Emergency Signaling Devices for the USCG Captain's License Exam

The Global Maritime Distress and Safety System (GMDSS) is an internationally standardized framework adopted by the International Maritime Organization (IMO) under SOLAS (Safety of Life at Sea). GMDSS replaced the old Morse code watch requirement in 1999 and mandates that vessels carry specific communications and distress equipment based on their geographic operating area, called a sea area.

The USCG requires compliance with GMDSS on all SOLAS vessels and sets parallel requirements for domestically inspected commercial vessels. Even if your vessel is not subject to GMDSS as a whole, the individual device requirements (EPIRBs, VHF DSC radios, visual distress signals) apply separately under Title 33 and Title 46 CFR.

The core philosophy of GMDSS is redundancy: a vessel in distress should be able to alert rescuers through at least two independent means. For a typical coastwise operator, that means a 406 MHz EPIRB (satellite path) plus a VHF DSC radio (radio path). For offshore voyages, additional MF/HF DSC or satellite communications are required.

An EPIRB (Emergency Position Indicating Radio Beacon) is a vessel-mounted distress transmitter. When activated, it broadcasts a unique 406 MHz digital signal encoded with a 15-hexadecimal-digit identification number to the Cospas-Sarsat satellite constellation. The signal is relayed through a Local User Terminal (LUT) to a Mission Control Center (MCC), then forwarded to the Rescue Coordination Center (RCC) — typically the U.S. Coast Guard — which dispatches SAR assets.

All current EPIRBs also transmit on 121.5 MHz, a homing frequency used by SAR aircraft and vessels once they are within line-of-sight range. Most modern units include an integrated GPS receiver that encodes the vessel's position directly into the 406 MHz message, reducing the search circle to approximately 100 meters (vs. several kilometers without GPS).

Cospas-Sarsat is an international satellite-based SAR system operated jointly by Canada, France, Russia, and the United States. It consists of instruments aboard LEO (low-earth orbit), MEO (medium-earth orbit), and GEO (geostationary) satellites that detect and process distress signals from 406 MHz beacons.

LEO satellites (altitude ~800 km) provide the most accurate position through Doppler processing, but a satellite may take up to 90 minutes to pass over the distress position. GEO satellites detect alerts almost instantly but cannot determine position via Doppler — they rely entirely on GPS data embedded in the beacon signal. MEO satellites (part of GPS and GLONASS constellations) combine near-instant detection with good Doppler positioning.

The chain: EPIRB activates → satellite receives 406 MHz signal → Local User Terminal (LUT) on the ground processes the signal → Mission Control Center (MCC) identifies the beacon's 15-digit hex ID and looks up the NOAA registration database → alert forwarded to Rescue Coordination Center (RCC) → USCG/Coast Guard SAR response dispatched.

Every 406 MHz EPIRB has a unique 15-hexadecimal-digit identification number (the hex ID) programmed at the factory. Federal regulations require that all 406 MHz beacons used in U.S. waters be registered with NOAA's Beacon Registration Database at beaconregistration.noaa.gov. Registration is free and takes approximately five minutes.

When your beacon activates, the MCC uses the hex ID to retrieve your registration record, which includes: vessel name, vessel type and description, home port, owner name and phone, emergency contacts, and up to three alternative contact numbers. The Coast Guard calls these contacts to (1) confirm whether a genuine distress exists, and (2) get information about the vessel and persons on board before dispatching assets.

An unregistered beacon still triggers a SAR response, but rescuers arrive with no information about who is in distress, how many persons are aboard, or who to notify. This significantly complicates the response. Failure to register is a civil violation. Failure to update a registration (e.g., new vessel, new phone number) can be equally dangerous.

The HRU is a small mechanical or hydraulic device that holds the EPIRB in its bracket under normal conditions but releases it when submerged to a depth of approximately 1–4 meters. As the bracket releases, the EPIRB floats upward (it is positively buoyant), exits the bracket, and automatically activates. This sequence is why a vessel can sink with the crew below decks — or the captain incapacitated — and the EPIRB will still deploy and transmit.

HRU expiration is separate from battery expiration.The HRU has its own stamped expiration date — typically every two years. A USCG inspector will check both the battery label and the HRU label. Both must be current for the EPIRB to be in compliance. This is a high-frequency exam question.

All 406 MHz EPIRBs have a self-test function activated by pressing and holding a dedicated test button. During the self-test, the beacon transmits a brief encoded test signal that Cospas-Sarsat satellites are programmed to recognize as a test — not a real distress. The beacon's indicator LED and sometimes an audible beep confirm the unit is functional.

The Cospas-Sarsat test recognition window is the first five minutes of any UTC hour(i.e., 00:00–00:04 UTC, 01:00–01:04 UTC, etc.). Self-tests performed outside this window may be processed as real distress alerts. Some newer EPIRBs suppress the 406 MHz transmission entirely during self-test and only test the internal electronics — check your owner's manual. Never test by fully activating the beacon outside of authorized test conditions.

A Personal Locator Beacon (PLB) uses the same 406 MHz Cospas-Sarsat system as an EPIRB but is designed for individual persons rather than vessels. PLBs are compact — many fit in a shirt pocket or clip to a life jacket — and must be manually activated. They have no float-free capability. Like EPIRBs, PLBs must be registered with NOAA.

Key differences between PLBs and EPIRBs:

• PLBs require a minimum battery life of 24 hours; EPIRBs require a minimum of 48 hours.
• PLBs are personal — they are registered to an individual, not a vessel. A PLB is not a substitute for a vessel-mounted EPIRB when carriage is required.
• PLBs are commonly used by fishermen, offshore racers, and commercial mariners as a personal backup device worn on the body, so the beacon stays with the person if they go overboard.
• PLBs must be user-serviceable or sent to the manufacturer for battery replacement. Many models require factory service; check the label.
• A PLB worn clipped to a life jacket is one of the most effective ways to ensure that a man-overboard victim can be located even if they are swept away before being spotted.

On the USCG exam, the PLB-vs-EPIRB distinction most frequently tested is: (1) PLBs are personal / EPIRBs are vessel-mounted; (2) PLBs have 24-hour minimum battery life / EPIRBs have 48-hour minimum; (3) a PLB does not satisfy vessel carriage requirements.

A Search and Rescue Transponder (SART) is a handheld electronic device designed for use in survival craft after abandoning ship. A radar SART operates in the X-band (9 GHz) radar frequency range. When interrogated by a ship's or aircraft's X-band radar pulse, the SART automatically responds by transmitting a series of 12 sweeps that appear on the interrogating radar display as a line of 12 dots extending approximately 8 nautical miles radially toward the SART.

The innermost dot is closest to the SART's actual position. As the rescuing vessel approaches, the line of dots begins to arc into concentric circles (because the Doppler shift changes with close proximity), indicating that the SART is very close — typically within 1 nm. SARTs must be held as high as possible (above the survival craft or on a mast) to maximize radar line-of-sight range.

Detection ranges:

• From a ship's radar (at 15 m antenna height): approximately 8 nautical miles
• From an aircraft radar (at 1,000 feet altitude): approximately 30+ nautical miles

SOLAS vessels are required to carry at least one SART (or AIS SART) per survival craft. Radar SARTs have a standby life of approximately 96 hours and a minimum active transmission life of 8 hours.

An AIS SART replaces the radar response mechanism with an Automatic Identification System (AIS) transmission. When activated, the AIS SART broadcasts a distress message on AIS channels 1 (161.975 MHz) and 2 (162.025 MHz), with an MMSI number that always begins with 970— a reserved prefix that all AIS software recognizes as a SART distress target.

The AIS SART includes an integrated GPS, so it transmits an accurate position that appears directly on the rescuing vessel's electronic chart system or chartplotter as a distress symbol. Any vessel with an AIS receiver within approximately 5–10 nautical miles (VHF line-of-sight) can detect and display the AIS SART — they do not need to be actively radar-scanning.

Advantages of AIS SART over radar SART:

• Works on all vessels with AIS, not just those actively radar-scanning
• GPS-accurate position displayed directly on the chart
• Vessel name and MMSI identify the survivor
• Not affected by S-band (3 GHz) radar — compatible with most navigation radars

Disadvantage: aircraft typically do not carry AIS receivers, so the radar SART may be more effective for air SAR operations. Some survival equipment carries both types.

Digital Selective Calling (DSC) is a digital protocol built into modern VHF (and MF/HF) marine radios that allows a vessel to send automated distress, urgency, safety, or routine calls to all DSC-equipped vessels and coast stations simultaneously — without voice communication. DSC is a required component of GMDSS.

VHF DSC operates exclusively on Channel 70 (156.525 MHz). No voice communications are ever permitted on Channel 70. All DSC-equipped VHF radios continuously monitor Channel 70 in the background, even when set to another working channel.

Every DSC radio must be programmed with the vessel's Maritime Mobile Service Identity (MMSI)— a unique 9-digit number assigned to the vessel. The MMSI is transmitted with every DSC call, allowing all receiving stations to identify the vessel. MMSIs can be obtained free from Sea Tow, Boat U.S., or the FCC (for licensed stations).

When a vessel is in distress, the DSC distress alert is transmitted by pressing and holding the dedicated red distress button (often protected by a cover) for approximately 5 seconds. The radio automatically transmits the following data on Channel 70:

• Nature of distress (can be selected: undesignated, fire, flooding, collision, grounding, listing, sinking, disabled, abandoning ship)
• Vessel MMSI number
• UTC time of activation
• Vessel position (if an external GPS is connected to the radio)

After the DSC alert is transmitted, the radio automatically switches to Channel 16 for voice communications. The captain then broadcasts the MAYDAY message on Channel 16 to provide additional details to responding vessels and the Coast Guard.

If a DSC-equipped radio receives a distress alert from another vessel, it sounds a visual and audible alarm and displays the distressed vessel's MMSI and position. The receiving vessel must acknowledge the alert on Channel 70 (DSC acknowledgment) and then switch to Channel 16 to render or coordinate assistance.

MMSI numbers follow an internationally standardized format:

• MID 338 identifies United States vessels
• MMSIs beginning with 970 are reserved for AIS SARTs (survival craft transponders)
• MMSIs beginning with 972 are for EPIRBs
• MMSIs beginning with 974 are for AIS EPIRBs
• MMSIs beginning with 00 are reserved for coast stations and groups

Recreational vessels may obtain an MMSI from the FCC (requires a ship station license), Sea Tow, or Boat U.S. (for U.S.-only operations). Commercial vessels must obtain their MMSI through the FCC ship station license process.

If you receive a MAYDAY from a vessel in distress and the Coast Guard does not respond within a reasonable time, you are obligated to relay the MAYDAY on behalf of the distressed vessel. The relay format is:

You must also continue listening on Channel 16 and provide whatever assistance you can — including proceeding to the distress position if safely possible. Every vessel has a legal duty to render assistance to persons in distress under maritime law (46 U.S.C. 2304).

Visual Distress Signals (VDS) are required by 33 CFR Part 175 for recreational vessels 16 feet and over operating on coastal waters, the Great Lakes, territorial seas, and those waters connected to them up to a point where a body of water is less than two miles wide. VDS are separate from the EPIRB and radio requirements — they are visual signals intended to attract the attention of nearby vessels and aircraft.

Vessels must carry an approved combination that satisfies both daytime and nighttime requirements. The minimum combination is at least three pyrotechnic devices that are Coast Guard-approved for both day and night use — such as three Type B aerial flares or three Type C handheld flares with both day and night approval.

Pyrotechnic flares have an expiration date stamped on the casing. USCG regulations require that flares not be past their expiration date to count as approved VDS. The typical service life of a pyrotechnic flare is 42 months (3.5 years) from the manufacture date. The expiration date is printed as month/year.

You may carry expired flares as supplemental equipment, but they cannot count toward your required three approved VDS. The practical recommendation: replace your flares every three years, and keep the old (not yet expired) set as a backup until they expire.

Disposal of expired pyrotechnic flares is regulated — they cannot simply be thrown in the trash. Options include local USCG auxiliary flare exchange programs, hazardous waste disposal facilities, or contacting local fire departments that sometimes accept expired marine flares for training.

The international distress signal SOS(• • • — — — • • •) was adopted in 1906 not because it stands for any specific phrase, but because it is a simple 9-element Morse sequence that is easy to transmit and recognize. While modern GMDSS has largely replaced Morse code requirements for commercial mariners, SOS can be transmitted by any means: light (flashlight, mirror), sound (horn), radio (CW transmitter), or even tapping on a hull.

On the USCG exam, SOS knowledge appears in questions about the electric SOS distress light, about mirror signaling, and about the historical context of distress signals. The SOS pattern:

Other internationally recognized distress signals (per COLREGS Annex IV) include:

• A gun or explosive signal fired at intervals of approximately 1 minute
• Continuous sounding of any fog-signaling apparatus
• Rockets or shells throwing red stars fired one at a time at short intervals
• Flames on the vessel (burning tar barrel or oil barrel)
• A rocket parachute flare or a hand flare showing red light
• An orange smoke signal
• Slowly and repeatedly raising and lowering arms outstretched to each side
• Radiotelegraph alarm signal (12 four-second dashes on 500 kHz)
• EPIRB/SART transmissions
• Approved signals transmitted by radio communication systems (DSC, MAYDAY)
• A square flag having above or below it a ball (or anything resembling a ball)
• A dye marker on the water

The square flag + ball combination is a classic exam question. An inverted national ensign (flying the flag upside down) is a recognized distress signal in some jurisdictions but is not in the official COLREGS list — know that the flag + ball signal is the official COLREGS non-pyrotechnic daytime signal.

Understanding the full chain from EPIRB activation to SAR dispatch is tested on the exam and is essential operational knowledge for any captain operating beyond coastal waters.

False distress alerts are a serious problem in maritime safety. The National Oceanic and Atmospheric Administration (NOAA) and U.S. Coast Guard estimate that the vast majority of EPIRB and DSC alerts are accidental. Each false alert diverts SAR assets, endangers rescue crews, and delays responses to genuine emergencies.

Legal Penalties

• Federal criminal penalty: Knowingly and willfully transmitting a false distress signal is a federal crime under 18 U.S.C. 1038 — punishable by up to 6 years imprisonment and fines up to $250,000.
• Civil penalty: The USCG may assess civil penalties and seek reimbursement for SAR costs expended in response to a false alert under 14 U.S.C. 521.
• Repeat offenders: Vessels with multiple false alerts may face suspension of vessel documentation, MMSI revocation, or other administrative action.

Cancellation Procedure — EPIRB Accidental Activation

1. Immediately turn off the EPIRB using the OFF/ARM switch.
2. Contact the nearest Coast Guard Sector or Group by phone as soon as possible. The USCG maintains a 24-hour watchstander who can cancel the alert in the MCC system before SAR assets are dispatched.
3. Alternatively, contact the Coast Guard on VHF Channel 16 and report the false activation immediately.
4. Provide: vessel name, MMSI, EPIRB registration hex ID, vessel location, and nature of the false activation.
5. Do not simply turn the EPIRB off and hope the alert was not received. The satellite system processes activations within minutes — if you do not cancel promptly, a SAR response will already be launched.

Cancellation Procedure — DSC Accidental Activation

1. Modern DSC radios have a cancellation timer — the distress button must be held for approximately 5 seconds before transmitting. If you release the button before the countdown completes, the alert is cancelled.
2. If the DSC alert was transmitted, switch the radio to Channel 16 immediately and broadcast a voice cancellation: state the vessel name, MMSI, that the distress call was sent in error, and that all is safe on board.
3. Contact the USCG on Channel 16 or by phone to confirm the cancellation and prevent a SAR response.