The lesson in plain words
- 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.
Step by step
Choosing the medium for the voyage
- Identify the route and its changes in coverage. The most demanding section can change the available service.
- Consult current coast-station and recognized satellite-service information. Published coverage and delivered services must fit the voyage.
- Match coverage to approved equipment and required watches. The ship's installation and requirements answer a separate question.
- Assess alternatives and service, antenna or power-supply failures. A failure changes capability, not the area's label.
- Check how maritime safety information will continue arriving. The voyage must maintain MSI reception as well as distress communication.
Key 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
Frequent 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.
Exam keys & interpretation
- 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.
Key points
- Sea area = coverage + continuous distress alerting.
- A3 depends on the recognized satellite service carried onboard.
- MF failure does not turn A2 into A4.
- Propagation limits paths; no fixed distance guarantees coverage.
Test your knowledge
What defines A1?
Go deeper: GOC exam level
The detail behind each step for the General Operator's Certificate: the rules, the radio technique and worked cases.
Full 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 is not a distance. It is a promise from a government: inside this line, a station of a given type keeps a continuous DSC (or satellite) watch for your alert. A1 is the area covered by at least one VHF coast station with continuous DSC alerting, A2 the area covered by an MF coast station, and A3 the area covered by a recognized mobile satellite service. A4 is everything left over. Distances such as "about 30 NM" for A1 describe what a typical VHF station achieves. They are not the definition, which is why two coasts can have very different A1 widths.
A3 is service-dependent. Since 1 January 2024 A3 means the coverage of the recognized mobile satellite service supported by the ship earth station you actually carry. Take two ships at 80°N north of Svalbard. Ship X carries only Inmarsat C: geostationary satellites sit low on the horizon or below it at that latitude, so she is outside her service's coverage and in A4, relying on HF. Ship Y carries an approved Iridium GMDSS terminal: the low-orbit constellation covers the poles, so the same water is A3 for her. Same position, two answers. That is why "polar means A4" is wrong.
Why VHF, MF and HF reach so differently
VHF is mainly a line-of-sight service, so antenna height decides range. A handy estimate is d (NM) ≈ 2.2 × (√h₁ + √h₂), heights in metres. Your antenna at 25 m (√25 = 5) talking to a coast-station mast at 100 m (√100 = 10) gives 2.2 × 15 ≈ 33 NM. Lower the coast mast to 25 m and you get 22 NM. A small yacht with its antenna at 9 m talking to another yacht at 9 m reaches only about 13 NM. More watts barely change these figures; height does.
MF at 2 MHz travels as a ground wave that hugs the sea, which conducts well, so a coast station often reaches a hundred miles or more by day, and further at night when the sky wave adds in. HF bounces off the ionosphere and can reach thousands of miles, but the best band shifts with time of day, season and path length. A higher band is not automatically better, and dropping to a lower band does not always fix a long night-time path.
| Leg of a passage | Typical area | Main alerting path you would plan on |
|---|---|---|
| Leaving port, 15 NM off a VHF coast station | A1 | VHF DSC Channel 70 |
| 120 NM offshore, inside a declared MF coverage | A2 | MF DSC 2187.5 kHz, VHF still useful ship-to-ship |
| Mid-ocean, 800 NM out | A3 for a ship with a recognized satellite terminal | Satellite distress alert and MF/HF DSC |
| High Arctic, outside your terminal's service | A4 | HF DSC |
Plan the whole voyage before you sail
Plan the passage leg by leg, the way you plan fuel. For each leg ask four questions: which area it is in, which equipment your certificate says you need there, which path you would alert on first, and how MSI keeps reaching you (NAVTEX near the coast, SafetyNET or SafetyCast offshore).
- 1Mark where the route leaves each coast station's declared coverage.
- 2Check that the ship is certified and equipped for the most demanding area on the route.
- 3Note where NAVTEX coverage ends and satellite MSI takes over.
- 4Decide what you will do if one path fails on each leg.
When equipment fails at sea
Suppose the MF/HF set dies 300 NM offshore on an A3 ship. The area is unchanged; your options have shrunk. The satellite terminal is now your only long-range path, so check it is logged in, tell the company and the coast station or RCC, log the defect and the time, and plan repairs or a route change if the remaining means cannot cover the rest of the voyage.