GMDSS Radio Procedures for USCG Captain Exam | Complete Study Guide

Ch 16

Primary distress and calling channel

406 MHz

EPIRB COSPAS-SARSAT frequency

4 Areas

GMDSS sea areas (A1 through A4)

9 digits

MMSI number length for DSC registration

What Is GMDSS?

The Global Maritime Distress and Safety System (GMDSS) is an internationally agreed set of safety procedures, equipment, and communication protocols adopted by the International Maritime Organization (IMO) under SOLAS (Safety of Life at Sea). It replaced the traditional Morse code radio watch that ended in 1999 and mandates automated, multi-layer distress alerting.

GMDSS ensures that a vessel in distress can alert rescue authorities and nearby vessels through multiple redundant systems — satellite, MF, HF, and VHF — without relying on a dedicated radio operator. The system is built around four sea areas that define what equipment a vessel must carry based on its operating range.

For the USCG captain's exam, you need to understand the four sea areas, the equipment associated with each, how DSC works, the correct distress call formats, and the radio watch obligations that apply to different vessel classes.

GMDSS Functional Requirements

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Transmit ship-to-shore distress alerts

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Receive shore-to-ship distress alerts

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Transmit and receive ship-to-ship distress alerts

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Transmit and receive SAR coordinating communications

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Transmit and receive on-scene communications

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Transmit and receive signals for locating (homing)

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Transmit and receive maritime safety information (MSI)

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Transmit and receive general radiocommunications

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Transmit and receive bridge-to-bridge communications

GMDSS Sea Areas: A1, A2, A3, and A4

Sea areas determine what communications equipment a vessel must carry. Know the coverage, range, and equipment requirements for each area cold — they appear on virtually every captain's license exam.

Area	Coverage	Approx. Range	Required Equipment
A1	Within VHF DSC range of a coast station	20-30 nm from shore	VHF radio with DSC (Class D)
A2	Within MF DSC range (excluding A1 areas)	100-150 nm offshore	MF/HF radio with DSC + VHF DSC
A3	Inmarsat satellite coverage (70N to 70S)	Most of the open ocean	Inmarsat terminal or MF/HF + VHF DSC
A4	Polar regions beyond Inmarsat coverage	Above 70N or below 70S latitude	HF radio with DSC + LEO satellite

Exam note: Area A1 equipment is the minimum for any GMDSS vessel. Moving from A1 to A2 adds MF radio with DSC. Moving to A3 adds Inmarsat or HF. A4 requires HF radio because Inmarsat satellites are geostationary and do not cover the poles.

U.S. Application: Most OUPV operators work in Area A1 (coastal). Master license holders working offshore may operate in A2 or A3. Understanding all four areas is tested on both OUPV and Master exams.

VHF Channel Reference

Know these channels. Channel 16, 22A, 13, and 70 appear on nearly every exam.

Channel	Frequency	Primary Use	Type
16	156.800 MHz	Distress, safety, and calling — mandatory watch	Critical
6	156.300 MHz	Inter-ship safety communications	Safety
9	156.450 MHz	Recreational calling channel (alternative to 16)	Calling
13	156.650 MHz	Bridge-to-bridge navigation (1 watt only)	Navigation
22A	157.100 MHz	U.S. Coast Guard liaison and working channel	USCG
67	156.375 MHz	U.S. inland waters bridge-to-bridge (certain areas)	Navigation
70	156.525 MHz	DSC only — NO voice transmissions permitted	DSC
WX1	162.550 MHz	NOAA weather broadcasts (receive only)	Weather
WX2	162.400 MHz	NOAA weather broadcasts (receive only)	Weather
WX3	162.475 MHz	NOAA weather broadcasts (receive only)	Weather

Channel 16 rule: Monitor Channel 16 at all times while the radio is on and you are underway. Leave Channel 16 only to communicate on a working channel, then return immediately. Channel 70 is DSC alerting only — no voice, ever.

Digital Selective Calling (DSC)

DSC is the digital distress alerting backbone of GMDSS. A Class D VHF radio (the minimum standard for recreational and many commercial vessels) transmits a digital distress alert on Channel 70 when the DSC distress button is pressed and held for 5 seconds. The transmission includes your MMSI number, GPS position (if a GPS is connected to the radio), UTC time, and the nature of the distress.

All coast stations and DSC-equipped vessels monitoring Channel 70 receive the alert simultaneously. After sending the DSC alert, the radio automatically switches to Channel 16 for voice communication. A DSC alert without a follow-up voice call on Channel 16 is incomplete — both elements are required.

Critical rule: Never transmit voice on Channel 70. It is reserved exclusively for DSC digital transmissions. Using Channel 70 for voice is an FCC violation and can block distress alerts from other vessels.

MMSI Number

A Maritime Mobile Service Identity (MMSI) is a unique 9-digit number assigned to your vessel's DSC radio. It must be programmed into the radio before use and registered so rescue authorities can identify your vessel.

▸9 digits — unique to each vessel
▸Free registration: BoatUS, Sea Tow, or FCC (Form 605)
▸Link to vessel name, type, length, color, and emergency contacts
▸U.S. vessel MMSIs begin with 338
▸Coast Guard MMSIs begin with 003
▸Update registration if vessel is sold or MMSI changes

DSC Call Types

Distress

Grave and imminent danger — highest priority, broadcasts to all stations

Urgency

Safety at risk but not immediately life-threatening

Safety

Navigation or weather hazard warning

Routine

Position report or calling a specific vessel by MMSI

MAYDAY Distress Call — Complete Script

Use MAYDAY only for grave and imminent danger to life or vessel. Transmit on Channel 16 (VHF) or 2182 kHz (MF/HF SSB). Say each element clearly and slowly.

1.MAYDAY MAYDAY MAYDAY

2.THIS IS [vessel name] [vessel name] [vessel name]

3.MAYDAY [vessel name]

4.[Position: latitude/longitude OR bearing and distance from landmark]

5.[Nature of distress: sinking, fire, medical, flooding, etc.]

6.[Number of persons on board]

7.[Other useful information: vessel type, color, taking on water, life raft deployed]

8.OVER

After Transmitting

Wait at least 2 minutes for a response. If no reply, repeat the full MAYDAY call. If still no response, try on 2182 kHz (MF/HF). Repeat as necessary until contact is made or you must abandon ship.

MAYDAY Relay

If you receive a MAYDAY from a vessel you cannot confirm the Coast Guard heard, relay it: "MAYDAY RELAY MAYDAY RELAY MAYDAY RELAY, ALL STATIONS, THIS IS [your vessel], RECEIVED THE FOLLOWING MAYDAY FROM [vessel in distress]..."

SEELONCE MAYDAY

When a MAYDAY is in progress, rescue authorities may transmit SEELONCE MAYDAY to silence all other traffic on the channel. Do not transmit anything until SEELONCE FEENEE (silence lifted) is announced.

PAN PAN Urgency Call

PAN PAN (pronounced pahn-pahn) is the urgency signal — second in priority only to MAYDAY. Use it when a person or vessel is in difficulty but not in immediate mortal danger: a medical situation, lost steering, engine failure in a strong current.

1.PAN PAN PAN PAN PAN PAN

2.ALL STATIONS ALL STATIONS ALL STATIONS (or specific station)

3.THIS IS [vessel name] [vessel name] [vessel name]

4.[Position]

5.[Nature of urgency: medical situation, loss of propulsion, etc.]

6.[Assistance required]

7.[Number of persons on board]

8.OVER

PAN PAN examples: Person overboard (when vessel can still maneuver — MAYDAY if life is immediately at risk), medical emergency requiring assistance, loss of steering, engine failure in a shipping lane, vessel taking on water but not yet in grave danger.

SECURITE Safety Call

SECURITE (pronounced say-cure-ee-tay) is the safety signal used for navigation hazard warnings and important meteorological warnings. Third in priority after MAYDAY and PAN PAN. Typically broadcast by the Coast Guard or port authority.

1.SECURITE SECURITE SECURITE

2.ALL STATIONS ALL STATIONS ALL STATIONS

3.THIS IS [Coast Guard or transmitting station]

4.[Message: derelict vessel, dangerous debris, weather warning, etc.]

5.OUT

Radio Priority Order

MAYDAY

Grave and imminent danger

PAN PAN

Urgency — safety at risk

SECURITE

Safety and nav hazards

Routine

All other communications

EPIRB — Emergency Position Indicating Radio Beacon

An EPIRB transmits a 406 MHz digital distress signal to the COSPAS-SARSAT satellite network, which relays the alert to a ground station and then to the nearest Coast Guard Rescue Coordination Center. Modern units include an integrated GPS receiver that encodes the vessel's position directly into the signal, reducing the search area to approximately 100 meters.

CAT I

Category I

Auto float-free via hydrostatic release
Also manually activatable
HRU expires every 2 years
Required for inspected vessels offshore

CAT II

Category II

Manual activation only
No hydrostatic release unit
Less expensive
Cannot substitute Cat I where required

Key frequencies: 406 MHz for satellite distress alerting (COSPAS-SARSAT), 121.5 MHz for homing (SAR aircraft use this to close in on position). Know both frequencies for the exam.

SART — Search and Rescue Transponder

A SART responds to X-band (9 GHz) radar pulses by transmitting a series of 12 equally-spaced dots on the rescuer's radar display, extending in a straight line toward the SART. As range closes, dots become arcs and then concentric circles.

▸Detection range: ~8 nm from ship, ~30-40 nm from aircraft
▸X-band (9 GHz) only — not S-band radar
▸Hold at least 1 meter above sea level for best range
▸12 dots on radar = SART is within range
▸96 hours standby battery, 8 hours active transmit
▸SOLAS: required in survival craft on applicable vessels

AIS-SART

An AIS-SART transmits a distress message on AIS frequencies (VHF Ch 87B and 88B), appearing as a distinctive AIS target with GPS-accurate position on any AIS-equipped chartplotter within VHF range. Does NOT appear on radar like a traditional SART. Range is approximately 20-30 nm (VHF radio range).

NAVTEX — Navigational Telex

NAVTEX is an automated system that broadcasts Maritime Safety Information (MSI) including navigational warnings, meteorological forecasts, ice reports, SAR information, and pilot messages. The receiver automatically captures broadcasts and displays or prints them without operator action.

NAVTEX Frequencies

518 kHzPrimary

International NAVTEX — English language broadcasts

490 kHz

National NAVTEX — local language broadcasts

4209.5 kHz

HF NAVTEX for long-range (SafetyNET supplement)

How NAVTEX Works

▸Each station identified by a single character (A-Z)
▸Each message type identified by a second character (A = navigational warnings, B = met, etc.)
▸Receiver can be programmed to reject distant stations or unwanted message types
▸Duplicate messages are automatically rejected by the receiver
▸Range: approximately 400 nm on 518 kHz
▸GMDSS requirement: vessels in A2 and beyond must carry NAVTEX or equivalent

SafetyNET (Inmarsat)

SafetyNET is the Inmarsat satellite equivalent of NAVTEX for Area A3 and A4 vessels. It broadcasts MSI from national authorities and the IMO via the Inmarsat C terminal, covering ocean areas beyond NAVTEX range.

Inmarsat & Satellite Communications

Inmarsat operates a fleet of geostationary satellites that provide reliable two-way voice, data, and distress communications for vessels in Areas A3. Because geostationary satellites orbit directly above the equator, they cannot provide coverage above approximately 70 to 76 degrees North or South latitude — which is why Area A4 (polar regions) requires HF radio instead.

Inmarsat Terminal Types

Inmarsat C

Data and telex only — no voice. Used for GMDSS distress alerting and SafetyNET. Most common GMDSS terminal.

Inmarsat B

Voice, fax, and data. Legacy high-gain system.

Inmarsat Fleet F

Modern broadband: FleetBroadband provides voice and high-speed data over smaller antenna.

Inmarsat Mini-C

Compact low-power version of Inmarsat C for smaller vessels.

Inmarsat Distress Alerting

An Inmarsat distress alert is triggered by pressing the red distress button on the terminal. The alert is sent to an LES (Land Earth Station) and forwarded to the nearest MRCC (Maritime Rescue Coordination Center). The vessel can then communicate by voice or data through the Inmarsat channel.

▸Priority: highest — distress calls preempt all other traffic
▸MMSI used for identification (same as DSC)
▸GPS position encoded in distress alert if integrated
▸Follow-up voice communication possible through satellite link

Coverage limit: Inmarsat geostationary satellites cannot reach polar latitudes above approximately 76 degrees North or South. Vessels operating beyond this require HF radio with DSC as their primary distress means.

SSB HF Radio for Offshore Communications

Single Sideband (SSB) High Frequency (HF) radio is the backbone of long-range communications for offshore vessels in Areas A2, A3, and A4. Unlike VHF, HF signals propagate via ionospheric skip, allowing communications over thousands of miles. Propagation varies by frequency, time of day, and solar activity.

HF Distress Frequencies

Band	Frequency	Primary Use
MF	2182 kHz	International distress and calling (voice)
MF DSC	2187.5 kHz	MF DSC distress alerting
4 MHz	4125 kHz	Distress and calling — daytime, shorter range
6 MHz	6215 kHz	Distress and calling
8 MHz	8291 kHz	Distress and calling — primary offshore band
12 MHz	12290 kHz	Distress and calling — long range daytime
16 MHz	16420 kHz	Distress and calling — very long range

HF Propagation Tips

▸Lower frequencies (4-6 MHz) work better at night and for shorter distances
▸Higher frequencies (12-16 MHz) work better during daylight for long range
▸8 MHz (8291 kHz) is often the best all-around offshore distress frequency
▸2182 kHz (MF) is the traditional distress frequency — USCG still monitors
▸MF DSC distress channel is 2187.5 kHz (not 2182 kHz)
▸The USCG Rescue 21 system monitors MF and VHF DSC 24/7
▸HF DSC distress frequencies parallel the voice distress frequencies

FCC Ship Station License

Operating an HF/SSB radio (as opposed to VHF alone) requires an FCC Ship Station License for the vessel. This is in addition to the operator's personal license (Restricted Radiotelephone Operator Permit).

Radio Watch Requirements

Vessel Class	Required Watch	Notes
All vessels with VHF radio	Channel 16 — whenever radio is on	Required under FCC rules and COLREGS. No exceptions while underway.
Vessels over 26 ft in certain waters	Channel 13 — bridge-to-bridge	Required under Bridge-to-Bridge Radio Act in regulated waters.
GMDSS vessels — compulsory	Channel 70 (DSC) + Channel 16 + MF DSC if A2+	Automated DSC watch maintained continuously. Manual Ch 16 watch also required.
Inspected vessels over 300 GT (international)	Continuous radio watch — GMDSS compliant	SOLAS Chapter IV requires full GMDSS installation and watch.
Recreational vessels (uninspected)	No mandatory watch — but must monitor Ch 16 if radio is on	VHF radio not federally required, but watch is required if equipped.
Commercial vessels under 100 GT carrying passengers	Channel 16 mandatory; GMDSS if applicable	COI may impose additional radio requirements based on route.

FCC Radio Licensing for Captains

REQUIRED

Restricted Radiotelephone Operator Permit (RR)

▸Required for any captain who operates a ship station radio
▸No examination required — pass FCC Form 605 application
▸One-time fee (currently around $35)
▸Lifetime validity — never expires
▸Authorizes operation of VHF marine radios on commercial vessels
▸Also required for HF/SSB radio operation
▸Carries legally even if vessel is in foreign waters

GROL

General Radiotelephone Operator License

The GROL is a higher-level FCC license required for those who maintain, repair, or adjust radio transmitters on ships or vessels. It is also required for operating certain radar equipment. Captains do not typically need a GROL unless they also maintain the radio equipment themselves — the RR is sufficient for radio operation.

Ship Station License

Required for the vessel (not the operator) when traveling to foreign ports, using MF/HF (SSB) radio, or for commercial operations. Filed by the vessel owner on FCC Form 605. 10-year renewable term. Recreational vessels in U.S. waters using only VHF are exempt from ship station licensing.

Marine Radio Operator Permit (MROP): The MROP is an older FCC license that preceded the RR. If you see MROP on an exam question, treat it as equivalent to the RR Permit — both authorize operation of marine radio on commercial vessels. The exam may use either term.

NATO Phonetic Alphabet

Used on all marine radio communications to spell out vessel names, positions, and critical information clearly. Memorize all 26 — position reporting and vessel name spelling appear on the exam.

Alpha

Bravo

Charlie

Delta

Echo

Foxtrot

Golf

Hotel

India

Juliet

Kilo

Lima

Mike

November

Oscar

Papa

Quebec

Romeo

Sierra

Tango

Uniform

Victor

Whiskey

X-ray

Yankee

Zulu

Numbers: Speak digits individually. "Position: NOVEMBER FOUR TWO, DECIMAL THREE FIVE NORTH, WHISKEY ZERO SEVEN THREE, DECIMAL TWO ZERO WEST." Say "NINER" (not nine) to distinguish from German "nein" on international channels.

Radio Terminology

Term	Meaning
OVER	Transmission complete — reply expected
OUT	Conversation complete — no reply expected (never say OVER AND OUT)
ROGER	Message received and understood
WILCO	Will comply (understood and will comply with instructions)
AFFIRMATIVE	Yes
NEGATIVE	No
SAY AGAIN	Please repeat your last transmission (never say REPEAT — military connotation)
STAND BY	Wait — I will call you back
BREAK	Separating sections of a long message
SEELONCE MAYDAY	Issued by rescue authority: silence all traffic except distress
SEELONCE FEENEE	Silence lifted — normal communications may resume
MAYDAY RELAY	Relaying a distress call on behalf of another vessel

High-Yield Exam Tips

Sea area A4 = HF only, no Inmarsat

Inmarsat geostationary satellites cannot cover polar regions (above ~70-76 degrees lat). A4 vessels must use HF radio and/or LEO satellite systems. This is a very common exam question.

Channel 70 = no voice, ever

Channel 70 is exclusively for DSC digital transmissions. No voice communications permitted under any circumstances. Using Ch 70 for voice is an FCC violation.

DSC alert is NOT the complete distress call

After pressing the DSC distress button, you MUST follow up with a voice MAYDAY on Channel 16. The DSC alert alone is not sufficient. The radio automatically switches to Ch 16 after sending the DSC alert.

MMSI must be registered before use

Programming an MMSI that is not registered does more harm than good — rescuers cannot identify the vessel. Register free through BoatUS, Sea Tow, or the FCC before your first offshore trip.

Captains need the Restricted Radiotelephone Operator Permit

A USCG captain's license does not automatically authorize radio operation. You need the FCC Restricted Radiotelephone Operator Permit (RR) separately — Form 605, no exam, about $35.

2182 kHz = MF voice distress; 2187.5 kHz = MF DSC distress

These are two different frequencies for two different types of distress calls. 2182 kHz is the traditional voice calling and distress frequency. 2187.5 kHz is the MF DSC digital alerting channel.

SART shows 12 dots on X-band radar

A SART transponder responding to radar interrogation appears as 12 equally-spaced dots on the radar display, extending in a line. As you close in, dots become arcs, then concentric circles around the SART.

SECURITE is broadcast, not sent to a specific vessel

SECURITE calls are transmitted to ALL STATIONS, not directed to a specific vessel. They warn the entire area of a navigational hazard or weather warning. You listen for SECURITE; you may transmit it if you spot a hazard others may not know about.

Frequently Asked Questions

What are the four GMDSS sea areas?

Area A1 is within range of at least one VHF coast station offering continuous DSC alerting, approximately 20-30 nautical miles from shore. Area A2 is within range of an MF coast station with DSC, roughly 100-150 nautical miles offshore. Area A3 covers latitudes between 70 degrees North and 70 degrees South served by Inmarsat geostationary satellites, encompassing most of the ocean. Area A4 is the polar regions beyond Inmarsat satellite coverage (above 70 degrees North or below 70 degrees South), where HF radio and LEO satellite systems are the only communications options.

What is the exact MAYDAY format for marine radio?

Transmit on Channel 16 (or 2182 kHz on MF/HF): (1) MAYDAY MAYDAY MAYDAY, (2) THIS IS [vessel name spoken three times], (3) MAYDAY [vessel name], (4) your position in latitude/longitude or bearing and distance from a known landmark, (5) nature of distress such as sinking, fire, or medical emergency, (6) number of persons on board, (7) any other useful information such as vessel description, color, or condition, (8) OVER. Wait for acknowledgment. If none received within a reasonable time, repeat the call. MAYDAY is reserved for grave and imminent danger to life or vessel only.

What is the difference between MAYDAY, PAN PAN, and SECURITE?

MAYDAY (from French m'aider, help me) indicates grave and imminent danger to life or vessel — the highest priority distress signal. PAN PAN (pronounced pahn-pahn) is the urgency signal, used when the safety of the vessel or a person is at risk but the situation is not immediately life-threatening, such as a medical situation that needs assistance or a vessel that has lost steering but is not sinking. SECURITE (pronounced say-cure-ee-tay) is the safety signal used for navigation hazard warnings, typically broadcast by the Coast Guard to announce dangerous debris, derelict vessels, or weather warnings. The order of priority on radio is: Distress (MAYDAY) over Urgency (PAN PAN) over Safety (SECURITE) over all other traffic.

What is DSC and what channel does it use?

Digital Selective Calling (DSC) is a digital protocol built into Class D and higher VHF radios that allows automatic distress alerting. Channel 70 on VHF is used exclusively for DSC — no voice transmissions are permitted on Channel 70. When you press and hold the DSC distress button for 5 seconds, the radio automatically transmits your MMSI number, GPS position (if a GPS is connected), time, and nature of distress to all DSC-equipped radios and coast stations monitoring Channel 70. After sending the DSC alert, the radio switches to Channel 16 for voice communication. An MMSI (Maritime Mobile Service Identity) is a 9-digit number that must be registered before use.

Do I need an FCC license to operate a marine radio as a captain?

Yes. As a licensed captain operating a commercial vessel, you are required to hold a Restricted Radiotelephone Operator Permit (RR) issued by the FCC to legally operate the ship station radio. This requires filing FCC Form 605, paying a one-time fee, and there is no exam required. The vessel itself also needs a Ship Station License if it travels to foreign ports, uses MF/HF (SSB) radio, or is a commercial vessel. Recreational vessels operating exclusively in U.S. waters on VHF radio are generally exempt from the ship station license, but the moment you carry passengers for hire, the recreational exemption does not apply.

What is NAVTEX and how is it used?

NAVTEX is an automated system for broadcasting maritime safety information (MSI) including navigational warnings, meteorological forecasts, ice reports, search and rescue information, and pilot messages. NAVTEX receivers automatically receive and print or display broadcasts on 518 kHz (international, in English) and 490 kHz (national, in local language). Each transmitting station is identified by a single letter code, and each message type is identified by another letter code. Mariners can program their NAVTEX receiver to reject messages from distant stations or message types they do not want, reducing clutter. NAVTEX is part of the GMDSS communications suite required on vessels operating in A2 and beyond.

What SSB HF radio frequencies are used for offshore distress communications?

The international distress frequency on MF (Medium Frequency) is 2182 kHz, which is the equivalent of Channel 16 for single sideband radio. For HF (High Frequency) SSB, the primary working and distress frequencies by band are: 4 MHz band at 4125 kHz, 6 MHz band at 6215 kHz, 8 MHz band at 8291 kHz, 12 MHz band at 12290 kHz, and 16 MHz band at 16420 kHz. The USCG maintains a radio watch on 2182 kHz as well as these HF distress frequencies. Propagation characteristics vary by time of day and season, so offshore mariners typically scan multiple bands. In GMDSS systems, MF DSC is conducted on 2187.5 kHz, which is the DSC equivalent of 2182 kHz.

What is the radio watch requirement for different vessel classes?

All vessels equipped with VHF radio must maintain a watch on Channel 16 whenever the radio is on, except when communicating on a working channel. Compulsory ship station vessels (generally those over 300 gross tons on international voyages or carrying more than 12 passengers on international voyages) must maintain continuous radio watch. Under GMDSS, continuous watch is maintained automatically by DSC on Channel 70 and 2187.5 kHz, supplementing the voice watch on Channel 16. Recreational vessels are not federally required to carry VHF radio but if equipped, must monitor Channel 16 when underway. Bridge-to-bridge communications on vessels over 26 feet operating in certain waters require monitoring Channel 13.

Related Study Guides

Captain License Hub

Full overview of USCG licensing requirements, exam topics, and study resources.

EPIRB Guide

Category I vs II, NOAA registration, hydrostatic release, SART, and PLB comparison.

VHF Marine Radio Guide

Channel assignments, calling procedure, MAYDAY protocol, DSC, and FCC licensing.

Practice GMDSS & Radio Questions

Test your GMDSS knowledge with real USCG-style exam questions — DSC procedures, sea area identification, MAYDAY format, and radio licensing. Instant feedback and AI explanations on every answer. Free on NailTheTest.

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