixed satellite latitude limits cannot define A3 for every recognized mobile satellite service. Examine the recognized service supported by the carried ship earth station and the continuous alerting coverage available for the voyage.
Separate classification from a practical communication choice
| Question | What answers it |
|---|---|
| Which sea area applies? | The formal coverage/service definition and applicable information |
| What equipment must this ship carry? | Its applicable SOLAS/flag requirements and approved arrangement |
| Which path is usable now? | Position, service availability, fitted equipment and current conditions |
| What happens after an alert? | The corresponding communication procedure and receiving station |
These questions are connected but are not interchangeable. A satellite terminal being carried does not automatically prove that it is a recognized GMDSS ship earth station. Likewise, being in a published coverage area is not proof that a particular antenna, receiver or network connection is functioning at this moment.
Understand why propagation matters
VHF is mainly a line-of-sight service. Antenna height, obstruction and the relative positions of the stations influence the path. The useful lesson is to examine whether the stations can communicate, rather than memorize one guaranteed range.
MF can support ground-wave communication, while HF can use ionospheric propagation over longer paths. HF performance changes with the path, time and ionospheric conditions. A higher frequency is not automatically a better choice, and a lower frequency is not automatically a safe fallback for every distance.
Work through the voyage before choosing equipment
- 1Identify the route, including its most demanding portion and any expected change of service coverage.
- 2Consult current coast-station and recognized satellite-service information.
- 3Match that information to the ship's approved equipment and required watches.
- 4Identify alternative means and the consequences of a service, antenna or supply failure.
- 5Check how maritime safety information will continue to be received along the route.
An equipment or service failure should trigger a capability assessment and the appropriate bridge action. It should not be explained away by inventing a new sea-area label. In an incident, use the appropriate available path that can reach assistance, without allowing an academic classification exercise to obstruct urgent communication.
Apply the distinction to polar routes
Polar location alone does not establish A4. A suitable approved global satellite service may provide the continuous alerting coverage relevant to A3. The learner still has to check the actual recognized service, terminal, voyage and applicable requirements. “Global” in a consumer product advertisement is not a substitute for that evidence.
Official references · 2026-09-09
Apply what you learned
A ship in a published A2 coverage area reports a failed MF installation. A trainee proposes changing the chart annotation from A2 to A4 and continuing on that basis.
Which part of the reasoning is wrong, and what needs reassessment?
Suggested answer: The area does not become A4 because equipment has failed. Reassess the ship's available and required communication capability for the voyage.
Why this works: Coverage classification and the ship's equipment condition are separate questions. The bridge must assess the failure, alternatives and applicable operating requirements using current information. Relabelling the area does not repair the radio or establish a suitable alternative.
Continue with guided radio exercises.
Practise in the radio simulatorKey Takeaways
Sea areas are defined by specified services and continuous alerting coverage.
Modern A3 depends on the recognized satellite service supported by the carried station.
Equipment failure does not redefine A1 or A2 coverage.
Propagation models explain limitations; they do not guarantee a fixed radio range.
Assess both distress communication and continuing MSI reception for the voyage.
Common Mistakes
Calling every polar route A4 without checking the recognized satellite service.
Describing an A2 area as A4 because the ship lacks working MF equipment.
Choosing a radio band solely from distance or the UTC hour.
Test Your Knowledge
Practise this topic and review the explanation for each answer.
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