Distress and safety frequencies

Under SOLAS Chapter IV every passenger ship and every cargo ship of 300 GT or more on international voyages must carry a Global Maritime Distress and Safety System (GMDSS) installation. The carriage requirement depends on the sea area in which the vessel operates.

GMDSS sea areas

Area A1 — within VHF DSC range of at least one shore station providing continuous DSC alerting (typically ~20–30 nm). Minimum kit: VHF + DSC.
Area A2 — within MF DSC range (typically ~100–150 nm), excluding A1. Adds MF DSC and radiotelephony on 2182 kHz / 2187.5 kHz.
Area A3 — within Inmarsat geostationary satellite coverage (roughly 76°N to 76°S), excluding A1 and A2. Adds Inmarsat-C SafetyNET, or HF DSC.
Area A4 — the remaining polar sea areas outside Inmarsat geostationary coverage. HF DSC is mandatory.

VHF marine band

Channel	MHz	Use
16	156.800	Distress, urgency, safety, calling. Continuous listening watch required by SOLAS.
70	156.525	DSC distress, urgency, safety, calling. Voice strictly not permitted.
13	156.650	Bridge-to-bridge / navigation safety. Mandatory in many port approaches.
06	156.300	Intership safety, SAR coordination, AIS-SART test.
67	156.375	Intership safety, SAR coordination (UK / many regions).
15	156.750	Onboard communications (1 W limit).
17	156.850	Onboard communications (1 W limit).
2087B	161.975	AIS 1.
2088B	162.025	AIS 2.

MF distress & safety

2182 kHz — MF radiotelephony distress, urgency, safety. Two 3-minute silence periods every hour (H+00 and H+30) for monitoring weak distress traffic.
2187.5 kHz — MF DSC distress, urgency, safety, calling.
490 kHz — NAVTEX international (English) primary.
518 kHz — NAVTEX international (English) primary global frequency.
4209.5 kHz — NAVTEX HF (tropical / equatorial use).

HF distress & safety

Band	DSC (kHz)	Radiotelephony (kHz)
4 MHz	4207.5	4125
6 MHz	6312	6215
8 MHz	8414.5	8291
12 MHz	12577	12290
16 MHz	16804.5	16420

EPIRB, SART, AIS-SART

406.0–406.1 MHz — Cospas-Sarsat EPIRB. Digital identification + GNSS position relayed via LEOSAR/GEOSAR/MEOSAR satellites.
121.5 MHz — EPIRB homing beacon (analogue AM). For SAR aircraft direction-finding once on scene.
9200–9500 MHz — SART (Search and Rescue Transponder), X-band. Shows a line of 12 dots on nearby shipboard radar.
161.975 / 162.025 MHz — AIS-SART. Transmits position via AIS for plotting on ECDIS / shipboard AIS.
156.525 MHz — Man Overboard (MOB) DSC alerts — same channel as DSC ch 70.

Voice procedure on channel 16

For a distress call: depress the DSC distress button first (continuous press ≥5 s), then transmit the spoken Mayday on VHF 16 or MF 2182 kHz. Speak slowly. Stations not involved must remain silent during distress traffic and shall not break the silence except to assist or to hand over. The on-scene coordinator may impose silence with "SEELONCE MAYDAY" (or, by another station, "SEELONCE DISTRESS") and lift it with "SEELONCE FEENEE".