4.4 VHF Channel Selection & Channel Assignments

4.4 VHF Channel Selection & Channel Assignments

Quick Answer: Ch-16 — monitored at all times at sea. Ch-70 — digital selective calling only, never voice. Ch-13 — required bridge-to-bridge watch in most of the continental US. Ch-08 — routine ship-to-ship working traffic. Ch-28 — call to a shore telephone. 121.500 MHz, A3E — aircraft distress.

Section 4.1 covered what the knobs on a marine VHF do. Official sub-topic 1-C-14 (VHF Channel Selection) is about which channel, and it is almost entirely a recall exercise. Six pool items, six numbers. There is no reasoning shortcut—learn the table.

The core channel table

Channel 16 — the continuous watch

What channel must VHF-FM-equipped vessels monitor at all times when at sea? Channel 16. This is the single most repeated fact in Element 1 and it appears in the watchkeeping sub-topic as well. Channel 16 is simultaneously the distress channel, the safety channel, and the calling channel—which is exactly why traffic must move off it immediately once contact is established.

Channel 70 — DSC only

Which VHF channel is used only for digital selective calling? Channel 70. Note the word only. Channel 70 is not a voice channel that also carries DSC; it is a data channel reserved for DSC alerting and acknowledgement. Modern radios physically prevent voice transmission on 70, and a set that lets you key voice onto it is faulty or misconfigured.

The relationship between 16 and 70 is the heart of modern GMDSS VHF working:

1. DSC alert goes out on Ch-70 — digital, addressed, carries your MMSI and position
2. Voice follow-up happens on Ch-16 — the radio typically switches automatically after the alert

Channel 13 — bridge-to-bridge

Which channel is used for the required bridge-to-bridge watch? VHF-FM on Ch-13 in most areas of the continental United States. Section 3.1 covered the bridge-to-bridge rules themselves; here the point is just the number, and the qualifier "in most areas of the continental United States" matters because some regions designate an alternative. Channel 13 is a low-power, short-range navigational-safety channel—the channel two vessels use to agree a passing arrangement, not a general chat channel.

Recall from section 5.5 that Ch-13 is specifically not a required survival-craft portable VHF capability. The pool likes to place Ch-13 as a distractor in survival-craft questions precisely because candidates learn it as an important number.

Channels 06, 08 and 28 — the working channels

These three are the "routine traffic" answers:

• Ch-06 — intership safety communications
• Ch-08 — the pool's answer for routine ship-to-ship voice traffic
• Ch-28 — the pool's answer for placing a call to a shore telephone (public correspondence through a coast station)

Ch-28 is a duplex public-correspondence channel, which is where the USA–INT control (section 20.3) becomes relevant: that control changes some channels that are normally duplex into simplex, reflecting differences between the international and US domestic channel plans.

The aviation outlier: 121.500 MHz, A3E

One item in this maritime-flavoured sub-topic is about aircraft: the aircraft frequency and emission used for distress communications is 121.500 MHz, A3E.

Two halves, both testable:

That emission designator is the giveaway. Marine VHF is FM (G3E); aviation VHF is AM (A3E). Aviation kept AM partly because overlapping AM transmissions produce an audible heterodyne that alerts controllers to a blocked call, whereas FM capture effect would silently hide the weaker station. You will meet 121.5 MHz again in section 5.2 as a legacy EPIRB homing frequency alongside the 406 MHz satellite alert.

Drill list

Cover the right column and recite:

1. Monitored at all times at sea → 16
2. DSC only → 70
3. Required bridge-to-bridge watch → 13
4. Routine ship-to-ship voice → 08
5. Call to a shore telephone → 28
6. Aircraft distress → 121.500 MHz, A3E