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 an alerting beacon. Its 406 MHz transmission alerts the Cospas-Sarsat system, while a 121.5 MHz homing signal can help nearby rescue units locate the beacon. Modern approved EPIRBs may also provide GNSS and AIS locating functions, depending on the applicable standard and installation date. Registration, identity coding, battery status, hydrostatic release and national approval are practical parts of readiness.
Activation and release
The float-free arrangement is intended to release the beacon when the vessel sinks, but the crew must not wait for sinking if distress assistance is already needed. Keep the release unobstructed and do not secure the lanyard to the ship in a way that prevents separation. Once activated, follow the beacon’s operating instructions and keep it upright and visible as far as conditions permit.
| Signal or feature | Operational role |
|---|---|
| 406 MHz | Alerts the Cospas-Sarsat system. |
| 121.5 MHz | Supports local homing by suitable rescue equipment. |
| GNSS/AIS, where fitted | Adds locating information to the alert or nearby picture. |
A deliberate beacon workflow
- 1Decide whether the situation needs distress assistance now.
- 2Take the beacon from its assigned location or allow approved float-free release.
- 3Activate it using the marked procedure and move it clear of obstructions.
- 4Keep watch for rescue communications and use other available means to provide details.
- 5If activated accidentally, stop transmission only through the supported procedure, notify the RCC promptly with identity, position and time, and log the event.
Worked insight: alerting is not the whole rescue
An EPIRB alert can begin the rescue chain, but the crew still needs to communicate the situation, prepare survival equipment and follow instructions. GNSS availability improves location information but should not become a reason to delay a needed alert. A beacon’s signal, battery and antenna condition affect real performance, so readiness checks should examine context rather than promise one response time.
The beacon should be part of a wider abandonment plan: know who carries it, and keep approved battery, release and registration maintenance records current. Those details make the alert useful after it leaves the vessel.
The IMO EPIRB performance standard MSC.471(101) should be read with the vessel’s fitted equipment and manufacturer instructions. Do not infer that every recreational beacon has the same approval, registration or feature set as a SOLAS installation.
Official references · 2026-09-09
Apply what you learned
A crewmember presses the EPIRB test control during a drill, then sees no rescue response and proposes activating it again.
What should the instructor reinforce?
Suggested answer: Use only the approved self-test and never repeat live activation to seek a response.
Why this works: A self-test checks the beacon according to its manufacturer procedure; it is not expected to create a distress case. A real activation is reserved for distress and starts an alerting chain. The crew should verify the test result, registration and readiness, and use a controlled training scenario for the remaining drill actions.
Continue with guided radio exercises.
Practise in the radio simulatorKey Takeaways
406 MHz alerts Cospas-Sarsat and 121.5 MHz supports local homing.
GNSS and AIS locating features depend on the approved beacon and installation.
Activate when distress help is needed; do not wait for sinking.
Self-test is controlled maintenance, never a substitute for an alert procedure.
Accidental activation requires prompt RCC notification and a recorded cancellation.
Common Mistakes
Tying the float-free lanyard to the vessel.
Waiting for the vessel to sink before activating a needed beacon.
Using live activation as a test or assuming every EPIRB has identical features.
Test Your Knowledge
Practise this topic and review the explanation for each answer.
More GMDSS lessons
- Module 4 — MF/HF radiotelephony and DSCMF/HF DSC alerts and subsequent voice traffic use different frequencies.
- Module 5 — NBDP / radiotelexNBDP is narrow-band direct-printing telegraphy.
- Module 6 — Inmarsat C and EGCInmarsat-C provides data messaging, distress alerting and SafetyNET reception; it does not provide voice.
- Module 10 — GPS, antennas, batteries and reserve powerCheck GNSS position and UTC time, antenna connections, radio identities and power supplies.
Last reviewed: