Module 8 — EPIRB / 406 MHz beacon
406 MHz Cospas-Sarsat beacon: manual and automatic activation, hydrostatic release, approved self-tests and false-alert handling.
The EPIRB (Emergency Position-Indicating Radio Beacon) is the vessel's radio lifeline.
How it works
Transmits on 406.025 MHz to Cospas-Sarsat satellites with a coded signal that includes the beacon identifier. Adds a 121.5 MHz carrier for final homing by SAR aircraft.
Activation
Mandatory maintenance
Battery, hydrostatic release, antenna and mounting must be checked against expiry dates. The beacon identifier must be registered with the national authority.
Testing
Only the manufacturer-approved tests (self-test) — without transmitting to the real satellite. Activating the beacon outside a real emergency is forbidden.
False alerts
If activated by mistake: switch it off immediately and contact the local RCC / MRCC by the fastest means (VHF-DSC, Inmarsat C, telephone) giving the beacon identifier and the reason. Log the event.
Cospas-Sarsat: what it is and is not
Cospas-Sarsat is the global SAR system that detects distress beacons on 406.0–406.1 MHz through a constellation of LEOSAR (low-earth orbit), GEOSAR (geostationary) and MEOSAR (medium-earth orbit, fully operational since 2016). The signal is received by LUTs (Local User Terminals), processed by MCCs (Mission Control Centres) and distributed to the responsible RCC by position. It is a separate chain from terrestrial GMDSS, not a bridge radio.
406 message content
Each beacon transmits a unique 15-character HEX ID (or the new 23-hex format used by second-generation MEOSAR beacons) identifying beacon type, coding country, serial number and protocol. Modern EPIRBs include an internal GNSS that inserts position directly into the message, shortening the locating time. The manual points out that EPIRBs installed on or after 1 July 2022 must also provide an AIS locating signal (2.5.2.11).
Mandatory registration
Paragraph 3.3.4.23 states that Cospas-Sarsat maintains an international database (www.406registration.com) for beacon registration, accessible 24/7 to SAR authorities. An unregistered or out-of-date beacon multiplies the time the RCC needs to contact the owner and confirm whether the alert is genuine. Registration is not paperwork; it is part of the rescue chain.
Activation and testing (3.3.4.25)
406 MHz beacons have a self-test function that evaluates key performance characteristics without generating a live alert on the Cospas-Sarsat network. Real activation is by float-free hydrostatic release or manually. Additional 121.5 MHz homing lets SAR aircraft close on the beacon on scene.
False activation
If an EPIRB is activated accidentally, it must be switched off at once and the nearest RCC notified by any available means (VHF, MF/HF, Inmarsat, satellite phone) giving the HEX ID, time of activation and cause. Skipping that step triggers unnecessary SAR deployment and damages future credibility.
VHF EPIRB and 121.5 MHz homing
Legacy EPIRBs transmitted only on 121.5 MHz as an analogue homing signal for aircraft; they are no longer primary in the GMDSS and Cospas-Sarsat stopped processing them. A modern EPIRB transmits the distress on 406 MHz and keeps 121.5 MHz only as a final-approach signal for SAR aircraft. No administration today should accept a VHF-only / 121.5 MHz-only EPIRB as SOLAS compliance.
Registration, HRU and maintenance cadence
The beacon is registered with the national authority (in Spain the Cospas-Sarsat National Centre; for international fleets, www.406registration.com) and updated on any change of owner, flag or contact. It is mounted in a float-free bracket with a hydrostatic release unit (HRU) that releases the beacon at about 4 m depth at any angle, with a buoyant lanyard (manual 3.3.4.10). Maintenance cadence: monthly self-test run from the beacon itself, full annual inspection with battery and HRU expiry check, shore-based maintenance by a qualified technician every 5 years, and battery and HRU replacement before their expiry date (typically every 5 years, per manufacturer).
Technical structure of the 406 signal
The main carrier sits in the 406.025 / 406.028 / 406.031 / 406.037 MHz block depending on protocol and generation. Each beacon transmits a ~520 ms digital burst repeated roughly every 50 s; the frame carries the HEX ID (15 characters first generation or 23 characters second-generation MEOSAR), country code, protocol (serial, aviation, MMSI, radio call), activation flags, and — in GNSS-equipped beacons — encoded position to ~120 m accuracy. An analogue 121.5 MHz homing carrier is kept on in parallel throughout operation for SAR-aircraft final approach.
Space segment and Cospas-Sarsat routing
LEOSAR: about 7 satellites in polar orbit at 800-1000 km; Doppler location measured across a pass (~90 min per orbit); typical latency 30-90 min; no GNSS in the beacon required. GEOSAR: geostationary satellites over the equator; instant detection but no Doppler location — they need the GNSS-encoded position in the message. MEOSAR: transponders hosted on GPS/Galileo/GLONASS; near-real-time detection with accurate (~2 km) multilateration. Shore routing: the beacon is received by one or more LUTs (Local User Terminals), which relay to the national MCC (Mission Control Centre); the MCC routes to the RCC responsible by position and registration data.
Activation decision and deployment
Activate only in a real emergency — never as a drill. Abandon-ship sequence
- 1release manually if there is time, or let the HRU release as she goes down
- 2in the raft, secure the floating lanyard to keep the beacon from drifting away
- 3stand the antenna vertical and keep clear of metal masses and metal liferaft frames that can shadow the signal
- 4do not switch off: it must stay active until actual rescue (autonomy ≥48 h). False activation: switch off at once, note start and end time, contact the nearest RCC by any means with HEX ID, time and cause; some administrations require a written report within 24-72 h.
EPIRB false-alert cancellation script and AIS locating signal
(1) Cancellation script: on detecting an accidental activation, switch the beacon off and contact the nearest MRCC immediately by the fastest working means (satellite phone, VHF, MF, HF, landline in port). Verbatim script: 'This is [ship name], MMSI [MMSI], EPIRB HEX ID [15 hex characters], false activation at [UTC time], cause [accidental activation / improper test / maintenance], no distress, no distress. Current position [lat/long], crew status OK.' Enter activation time, cancellation time, cause and signature in the radio log.
(2) AIS locating signal on 406 EPIRBs: new-build beacons installed on or after 1 July 2022 (requirement set by IMO resolution MSC.471(101)) must additionally transmit AIS position messages alongside the 406 MHz Cospas-Sarsat carrier and the 121.5 MHz homing signal. This greatly speeds local detection by nearby AIS-equipped ships, complementing satellite location with a VHF on-scene track.
(3) Registration must be kept current on any change of contact, owner or flag: via www.406registration.com (international) or the Cospas-Sarsat National Centre (Spain). An out-of-date record delays the RCC's response because the call to the reference contact does not connect.
STCW Bridge Watch Lens
Decide applicability before manoeuvring: Rules 4-10 apply in any visibility, Rules 11-18 only when vessels are in sight, and Rule 19 governs radar-only encounters in restricted visibility.
Build the traffic picture with sight, hearing, radar/ARPA and chart context. Do not let AIS or one isolated bearing replace systematic observation.
After manoeuvring, keep monitoring bearing, range, CPA/TCPA and passing distance until the other vessel is finally past and clear.
Exam Focus
Start every scenario by classifying the encounter: overtaking, head-on, crossing, narrow channel, traffic separation, or restricted visibility.
If two rules seem to conflict, check the order carefully: overtaking duties still apply, and Rule 2 still requires ordinary seamanship.
Steady or nearly steady bearing is the classic trigger, but close range, a large vessel or a tow can still mean risk even when bearing change seems small.
Key Takeaways
406 MHz carries data to satellite; 121.5 MHz is the local SAR homing carrier.
The hydrostatic release frees the beacon between 1.5 and 4 m depth.
Every false alert must be reported to the RCC immediately.
The identifier must be registered and kept up to date.
Cospas-Sarsat detects 406.0–406.1 MHz beacons via LEOSAR, GEOSAR and MEOSAR satellites.
The unique HEX ID and registration at 406registration.com are part of the SAR chain.
EPIRBs installed on or after 1 July 2022 must also carry an AIS locating signal (2.5.2.11).
121.5 MHz homing enables final approach by SAR aircraft on scene.
121.5 MHz is now homing only; the primary distress emission is 406 MHz.
National registration or 406registration.com must be updated on every change of owner/contact.
Float-free bracket with HRU releases the EPIRB at about 4 m; buoyant lanyard prevents loss.
Cadence: monthly self-test, annual inspection, shore-based maintenance every 5 years, battery/HRU replaced before expiry.
406 frame: carrier 406.025-406.037 MHz, ~520 ms burst every 50 s, 15/23-character HEX ID.
LEOSAR Doppler (~30-90 min); GEOSAR instant detection (needs GNSS); MEOSAR multilateration on GPS/Galileo.
Route: beacon → LUT → national MCC → responsible RCC by position and registration.
Common Mistakes
Testing the beacon by actually activating it (never!).
Missing battery and hydrostatic release expiry dates.
Failing to cancel a false alert thinking "it will sort itself out".
Not updating registration on 406registration.com when owner, flag or contacts change.
Testing the EPIRB by actual activation instead of using the self-test.
Failing to tell the RCC about a false activation; the alert is treated as real until cancelled.
Treating the EPIRB as a substitute for all communications: it is alerting + locating, not traffic.
Accepting a 121.5 MHz-only beacon as GMDSS compliance.
Failing to replace battery or HRU before expiry and finding the float-free release inoperative.
Leaving the registration with a former owner's details so the RCC calls the wrong contact.
Deploying the beacon inside the raft with the antenna near metal structures that shadow the signal.
Switching the beacon off before rescue 'to save battery' and losing the 121.5 MHz final homing.
Test Your Knowledge
Test your knowledge and prove your mastery.