1The lesson in plain words
In one sentence…
A 406 MHz EPIRB alerts Cospas-Sarsat; 121.5 MHz helps nearby units locate it.
What does it do?
An EPIRB starts the rescue alert; it does not replace the survival plan or follow-up communication.
When do you activate it?
When distress assistance is needed; do not wait for sinking. The float-free release is intended to release it if the vessel sinks.
What do you check?
The coded identity and registration must match the real beacon; battery, release and national approval are part of readiness.
How do you test it?
Use the manufacturer’s self-test only. A self-test is not a distress transmission and live activation is not a test.
2Step by step
EPIRB activation and checks
- 1
Decide whether the situation needs distress assistance now.
Activate when help is needed; do not wait for sinking.
- 2
Take the assigned beacon or allow approved float-free release.
The float-free release is intended to release it as the vessel sinks.
- 3
Activate it using the marked procedure and clear its obstructions.
The release must remain clear so the beacon can separate.
- 4
Keep it upright and visible as far as conditions permit.
This is the stated way to keep it operating after activation.
- 5
Use the manufacturer’s self-test only to check it.
A self-test is not a live distress transmission.
- 6
After accidental activation, stop transmission through the supported procedure.
Cancellation must follow the procedure supported by the equipment.
- 7
Notify the RCC with identity, position and time, and record the cancellation.
The RCC needs to identify the accidental alert and its time.
3Key data
- Primary alert
- 406 MHz · Cospas-Sarsat
- Local homing
- 121.5 MHz
- Additional locating
- GNSS and AIS, where fitted
- First generation
- 15 hexadecimal characters
- Second generation
- 23 hexadecimal characters
- Release
- Before 4 m depth
4Key points
- 1.
Identity and registration must match the real vessel.
- 2.
Keep the release clear; do not tie the lanyard to the ship.
- 3.
Manufacturer self-test, never live activation as a test.
- 4.
Accidental activation: stop, notify the RCC and record the cancellation.
5Frequent mistakes
Wrong: "The lanyard should stay tied to the vessel."
Right: Keep the release clear and do not tie the lanyard so the beacon can separate.
Wrong: "I will wait for the vessel to sink before activating the EPIRB."
Right: Activate when distress help is needed; do not wait for sinking.
Wrong: "Live activation is a suitable test."
Right: Use the manufacturer’s self-test only; live activation is for distress.
6Practice case & exam keys
Situation
During a drill a crewmember presses the EPIRB self-test, sees the test light flash, notices no rescue response and proposes switching the beacon to ON to check that it really works.
What should the instructor reinforce?
Show the answer →
A self-test is never meant to bring a response; switching to ON sends a real distress alert. Record the self-test result and stop there.
Why: The self-test checks battery and circuits without transmitting an alert, so no response is the correct result. ON starts a real Cospas-Sarsat alert and a real SAR effort. The drill should continue with a check of the hex ID against the registration, the expiry dates and the abandon-ship task of taking the beacon to the raft.
Exam keys
When you see “406 MHz”, link it to the Cospas-Sarsat alert, not local homing.
When “121.5 MHz” appears, look for local homing with suitable rescue equipment.
If identity is asked, choose the real beacon: first generation 15; second generation 23 hexadecimal characters.
How to read it
Think of the EPIRB as the alert’s beginning: rescue still needs communication, survival and instructions.
The fitted equipment matters: GNSS, AIS, registration and approval depend on the real beacon and installation.
7Go deeper: GOC exam level
The detail behind each step for the General Operator's Certificate: the rules, the radio technique and worked cases.
The Cospas-Sarsat chain
An EPIRB (Emergency Position-Indicating Radio Beacon) exists to put a distress alert into the Cospas-Sarsat system. SOLAS ships carry a 406 MHz float-free EPIRB that activates automatically if the ship sinks, provided it is correctly installed and armed.
- 1The beacon transmits a coded digital message in the 406.0–406.1 MHz band.
- 2The satellite constellation (LEOSAR in low orbit, GEOSAR in geostationary orbit and MEOSAR on navigation satellites in medium orbit) relays or detects it.
- 3Local User Terminals (LUTs) on the ground process the satellite data.
- 4Mission Control Centres (MCCs) distribute the alert to the responsible RCC/MRCC.
- 5The RCC correlates the beacon identity, its registration, the position and any other sources before and during the response.
Position and homing
| Signal | What it gives rescuers |
|---|---|
| 406 MHz digital message | The alert, the beacon identity and, if fitted, an encoded GNSS position |
| Position from the satellite system | Doppler or MEOSAR location even without GNSS, with some delay and uncertainty |
| 121.5 MHz homing | A signal that aircraft and rescue units can direction-find in the final approach |
| AIS locating (EPIRB-AIS) | A position on AIS1/AIS2 visible to nearby ships and aircraft on their AIS displays |
Float-free and the hydrostatic release unit
The HRU frees the beacon when the ship sinks to a few metres. The installation must let the beacon float free without getting caught on structure, then activate as designed.
Registration, tests and maintenance
- Register the beacon with the Administration and keep vessel and emergency-contact details current. The RCC uses this data to verify the alert.
- Use only the manufacturer's self-test. Never generate a real alert to "see if it works".
- Check the battery and HRU expiry dates.
- SOLAS requires an annual test and shore-based maintenance at intervals not exceeding five years, under IMO guidance and the Administration's rules.
- After an accidental activation, switch the beacon off and tell the nearest RCC or coast station immediately. Do not just put it back in the bracket.
| Check | Typical interval |
|---|---|
| Visual check: bracket, HRU, antenna, no obstruction | Each watch round / weekly |
| Self-test per manufacturer | Monthly |
| Annual test (frequency, coding, registration, battery) | Every 12 months |
| Shore-based maintenance | Not more than 5 years |
| HRU replacement | At the expiry date, typically 2 years for disposable units |
Beacon identity and the second generation
Every 406 MHz beacon has a unique identity, shown on its label as a 15-hex ID (fifteen hexadecimal characters). Second-generation beacons carry a longer 23-hex ID. The ID is coded from the country code and the MMSI or another serial, and it is the key the RCC uses to find the registration.
| Feature | First generation | Second generation |
|---|---|---|
| Identity on label | 15-hex ID | 23-hex ID |
| Best satellite system | LEOSAR/GEOSAR, also detected by MEOSAR | Designed for MEOSAR |
| First alert and position | Can take longer without GNSS | Faster first alert and more accurate independent location |
| Return link | Only on some newer RLS models | Optional Return Link Service via Galileo: a light on the beacon confirms the alert was received |
Man-overboard devices are not EPIRBs. An AIS-MOB transmits on AIS1/AIS2 with an identity 972XXYYYY; some also send a DSC alert on Channel 70 to the own ship (closed loop) or to all stations (open loop). They help the ship recover its own crew; they do not replace the 406 MHz EPIRB.
References: MSC.471(101) — 406 MHz float-free EPIRB performance standards · SOLAS IV as amended by MSC.496(105)
GOC exam check
On which band does a Cospas-Sarsat EPIRB transmit its distress alert?
8Full lesson & sources
Open the full lesson (diagrams and official references)
A 406 MHz EPIRB alerts the Cospas-Sarsat system; its 121.5 MHz signal helps nearby rescue units home in. The applicable modern IMO standard adds GNSS and AIS locating capability for EPIRBs installed from 1 July 2022. First-generation beacon identifiers have 15 hexadecimal characters; second-generation identifiers have 23. Registration and national equipment approval must match the actual beacon.
Keep the float-free release unobstructed. The release mechanism must operate before 4 m depth; do not tie the lanyard to the ship. Use the beacon when distress assistance is needed, without waiting for sinking. Follow the manufacturer self-test procedure; coded test emissions are possible. After accidental activation, stop transmission, contact the RCC with identity, position and time, and record the cancellation.
What an EPIRB contributes
An EPIRB is a beacon that alerts rescuers when nobody on board can use a radio. Its 406 MHz transmission alerts the Cospas-Sarsat system. Every 50 seconds or so it sends a short digital burst containing its unique identity and, if it has a GNSS receiver, its position. Satellites pick up the burst, ground stations process it, and the alert reaches the responsible RCC. A separate low-power signal on 121.5 MHz lets aircraft and lifeboats home in on it during the final approach, and newer beacons also transmit an AIS position.
The RCC sees a beacon identity, not a ship name. It looks the identity up in the registration database to find out whose beacon it is, then often calls the emergency contact. That is why an unregistered or wrongly registered beacon slows down a rescue.
| Signal | Who uses it | Typical timing |
|---|---|---|
| 406 MHz with encoded GNSS position | RCC, via Cospas-Sarsat | Often within minutes on MEOSAR |
| 406 MHz without GNSS | RCC, after the system computes a position | Can take longer and is less precise |
| 121.5 MHz homing | Aircraft and rescue units close in | Final approach only |
| AIS (EPIRB-AIS models) | Nearby ships and aircraft with AIS | Shown on their AIS displays |
Activation and release
On a SOLAS ship the EPIRB sits in a float-free bracket. If the ship sinks, the hydrostatic release unit (HRU) cuts the beacon loose at a depth of a few metres, the beacon floats up and switches itself on. That covers the case where the ship goes down too fast for anyone to act.
Do not wait for that if you need help sooner. If you are abandoning ship, take the beacon out of the bracket, activate it by hand following the marked procedure, take it into the liferaft and tie it to the raft, not to the ship. Keep it upright, with the antenna clear of the canopy.
A deliberate beacon workflow
- 1Decide whether you need help now. If yes, alert by every means available; the EPIRB is one of them.
- 2Free the beacon by hand, or let the HRU release it if the ship sinks.
- 3Activate it using the marked procedure and check the indicator light.
- 4Keep it in the open, upright and secured to the liferaft.
- 5Listen on the survival-craft VHF for rescue units, and give details by voice when you can.
If the beacon goes off by accident, switch it off and tell the nearest RCC or coast station at once: beacon identity, position, time of activation and that there is no distress. Then log it. Putting it back in the bracket without telling anyone leaves an RCC searching for a ship that is fine.
Worked insight: alerting is not the whole rescue
At 0300 a ship sinks quickly in heavy weather. The HRU frees the EPIRB at about 4 m, it surfaces, and its GNSS position is in the first burst. The RCC may have it within minutes and calls the registered owner, who confirms the ship is on that route. An aircraft is sent, but the EPIRB cannot tell it there are 18 survivors in two rafts with one injured. The survival-craft VHF and the SART do that work.
Keep up the maintenance that makes the alert useful: a monthly self-test, an annual test, shore-based maintenance at least every 5 years, HRU replacement at its expiry date (typically 2 years) and up-to-date registration. The IMO EPIRB performance standard MSC.471(101) sets what an approved SOLAS beacon must do. A leisure beacon may have fewer features.
Official references · 2026-09-09
9Test your knowledge
Which system receives the 406 MHz alert?
Keep studying…
Reviewed by Ramon Curto, Nautical teacher at IECNPC · Last reviewed: