1The lesson in plain words
In one sentence…
A sea area is not a distance: choose the medium from available service and continuous distress coverage.
What defines A1–A4?
A1 is government-declared VHF with continuous DSC; A2 is MF outside A1; A3 depends on recognized satellite service; A4 is outside all three. Each area combines coverage and alerting.
Why does the area matter?
It describes coverage and service; to choose the medium, check position, availability, installed equipment and conditions. Do not confuse classification and choice before sailing.
Why does propagation matter?
VHF is mainly line of sight; MF can use ground-wave propagation and HF ionospheric propagation; conditions change the result.
How do you choose the medium?
Identify route and coverage, consult current services, match them to approved equipment, assess alternatives and check MSI reception; do not trust a fixed distance when choosing.
2Step by step
Choosing the medium for the voyage
- 1
Identify the route and its changes in coverage.
The most demanding section can change the available service.
- 2
Consult current coast-station and recognized satellite-service information.
Published coverage and delivered services must fit the voyage.
- 3
Match coverage to approved equipment and required watches.
The ship's installation and requirements answer a separate question.
- 4
Assess alternatives and service, antenna or power-supply failures.
A failure changes capability, not the area's label.
- 5
Check how maritime safety information will continue arriving.
The voyage must maintain MSI reception as well as distress communication.
3Key data
- A1
- Government VHF + continuous DSC alerting
- A2
- MF outside A1
- A3 since 2024
- Outside A1/A2; recognized satellite service
- A4
- Outside A1, A2 and A3
- Polar waters
- A3 with suitable approved global service
- VHF
- Mainly line of sight
- MF
- Can use ground-wave propagation
- HF
- Ionospheric propagation; conditions matter
- Fixed distance
- Does not guarantee coverage
4Key points
- 1.
Sea area = coverage + continuous distress alerting.
- 2.
A3 depends on the recognized satellite service carried onboard.
- 3.
MF failure does not turn A2 into A4.
- 4.
Propagation limits paths; no fixed distance guarantees coverage.
5Frequent mistakes
Wrong: "Every polar route is A4."
Right: Check the recognized service: polar waters can be A3 with a suitable approved global service.
Wrong: "If MF fails, A2 becomes A4."
Right: The failure changes the ship's capability; it does not redraw published coverage.
Wrong: "I choose a band only by distance or UTC hour."
Right: Assess route, service, equipment and current conditions; a fixed distance does not guarantee coverage.
6Practice case & exam keys
Situation
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?
Show the answer →
The area does not become A4 because equipment has failed. Reassess the ship's available and required communication capability for the voyage.
Why: 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.
Exam keys
If the question says "A3 since 2024," look for the recognized service and carried ship earth station.
If it says "MF failed," do not change A2 to A4: reassess capability and alternatives.
If an option promises a "guaranteed distance," reject it: VHF, MF and HF depend on path and conditions.
How to read it
Think of a sea area as a service definition, not a circle drawn at a fixed distance from the coast.
Separating coverage from equipment prevents inventing A4 after a failure or trusting an unchecked link.
7Full lesson & sources
Open the full lesson (diagrams and official references)
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
8Test your knowledge
What defines A1?
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