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 120 NM offshore inside a declared A2 area reports that its MF installation has failed. A trainee proposes writing A4 on the chart for this leg and carrying on.
What is wrong with the trainee's reasoning, and what should happen instead?
Show the answer →
The area is still A2. Reassess which means can still alert the shore, report and log the defect, and plan repair or a route change.
Why: Sea areas describe shore-side coverage, which has not changed. What changed is the ship's ability to use it. The bridge must identify the remaining means (VHF, a satellite terminal if fitted, the EPIRB as a last resort), tell the company and coast station, and decide whether the remaining equipment is adequate for the rest of the voyage.
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.
7Go deeper: GOC exam level
The detail behind each step for the General Operator's Certificate: the rules, the radio technique and worked cases.
- 1. Ground wave (MF): follows the sea and fades with distance
- 2. Sky wave (HF): bent back down by the ionosphere
- 3. Skip zone: no signal between the end of the ground wave and the first sky-wave return
- 4. D layer: absorbs lower frequencies by day
- 5. F layer: carries long HF links (the E layer lies between)
- 6. VHF line of sight: ends at the radio horizon
Sea areas by definition, not by distance
| Area | Current definition (simplified) |
|---|---|
| A1 | Within radiotelephone coverage of at least one VHF coast station with continuous DSC alerting |
| A2 | Outside A1, within radiotelephone coverage of at least one MF coast station with continuous DSC alerting |
| A3 | Outside A1 and A2, within coverage of a recognized mobile satellite service supported by the ship earth station carried on board, with continuous alerting |
| A4 | Everything outside A1, A2 and A3 |
Equipment logic by area
This is a teaching summary of SOLAS IV for Convention ships. Exact carriage depends on the ship, its tonnage and its approved configuration: always check the Safety Radio Certificate and the Administration's documents.
| Function / equipment | A1 | A2 | A3 | A4 |
|---|---|---|---|---|
| VHF with DSC on Ch 70 + DSC watch | Yes | Yes | Yes | Yes |
| VHF radiotelephony Ch 6, 13, 16 | Yes | Yes | Yes | Yes |
| MF DSC 2187.5 kHz + radiotelephony 2182 kHz | — | Yes | Yes (both A3 options include MF DSC and voice) | Part of MF/HF |
| MF/HF DSC + radiotelephony | Optional | May serve as additional means | Alternative to the RMSS ship earth station | Yes, the core equipment |
| RMSS ship earth station | Not required | Not required | Yes, for the service that defines A3 | May be fitted, but A4 lies outside its supported coverage |
| 406 MHz float-free EPIRB | Yes | Yes | Yes | Yes |
| MSI reception | Yes | Yes | Yes | Yes |
| Radar SART or AIS-SART | Yes | Yes | Yes | Yes |
| GMDSS portable VHF | According to tonnage/passenger status | Same | Same | Same |
The watches you must keep
- VHF DSC: continuous watch on Channel 70.
- MF DSC: continuous watch on 2187.5 kHz where MF is required.
- MF/HF (A3 with MF/HF, and A4): DSC watch on 2187.5 kHz and 8414.5 kHz, plus at least one of 4207.5, 6312, 12577 or 16804.5 kHz chosen for the time of day, position and propagation.
- MSI: reception on the system that serves the area the ship is in (NAVTEX, SafetyNET, SafetyCast...).
- VHF Channel 16: listening watch where practicable.
Radio from zero: frequency and wavelength
Frequency is how many cycles a wave completes per second, measured in hertz. 1 kHz = 1,000 Hz; 1 MHz = 1,000,000 Hz; 1 GHz = 1,000,000,000 Hz. Wavelength is the distance the wave travels during one cycle.
λ = c / f, with c ≈ 3 × 10⁸ m/s. The working shortcut: λ (m) ≈ 300 / f (MHz).
| Frequency | Approximate λ | What it tells you |
|---|---|---|
| 518 kHz (NAVTEX) | 579 m | Receive-only; a short active antenna is enough |
| 2182 kHz (MF voice) | 137 m | A resonant antenna would be impossibly long; hence the ATU |
| 8291 kHz (HF voice) | 36 m | A quarter-wave is about 9 m |
| 156.8 MHz (Ch 16) | 1.9 m | Compact VHF whips |
| 406 MHz (EPIRB) | 0.74 m | A short antenna built into the beacon |
| Band | Range | Typical GMDSS use |
|---|---|---|
| MF | 300 kHz – 3 MHz | 2182 kHz voice, 2187.5 kHz DSC, NAVTEX 518 kHz |
| HF | 3 – 30 MHz | Long-range voice and DSC on 4/6/8/12/16 MHz |
| VHF | 30 – 300 MHz | Maritime band around 156–174 MHz, AIS |
| UHF/SHF and above | Above 300 MHz | 406 MHz EPIRB, 9 GHz radar SART, satellite links |
Channel versus frequency, simplex versus duplex
A frequency is a physical quantity. A channel is a regulatory label that stands for one frequency (simplex) or a pair of frequencies (duplex). VHF Channel 16 is 156.800 MHz. On VHF you think in channels; on MF/HF you more often work with frequencies or ITU channel numbers.
In simplex, both stations take turns on the same frequency. In duplex, each direction uses its own frequency, and simultaneous talk is possible if the equipment and network allow it. "Duplex channel" in maritime use often just means a ship/coast pair, even if your handset works half-duplex.
Propagation: why the band decides the reach
- VHF is essentially line of sight. Antenna height and the radio horizon dominate. Raising the antenna usually helps more than adding power.
- MF travels well as a ground wave over sea water (a good conductor) and gains ionospheric propagation at night.
- HF relies on sky waves refracted by the ionosphere, giving links of hundreds to thousands of miles.
The ionosphere is not a mirror
It is a plasma whose electron density changes with the sun, local time, season, latitude and geomagnetic activity.
- D layer: present by day; absorbs strongly at the lower HF frequencies. It largely disappears at night.
- E layer: helps on shorter paths; sporadic E can appear.
- F layer (F1/F2 by day, merged at night): carries most long-distance HF links.
MUF, LUF, OWF and the skip zone
- MUF (maximum usable frequency): the highest frequency that still returns to earth over a given path at a given time.
- LUF (lowest usable frequency): the practical lower limit set by absorption, noise and signal-to-noise ratio.
- OWF / FOT (optimum working frequency): a reliable choice with a margin below the MUF.
- Skip zone: the gap between the end of the ground wave and the first return of the sky wave, where a station may hear nothing.
Channel plans: how the numbers are built
VHF (Radio Regulations Appendix 18). Channels run from 01 to 28 and 60 to 88. Some are simplex (both stations on one frequency), others duplex (ship and coast frequencies 4.6 MHz apart). Channels 6, 8, 72 and 77 are intership only; others are shared between port operations, ship movement and public correspondence according to the table and national use. Always check the local channel in the publications rather than guessing.
MF/HF (Radio Regulations Appendix 17). HF radiotelephone duplex channels carry a three- or four-digit number: the band in MHz followed by a two-digit serial. Channel 401 is the first channel in the 4 MHz band, 822 the 22nd in the 8 MHz band, 1601 the first in the 16 MHz band. Each number stands for a coast transmit and a ship transmit frequency.
| Communication type | Where it happens |
|---|---|
| Distress, urgency and safety | Protected DSC and voice frequencies on each band |
| Port operations and ship movement | Assigned VHF channels, VTS and pilot channels |
| Ship-to-ship | Intership VHF channels; Ch 13 for bridge-to-bridge safety |
| Public correspondence | Duplex VHF and MF/HF channels of coast stations, satellite services |
GOC exam check
Which is the definition of sea area A2?
8Full 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.
Official references · 2026-09-09
9Test your knowledge
What defines A1?
Keep studying…
Reviewed by Ramon Curto, Nautical teacher at IECNPC · Last reviewed: