- 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 |
Check this section
Which is the definition of sea area A2?
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.
Check this section
A ship in A3 relies on MF/HF. Which DSC watch must it keep on MF/HF?
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 |
Check this section
Roughly what is the wavelength of the 8291 kHz HF voice frequency?
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.
Check this section
Which ionospheric layer absorbs the lower HF frequencies strongly by day and largely disappears at night?
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 |
Check this section
What does the HF radiotelephone duplex channel number 822 stand for?
GOC case
Your ship is 400 NM offshore at 1400 local time, outside VHF and MF coast coverage, and needs a voice link with a coast station about 1,200 NM away. The MF/HF installation is working. Which band is the sensible first try?