Module 2 — Frequencies, sea areas and choosing the medium
Sea areas A1–A4, VHF / MF / HF / satellite bands, propagation, simplex vs duplex and how to pick the right equipment for distance and emergency type.
Before touching any specific equipment, you have to answer one question:
GMDSS sea areas
| Area | Typical coverage | Primary equipment | Backup |
|---|---|---|---|
| A1 | Up to ~20–30 NM offshore with VHF-DSC | VHF-DSC + channel 16 voice | EPIRB |
| A2 | Outside A1, up to ~150 NM with MF-DSC | MF-DSC + 2182 kHz voice | VHF + EPIRB |
| A3 | Outside A2, within recognized satellite coverage | Inmarsat C / Iridium + HF-DSC | MF + EPIRB |
| A4 | Polar areas, outside satellite coverage | HF-DSC + HF voice + EPIRB | No satellite backup |
Bands and how they propagate
| Band | Frequencies | Propagation | Typical range |
|---|---|---|---|
| VHF | 156–174 MHz | Direct wave (line-of-sight) | 20–40 NM |
| MF | ~1.6–4 MHz | Ground-wave + night-time sky-wave | 50–300 NM |
| HF | 4–27 MHz | Ionospheric refraction | 300 to >5000 NM |
| Satellite | 1.5–1.6 GHz / Ka | Up to satellite + down | Service footprint |
Simplex, duplex, semi-duplex
| Mode | Frequencies | Tx/Rx | Example |
|---|---|---|---|
| Simplex | 1 only | Alternating — only one speaks at a time | VHF channel 16 |
| Duplex | 2 (TX pair + RX pair) | Simultaneous — both can speak | Coast MF/HF channels |
| Semi-duplex | 2 frequencies | Alternating (like simplex) | Some VHF coast channels |
Critical distress frequencies — memorise them
| Frequency | Use |
|---|---|
| 156.800 MHz | VHF channel 16 — distress / calling voice |
| 156.525 MHz | VHF channel 70 — DSC (never voice) |
| 2182 kHz | MF — distress voice |
| 2187.5 kHz | MF — DSC distress |
| 4207.5 / 6312 / 8414.5 / 12577 / 16804.5 kHz | HF — DSC distress (5 bands) |
| 406 MHz | Satellite EPIRB (Cospas-Sarsat) |
| 9 GHz | Radar SART |
The mental rule for picking the medium
- 1A1 → start with VHF-DSC (channel 70).
- 2A2 → start with MF-DSC (2187.5 kHz).
- 3A3 → start with ship earth station (Inmarsat C) or HF-DSC.
- 4A4 → start with HF-DSC and 406 MHz EPIRB.
In any sea area, if the first medium fails, escalate to the next. The EPIRB is always the final safety net: it activates as the vessel goes down.
Sea area definitions in the 2024 edition
Section 2.4 defines the four GMDSS sea areas:
| Area | Definition |
|---|---|
| A1 | Coverage of at least one VHF coast station with DSC alerting |
| A2 | MF-DSC coverage (~100-150 NM) excluding A1 |
| A3 | Coverage of a recognized mobile-satellite service with continuous alerting |
| A4 | The rest (polar and anywhere outside A1/A2/A3) |
Boundaries are fixed by the coastal administration and published in ITU List IV.
What changed on 1 January 2024 (2.4.3)
Before 2024, A3 was identified with the Inmarsat geostationary footprint. After the SOLAS amendment entered into force that day, A3/A4 are defined ship-by-ship from the installed ship earth station:
- No SES: the ship is in A4 as soon as it leaves A1/A2.
- Iridium SES (global coverage): may have no A4 at all.
- Inmarsat-C: keeps the classic A3/A4 pattern.
- Inmarsat Fleet Safety or regional service: A3 limited to that footprint, A4 beyond it.
Full distress and safety frequency table
| Medium | DSC | Associated voice | NBDP |
|---|---|---|---|
| VHF | ch 70 · 156.525 MHz | ch 16 · 156.800 MHz | — |
| MF | 2187.5 kHz | 2182 kHz | 2174.5 kHz |
| HF 4 MHz | 4207.5 kHz | 4125 kHz | 4177.5 kHz |
| HF 6 MHz | 6312 kHz | 6215 kHz | 6268 kHz |
| HF 8 MHz | 8414.5 kHz | 8291 kHz | 8376.5 kHz |
| HF 12 MHz | 12577 kHz | 12290 kHz | 12520 kHz |
| HF 16 MHz | 16804.5 kHz | 16420 kHz | 16695 kHz |
Complementary frequencies:
- NAVTEX: 518 kHz international, 490 kHz and 4209.5 kHz national.
- 406 EPIRB: 406.0–406.1 MHz (digital burst) + 121.5 MHz (homing).
- AIS-SART: 161.975 MHz (AIS ch 1) and 162.025 MHz (AIS ch 2).
- SAR aircraft: 121.5 MHz AM (aeronautical emergency / homing) and 123.1 MHz AM (on-scene aircraft-to-ship).
Carriage by area (Part 6, Table 3)
| Area | Adds to A1 | Common to all areas |
|---|---|---|
| A1 | VHF-DSC | 406 EPIRB, SART or AIS-SART, 3 portable VHFs (≥500 GT), MSI receiver |
| A2 | MF-DSC | (same as A1) |
| A3 | MF/HF-DSC or recognized RMSS (Inmarsat-C / Iridium) + EGC | (same as A1) |
| A4 | MF/HF-DSC mandatory (RMSS alone is not enough, polar coverage) | (same as A1) |
Every SOLAS ship must also carry a second independent alerting means, reserve power for 1 h with an emergency generator or 6 h without, and two of the three maintenance methods in A3/A4.
Simplex, duplex and HF bands by time of day
- Simplex: one frequency, both directions alternating (ch 16, 2182 kHz).
- Duplex: two frequencies simultaneously (many port VHF channels, ch 23–28, 84–87).
- HF propagation: depends on the MUF (maximum usable frequency, rises by day with solar ionisation) and the LUF (lowest usable frequency, rises with daytime D-layer absorption).
| HF band | Best use |
|---|---|
| 4 / 6 MHz | Night, medium distance (800–1500 NM) |
| 8 MHz | Day/night transition, intermediate distance |
| 12 / 16 MHz | Day, long distance (2000+ NM) |
If f > MUF the signal punches through the ionosphere and is lost; if f < LUF it is absorbed. Picking the wrong band is equivalent to not transmitting.
MMSI: format and prefixes (Art. 19 RR, ITU-R M.585)
Nine-digit number whose first three digits are the MID (Maritime Identification Digits) identifying the responsible administration.
| Prefix | Meaning |
|---|---|
| MIDxxxxxx | Ship (6 digits after the MID) |
| 0MIDxxxxx | Coast station (single leading 0) |
| 00MIDxxxx | Group of coast stations |
| 111MIDxxx | SAR aircraft |
| 970xxxxxx | AIS-SART |
| 972xxxxxx | MOB device |
| 974xxxxxx | EPIRB with AIS |
| 99MIDxxxx | Aid to navigation (AtoN) |
Range estimation
- VHF (line of sight): d(NM) ≈ 2.2·(√h_tx + √h_rx) with heights in metres. Example: 15 m and 4 m antennas → ~8.5 + 4.4 ≈ 13 NM.
- MF (groundwave): ~150–200 NM by day, 300+ NM at night.
- HF (skywave): subject to MUF/LUF (see band table).
Teaching rule: trusting VHF beyond 25 NM between typical antennas is unrealistic.
Decision tree: which medium to use
Medium selection is not rote; it is worked step by step before pressing anything:
(1) Classify traffic priority: distress, urgency, safety or routine (ITU RR Articles 32-33). Priority drives frequency and procedure.
(2) Identify the current sea area by verifying actual coast-station or satellite-service coverage in the GMDSS Master Plan (GISIS), not from static maps that may be out of date.
(3) Check operational status of available gear: VHF-DSC, MF/HF-DSC, Inmarsat-C, Iridium SafetyCast, EPIRB. A set not exercised during the last watch does not count as available.
(4) Primary medium per matrix: A1 → VHF-DSC; A2 → MF-DSC; A3 → recognized RMSS or MF/HF-DSC per carriage; A4 → multi-band MF/HF-DSC mandatory (RMSS alone does not cover).
(5) Independent secondary medium: two separate and independent means (SOLAS IV/15). In A1 with dual coverage, MF-DSC may be backup; in A2, HF-DSC or RMSS; in A3, HF-DSC complements RMSS; in A4, EPIRB + multi-band HF.
(6) If the primary fails to acknowledge inside the expected window (1-2.75 min on MF/HF-DSC, 5-15 s on VHF-DSC), switch to the secondary — do not retry the same medium indefinitely: propagation or coverage will not improve by repetition.
STCW Bridge Watch Lens
Decide applicability before manoeuvring: Rules 4-10 apply in any visibility, Rules 11-18 only when vessels are in sight, and Rule 19 governs radar-only encounters in restricted visibility.
Build the traffic picture with sight, hearing, radar/ARPA and chart context. Do not let AIS or one isolated bearing replace systematic observation.
After manoeuvring, keep monitoring bearing, range, CPA/TCPA and passing distance until the other vessel is finally past and clear.
Exam Focus
Start every scenario by classifying the encounter: overtaking, head-on, crossing, narrow channel, traffic separation, or restricted visibility.
If two rules seem to conflict, check the order carefully: overtaking duties still apply, and Rule 2 still requires ordinary seamanship.
Key Takeaways
The sea area drives the mandatory equipment fit on board.
VHF = line of sight; MF = ground-wave; HF = ionospheric refraction.
VHF channel 70 is DSC only, not voice. Channel 16 is voice.
In A3 use the installed recognized satellite service; in A4 the main fallback is HF and EPIRB.
A1 is VHF-DSC range; A2 is MF-DSC range; A3 depends on installed RMSS; A4 is the remainder.
Since 1 January 2024 A3/A4 are defined ship-by-ship by satellite fit, not by a fixed map.
DSC VHF ch 70, MF 2187.5 kHz and HF 4207.5–16804.5 kHz are the six DSC distress frequencies.
Part 6 Table 3 turns each sea area into specific mandatory carriage items.
Simplex = one frequency alternating; duplex = two frequencies simultaneously.
HF at night favours 4/6 MHz; by day and over long distance, 12/16 MHz; 8 MHz is intermediate.
MMSI prefixes: 0 coast, 00 coast group, 111 SAR aircraft, 970 AIS-SART, 972 MOB, 974 EPIRB-AIS, 99 AtoN (M.585).
Aeronautical SAR: 121.5 MHz AM (emergency/homing) and 123.1 MHz AM (on-scene); AIS-SART on 161.975 / 162.025 MHz.
VHF range: d(NM) ≈ 2.2·(√h_tx + √h_rx) in metres; MF groundwave 150-200 NM; HF depends on MUF/LUF.
A3 needs MF/HF-DSC or a recognized RMSS; A4 always requires MF/HF-DSC because of polar coverage.
Common Mistakes
Selecting VHF when the vessel is 100 miles offshore in open waters.
Using HF in daytime on a closed band without checking alternatives.
Confusing RX and TX frequencies on a duplex channel.
Defining A3 from the old Inmarsat footprint without checking the vessel's installed satellite fit.
Assuming A4 means only 'polar regions' when it also appears anywhere RMSS coverage is lost.
Confusing DSC frequencies with the associated voice frequencies (ch 70 vs ch 16; 2187.5 vs 2182 kHz).
Calling on 12 MHz to a station 200 NM away at night: the band is closed; 4 or 6 MHz is the right choice.
Reading a 970xxxxxx MMSI on AIS as a normal ship instead of recognizing it as an AIS-SART.
Ignoring 121.5 / 123.1 MHz when planning comms with a SAR aircraft on scene.
Trusting VHF beyond 20-25 NM between typical antenna heights: the 2.2·(√h) rule disagrees.
Assuming an RMSS alone satisfies A4: SOLAS requires MF/HF-DSC in A4 for polar coverage.
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