EPIRB, SART and Receiving a DSC Distress Alert

EPIRB: what it is and how the alert reaches a rescue centre

In short: Read first: GMDSS sea areas and equipment and distress signals. An EPIRB (Emergency Position-Indicating Radio Beacon) is the ship’s last-resort alarm. It sends a distress alert by satellite, with the ship’s identity and position, to a rescue centre — even if nobody on board can use the radio.

• It works through the Cospas-Sarsat satellite system.
• It transmits on 406 MHz (406–406.1 MHz band) (IMO MSC.471(101)).
• Every SOLAS ship in every sea area carries one (SOLAS IV/7).

The alert chain

1. Beacon activated (floats free or switched on by hand).
2. Satellite picks up the 406 MHz burst.
3. LUT (Local User Terminal) — a ground station that receives the satellite data and works out the position.
4. MCC (Mission Control Centre) — checks the alert and routes it.
5. RCC/MRCC (Rescue Coordination Centre) — launches the search and rescue.

• Memory aid: Beacon → Sat → LUT → MCC → RCC.
• The message carries the beacon’s unique identity (country code + ship’s MMSI or serial number) (IMO MSC.471(101), Part B). That is why the EPIRB must be registered with the flag Administration: the RCC uses the registration to know which ship it is and whom to call.

Frequencies and position

• 406 MHz — the satellite distress alert.
• 121.5 MHz — a weak homing signal for the final search, mainly for aircraft (IMO MSC.471(101) 2.3.14). Short range only.
• GNSS (GPS) — most EPIRBs carry a GNSS receiver and send their position inside the alert. This gives a fast, accurate fix: alert times are measured in minutes. Since 1 July 2022, every newly installed SOLAS EPIRB must have GNSS, updated at least every 5 min (IMO MSC.471(101)).
• Without GNSS, satellites in low Earth orbit work out the position by the Doppler effect, to within about 3 NM. It takes longer.
• AIS locating signal — EPIRBs installed from 1 July 2022 also transmit on AIS, so ships nearby see “EPIRB ACTIVE” on AIS and the chart display (IMO MSC.471(101)).

Stowage, activation and registration

• Stowed in an easily accessible place, ready to be released by hand and carried to the liferaft.
• Held in a float-free bracket with a hydrostatic release unit (HRU). If the ship sinks, the HRU frees the EPIRB before it reaches 4 m depth, at any list or trim (IMO MSC.471(101) 2.6.3).
• Two kinds, often called categories:
  • Category I — float-free, activates automatically when it floats free in the water. This is the SOLAS type.
  • Category II — activated manually.
• A manual alert needs at least two separate actions, to stop false alerts (IMO MSC.471(101) 3.3).
• The lanyard is for tying the EPIRB to the liferaft or a person, never to the ship.
• Registered with the Administration. In the UK, the UK Beacon Registry (free, online).

EPIRB battery, tests and the end of 121.5 MHz satellite alerting

In short: The battery must last at least 48 h of transmission. Test the EPIRB every month with its self-test, never with a real alert. Only 406 MHz beacons are detected by satellite.

• Battery: at least 48 h of operation (IMO MSC.471(101) 2.4; the same figure was in the older IMO A.810(19), which still applies to EPIRBs installed before 1 July 2022).
• Monthly test: use the self-test (it checks the beacon without using the satellite system). At the same time check:
  • mounting and HRU, and the HRU expiry date;
  • battery expiry date (marked on the beacon);
  • lanyard, damage, and the registration label.
• Each self-test uses a little battery: do not overdo it. Record it in the GMDSS log.
• Yearly test and shore-based maintenance at least every 5 years (SOLAS IV/15), as part of the GMDSS routine checks.

Why old 121.5/243 MHz beacons are obsolete

• Old EPIRBs that alerted on 121.5/243 MHz were unreliable: the bands were crowded, propagation was poor, and there were many false alerts.
• Cospas-Sarsat stopped satellite processing of 121.5/243 MHz on 1 February 2009. Since then only 406 MHz beacons are detected by satellite.
• 121.5 MHz is still used — but only as the homing signal inside a 406 MHz beacon.
• Older texts give a different year for this change. The date to remember is 1 February 2009.

SART: making the liferaft show on radar

In short: A SART (Search And Rescue Transponder) sits in the survival craft. When a ship’s or aircraft’s X-band (9 GHz) radar sweeps it, it answers. The answer shows on the rescuer’s radar as a line of 12 dots pointing to the liferaft. More on the rescuer’s side in radar in search and rescue.

• Radar SART: answers X-band (3 cm, 9 GHz) radar only. It does not answer S-band (10 cm).
• AIS-SART: the other accepted type. It sends its GNSS position by AIS, so it shows as a special AIS target on the chart display.
• Carriage: 1 SART under 500 GT, 2 at 500 GT and over (SOLAS IV/7).

What you see on the radar screen

• The SART is at the first dot — the one nearest your own ship. Steer for it.
• Figures from IMO SN/Circ.197. Some references say the arcs appear from about 1–3 NM; the IMO guidance gives no fixed distance, only “as you approach”.
• Radar settings when looking for a SART: 6 or 12 NM range scale; gain at normal long-range setting; sea clutter to minimum; rain clutter as normal (it does not hide the dots). Detuning the radar can help in clutter, but retune as soon as possible (IMO SN/Circ.197).

Battery, range and use in the liferaft

• Battery: 96 h on standby, then 8 h transmitting while continuously interrogated (IMO MSC.510(105) 2.3, which replaced IMO A.802(19) from 1 January 2024 — same figures).
• Range: at least 5 NM from a ship’s radar with its antenna at 15 m; up to 30 NM from an aircraft at 3,000 ft (IMO MSC.510(105) 2.7).
• Mount the SART at least 1 m above sea level — use the pole and the antenna pocket in the liferaft (IMO MSC.510(105) 2.5). Higher = longer range.
• It beeps or flashes when a radar triggers it, so survivors know a ship is near.
• Testing on board: keep it to a few seconds — it shows on every radar nearby and drains the battery (IMO MSC.510(105)). Monthly: check stowage, damage and battery expiry date.

PLB and SART rules on yachts

In short: A PLB (Personal Locator Beacon) is a small 406 MHz beacon for one person. It does not replace the ship’s EPIRB. Yachts follow their yacht code, which sets EPIRB and SART numbers by operating category.

• PLB: works through Cospas-Sarsat on 406 MHz, like an EPIRB, with 121.5 MHz homing. It is activated by hand and registered to a person, not to a ship. In the UK it is registered with the UK Beacon Registry like an EPIRB.
• Typical large-yacht list:
  • EPIRB: 1 for Category 1 and 2; 2 for Category 0 (unrestricted).
  • SART: 1 for Category 0, 1 and 2.
  • In Category 0 and 1, a SART may not be required if the EPIRB has 121.5 MHz homing.
  • PLB: listed as an extra item for the crew.
• These numbers come from a yacht’s own equipment list. Always check the code that applies to your yacht.

Receiving a VHF or MF DSC distress alert

In short: Listen first, do not relay. Listen on VHF Ch 16 / 2182 kHz for 5 min. Let the coast station or RCC acknowledge. Acknowledge yourself by voice, and only if you can help or nobody else answers. Never send a DSC relay (IMO COMSAR/Circ.25, IAMSAR Vol. III).

The IMO flow diagram, step by step:

1. DSC distress alert received (VHF Ch 70 or MF 2187.5 kHz). Call the Master.
2. Listen on VHF Ch 16 / 2182 kHz for 5 min.
3. Has a coast station (CS) or RCC acknowledged the alert?
   • Yes → go to step 6.
   • No → step 4.
4. Is distress traffic in progress on Ch 16 / 2182 kHz?
   • Yes → go to step 6.
   • No → step 5.
5. Is the DSC distress call still continuing?
   • Yes → acknowledge by radiotelephony (voice) to the ship in distress on Ch 16 / 2182 kHz, then inform the CS and/or RCC (step 7).
   • No → inform the CS and/or RCC (step 7).
6. Can own vessel assist?
   • Yes → acknowledge by radiotelephony to the ship in distress on Ch 16 / 2182 kHz, then inform the CS and/or RCC.
   • No → go to step 8.
7. Inform the CS and/or RCC.
8. Enter the details in the log, then reset the system.

The two rules examiners want

• No DSC relay on VHF or MF. A ship is never allowed to send a DSC distress relay on receiving a DSC distress alert on VHF Ch 70 or MF 2187.5 kHz (IMO COMSAR/Circ.25, Note 2).
• Acknowledge by voice, not by DSC. A DSC acknowledgement is sent only if the alert keeps repeating, the ship in distress is beyond doubt in the vicinity, and after consulting an RCC or coast station — to stop the repeating call (IMO COMSAR/Circ.25, Note 1).
• Distress alerts, acknowledgements and relays are sent only with the Master’s permission (IMO COMSAR/Circ.25).
• Why so strict? Unneeded relays and acknowledgements flood the RCCs, confuse the picture and delay the rescue.

Receiving an HF DSC distress alert

In short: HF alerts can come from a ship thousands of miles away. Listen on the associated voice (RTF) or NBDP channel for 5 min. If no coast station acknowledges and no distress traffic follows, relay on HF to a coast station only and inform the RCC (IMO COMSAR/Circ.25, IAMSAR Vol. III).

1. HF DSC distress alert received. Call the Master.
2. Listen on the associated RTF or NBDP channel(s) for 5 min.
3. Has a CS or RCC acknowledged or relayed the alert?
   • Yes → go to step 5.
   • No → step 4.
4. Is distress communication in progress on the associated RTF channels?
   • Yes → step 5.
   • No → transmit a distress relay on HF to a coast station and inform the RCC. Start the relay by hand, not automatically.
5. Can own vessel assist?
   • Yes → contact the RCC by the most efficient means to offer help. Then talk to the station controlling the distress as directed and help as required.
   • No → no extra calls.
6. Enter the details in the log, then reset the system.

• If the casualty is clearly not in your area, or other craft are better placed, stay off the air: extra calls interfere with the rescue. Still log it.
• The difference from VHF/MF: on HF a relay is allowed — but only to a coast station, never to “all ships”.
• The only other case for a relay: you know the ship in distress cannot send its own alert and the Master thinks more help is needed. Then the relay may go to “all ships” or to a coast station (IMO COMSAR/Circ.25, quoting ITU-R M.541).

HF distress channels (kHz)

• Source: IMO COMSAR/Circ.25, Flow diagram 2.
• Since 1 January 2024, NBDP is no longer required for distress traffic in A3 and A4 (IMO MSC.496(105)). In practice you listen on the voice channel.