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MOB and Personal Emergency Radio Devices

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MOB and Personal Emergency Radio Devices

MCA issue guide to Personal Emergency Radio Devices

If you fall overboard or your boat sinks it may be difficult to find you, even in a moderate swell. It will be worse if no-one sees you go into the water.

Wearing a Personal Locator Beacon (PLB) or a Man Overboard (MOB) device will help raise the alarm so that rescuers can locate you. They can easily be attached to your PFD - Lifejacket - Buoyancy Aid - and should be worn at all times while you are on deck.



Will alert others to your distress.

BUT may not provide updated information regarding your location.


Will help rescuers to find you.

BUT will not necessarily alert them to your distress.



Use VHF which is limited by line of sight

A few miles of comms for person in the water.


Use SATELLITES which have limited range to shore.

Minimum comms with local vessels.


Use a combination of VHF and SATELLITE.

Personal emergency devices only work when aerial is above the water

Devices and beacons which rely on GMDSS communications (VHF DSC, 406MHz Cospas-Sarsat, AIS) may only be used for emergencies. Some of these technologies rely on receivers that are commonly found on other vessels, others require a special receiver which is carried by UK rescue helicopters and lifeboats.

Most MOBs are recovered by own vessel and nearby vessels often assist

If you are close to shore, and there are likely to be other boats nearby with receivers, you will benefit from a local device which alerts other nearby vessels to your distress. A local device will also alert your vessel’s crew should you go overboard. If the Coastguard receives your distress message they will try to locate nearby vessels that are in a position to assist. If, however, you are far offshore and out of range of the local technologies, any Coastguard receiver or other vessels, the 406 MHz Cospas-Sarsat technology is more suitable. All the more so if you are a lone sailor.

In deciding which device is most suitable for your needs you need to assess the risk. For example, after an incident, is it likely that your vessel is going to be left with no competent person on-board able to pick you up. In this situation at least one person should have a 406 MHz Cospas-Sarsat device available.Not everybody on board needs to have the same device. Most crew could carry local alerting and locating devices, providing there is a 406 MHz Cospas-Sarsat device available if required.

Learn about ALERTING and LOCATING Technology...



ALERTING Technology will alert others to your distress - but will not necessarily provide updated and ongoing information regarding your location.

Local devices which use VHF DSC

This is likely to be a DSC distress beacon which can send a distress message to your own boat and if no response, to nearby boats. The distress message contains an accurate position from an inbuilt GNSS (Global Navigation Satellite System such as GPS) receiver. It will cause an audible alarm in the receiving DSC radio. The message is repeated at intervals and stops once acknowledged. After this you may need to rely on a locating technology to help rescuers find you. VHF range limitations mean that a transmission from water level might have a five mile range but could be partially blocked by waves. VHF DSC is monitored by the UK Coastguard and all SOLAS vessels, and the radios are popular with small boat users. These DSC signals are only used for alerting and will be acknowledged.

Worldwide devices which use 406 MHz Cospas-Sarsat

This might for example be a personal locator beacon. The beacon will send a distress signal through the Cospas-Sarsat satellite system to a Rescue Coordination Centre (RCC). Some devices have an integral GNSS which significantly improves how quickly and accurately the casualty location is available. Without a built-in GNSS it can take up to 2 hours to achieve a position with an accuracy of 5 km. You need to ensure that your device is registered with the UK Coastguard so that your information is automatically available to rescue services.

Other Man Overboard Alerting Technology

Other technologies include a tag system which uses a base unit on the vessel and tags worn by all crew members. The alarm sounds on board your vessel only, when the base loses contact with a tag, when the tag is either immersed in water or when the distance between the tag and the base is too great. Other systems can alarm when they receive the AIS MOB message however this is not common or a recognised GMDSS method of raising the alarm and cannot be relied upon beyond your own vessel.

LOCATING Technology will help rescuers to find you - but will it not necessarily alert them to your distress.

Local devices which use AIS (Automatic Identification System)

All devices using AIS have an inbuilt GNSS receiver which provides an accurate position so that rescuers can locate you more easily. The beacon will broadcast your position via AIS to any vessel that has an AIS receiver and is within VHF range. AIS receivers are carried on all SOLAS vessels and are popular with small boat users. UK rescue helicopters and lifeboats also carry this equipment to assist with locating casualties.

Local devices which use 121.5 MHz Homing Signal

Your MOB device or PLB may also be able to transmit a 121.5 MHz homing signal. UK rescue helicopters and lifeboats are equipped with Direction Finding (DF) equipment so that they can track this signal. However, most other vessels do not carry equipment capable of receiving or locating the 121.5 MHz homing signal.

Worldwide devices which use 406 MHz Cospas-Sarsat

Having transmitted an alert, these devices will continue to transmit and provide the Rescue Coordination Centre (RCC) with updated position information. If the device has a built-in GNSS this position information will be far more accurate. However, vessels / aircraft on scene will not receive this directly, relying on the RCC to forward this information. UK rescue helicopters can use Direction Finding (DF) on the 406 MHz frequency.

Other Man Overboard Locating Technology

Although there are other technologies which may transmit a homing signal or the position from an inbuilt GNSS receiver to a specialist receiver, you need to consider who will have a specialist receiver with the capability to rescue you.

Personal Emergency Radio Devices - MCA - Maritime Coastguard Agency

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