Which PLB or AIS MOB is right for your situation?

PLB or AIS MOB: what is the difference?

A PLB (Personal Locator Beacon) sends a distress alert on 406 MHz through the Cospas-Sarsat satellite system to the responsible search and rescue organisation. An AIS MOB transmits a local man overboard message, allowing vessels with suitable AIS receiving equipment to see your position. Models with DSC can also alert compatible marine VHF radios.

A combined AIS–PLB beacon provides both alerting routes. Check which transmission technologies are included: the general term ‘personal distress beacon’ does not tell you who will receive your alert.

Sources: 3, 4, 5, 6.

What do the abbreviations mean?

PLB — Personal Locator Beacon
A personal distress beacon that, when activated, transmits a 406 MHz distress alert through the Cospas-Sarsat satellite system. You carry a PLB on your person. Not every PLB also has AIS or automatic activation.

AIS — Automatic Identification System
A radio system used by vessels to exchange information including their identity and position. An AIS distress beacon transmits a special message that allows compatible AIS equipment nearby to display your position. Reception and alarm presentation depend on range, equipment and settings.

MOB — Man Overboard
The international abbreviation for ‘man overboard’. An AIS MOB is a personal beacon used to locate someone in the water locally. It does not necessarily have a satellite connection.

DSC — Digital Selective Calling
A system for digital calls using marine radio. A MOB beacon with DSC can alert compatible marine radios within radio range. Which radios are called depends on the beacon and the selected alert mode. DSC does not provide voice communication.

VHF — Very High Frequency
The frequency band used for marine VHF communication, AIS and maritime DSC, among other services. Range is strongly influenced by antenna height and conditions. A beacon just above the water generally has a shorter range than a vessel’s elevated antenna.

MMSI — Maritime Mobile Service Identity
A nine-digit identification number for maritime radio equipment. Your vessel’s MMSI number and a MOB beacon’s own identity are not the same. Entering the vessel’s MMSI does not replace the beacon’s factory-programmed identity.

GNSS and GPS — satellite positioning
GNSS is the collective term for satellite navigation systems, including GPS and Galileo. The receiver in the beacon uses these systems to determine your position. The distress alert is then transmitted through a separate system, such as AIS or the 406 MHz satellite link.

Cospas-Sarsat — satellite distress alerting system
The international system that receives distress signals from devices including PLBs and EPIRBs and forwards them through ground stations to the responsible search and rescue organisation.

RLS — Return Link Service
A return message via Galileo that confirms, on compatible beacons, that the distress alert has been received and the location determined. RLS does not confirm that a rescue unit is on its way and does not provide an arrival time.

EPIRB — Emergency Position-Indicating Radio Beacon
A satellite distress beacon associated with a vessel. Some versions can release and activate automatically if the vessel sinks. A PLB is personal equipment and does not replace an EPIRB where one is required.

121.5 MHz — homing signal for the final stage of a search
A radio signal that rescue units with suitable direction-finding equipment can use to locate the beacon. On its own, it does not provide worldwide satellite alerting.

NFC — Near Field Communication
A very short-range connection. On compatible beacons, you can use a compatible smartphone to read information such as test results and battery data. NFC does not transmit a distress alert.

Choose according to your cruising area and crew

These are starting points for choosing equipment, not a guarantee of rescue. A beacon helps raise the alarm and locate you. A suitable lifejacket with a harness, a safety tether, preventing falls overboard and a workable, practised man overboard procedure remain essential.

Selection guidance based on technical functions, not a universal product ranking. Sources: 3 - 8.

Current models and their main differences

These examples show which functions can be combined. Discuss availability, country coding and suitability for your lifejacket before ordering.

For the PLB3 and ResQLink AIS, the battery must be replaced no later than 6 years after manufacture or 5 years after entering service, whichever comes first. The manufacturer’s replacement instructions also apply after emergency activation. The stated operating duration is not a rescue time.

Specifications based on manufacturer information, not independent performance testing. Sources by model: 3 - 8; PLB3 battery/operation: 14. easy2-MOB operating durations have not been compared because specifications and test conditions differ.

Ocean Signal: personal beacon comparison

Four points often overlooked when buying

1. Automatic does not always mean water-activated

Some beacons activate when the lifejacket inflates; others activate on contact with water. Correct installation and arming are required. Check the combination of beacon and exact lifejacket model, and follow both manuals.

2. Waterproof is not the same as buoyant

A flotation pouch can prevent loss, but does not automatically make a beacon suitable for transmitting while floating freely. Attach it to the wearer or lifejacket as instructed in the manual. Keep the antenna and satellite reception area unobstructed and above water as specified.

3. AIS range is not a fixed distance

Waves, antenna height and the receiving installation affect range. Also check that your chartplotter clearly identifies the MOB target and that alarms can actually be heard. Receiving a position and noticing an alarm are two different things.

4. Acknowledgement is not an arrival time

Return Link Service (RLS) confirms receipt of the satellite alert and determination of the location. It does not confirm that a lifeboat or helicopter is already on its way. NFC and Bluetooth are used for checks and configuration; your phone is not the beacon’s satellite transmitter.

Installation and alert behaviour depend on the model. Sources: 6, 8, 9, 13.

Will the beacon fit your lifejacket?

The PLB3 weighs 190 grams and measures 200 × 36 × 22 mm. Check the available space, comfort and unobstructed deployment of the lifejacket and antenna. 

Differing practical experiences: Yachting World found the PLB3 a tight fit in a Spinlock Vito and noticed additional stiffness on one side. PBO fitted it into Crewsaver and TeamO lifejackets with some effort. PBO also highlighted the difficulty of interpreting the LED patterns. These are experiences with specific combinations, not evidence that every compact lifejacket is unsuitable.

Have the combination checked against both manufacturers’ instructions. Familiarise yourself with activation and status indicators beforehand, and practise operation without transmitting a real distress alert.

Dimensions and reported experiences; fit depends on the model. Reviews differ. Sources: 14, 26, 27.

ResQLink AIS and ResQLink View are different models

The ResQLink AIS (PLB-450) has AIS and RLS, but no text display or built-in buoyancy. The ResQLink View (PLB-425) has a display and built-in buoyancy, but no AIS. The View RLS is another separate version. Specifications and review findings are often confused between these models.

The display can make status information and GPS coordinates easier to read. However, buoyancy does not mean a beacon can operate without attention to antenna position. Follow the placement instructions for the specific model.

Manufacturer specifications: 5, 28. The HikingGuy review of the View (30) is not a test of the PLB-450.

Adding a PLB or lifejacket light yourself: choice and responsibility

Consumers have the right to choose an additional PLB, AIS MOB or lifejacket light for their own lifejacket. An accessory’s absence from the manufacturer’s original type approval does not automatically mean that using it in that combination is unlawful or that it must always be removed. It means that the combination has not been tested by the manufacturer. This is our interpretation of the product rules for private use; it does not imply an unrestricted right to use any combination or have it accepted by every servicing station.

Where possible, choose a lifejacket model with your preferred integrated PLB and/or lifejacket light, with manufacturer type approval covering the combination. This gives you assurance that the combination has been tested and approved against the applicable standards and is supported by the manufacturer.

If you depart from this configuration, you assume responsibility as the owner for the chosen addition and its safe use. The original type approval does not automatically cover a modified combination. Consult the instructions for both the lifejacket and the accessory, and check—or ask us to check—that the accessory can be fitted as specified without impairing the lifejacket’s safe operation.

Can a servicing station leave the accessory in place?

It may be possible to retain it where a competent station can adequately substantiate that the combination operates safely and the applicable servicing requirements, standards and conditions of use permit this. Simply observing that the accessory does not visibly obstruct anything is insufficient. Servicing requires assessment and documentation of:

• the accessory’s make, model, mounting position and attachment;
• unobstructed unfolding and inflation, accessible controls, and the absence of chafing or damage risks to the bladder;
• effects on fit, flotation position and turning performance in the water, as well as beacon accessibility, activation and antenna position;
• the instructions used, checks, any tests and the limits of the assessment.

A service inspection does not confer new type approval.

Agree on servicing and accessories in advance

A manufacturer-affiliated servicing station may make an approved configuration a condition of its service. It may state that the addition has not been tested under the product’s type approval, that it breaches the warranty conditions, and therefore remove your accessory. If, following an incident, liability is denied on the basis of an unapproved accessory being present, a direct causal link between the addition and the lifejacket’s functioning will also have to be demonstrated. The practical position is therefore more nuanced.

This is different from a general legal ban on consumers adding accessories. Before handing over your lifejacket, discuss whether your accessory can remain fitted and agree that any removal will be discussed with you beforehand.

Have your lifejacket and preferred accessory assessed together before purchase.

Conditional legal and technical explanation; not general approval of untested combinations. Sources: 31-34. 

Registration, licences and operator certificates in the Netherlands and Belgium

Beacon registration, a radio equipment licence and an operator certificate are separate matters. The country of registration and the vessel’s flag also matter: simply using a vessel in Belgian or Dutch waters does not automatically determine which registration procedure applies to you.

Costs and valid registration

In 2026, the RDI charges €50 to issue a PLB licence and €50 annually. Additional charges may apply for extra units. These are government charges, not a satellite subscription. Keep emergency contacts and beacon identification details up to date.

Which operator certificate is required for DSC?

For the DSC function of a Dutch DSC marine radio, the RDI specifies at least Marcom-B. Its certificate overview distinguishes DSC Class C from the other classes; the claim that the basic certificate is sufficient for all DSC is therefore incorrect. For a Belgian Class D DSC marine radio, BIPT specifies the SRC; an ordinary VHF certificate is insufficient.

These marine radio rules do not, by themselves, establish the certification requirement for every wearer of a personal Class M beacon. Before purchase, ask the RDI or BIPT to confirm what applies to your beacon and its use. The Dutch registration requirement for Class M has not been directly verified here against an explicit RDI publication; the registration recommendation cited comes from the KNRM.

Sources: 10, 11, 20-25, 29. Do not infer a general exemption from the absence of DSC.

Programming an MMSI: beacon identity and vessel number are different

The MOB2 has a fixed, factory-programmed identity beginning with 972. This is not a Belgian country code and is not replaced by your boat’s MMSI number. According to the manual, the MOB2 defaults to DSC open-loop operation, alerting all compatible stations within range. The vessel’s MMSI is recommended for testing and is required when configuring targeted closed-loop alerting. Check the configuration of your version.

Source: MOB2 manual, version 01.05, §3.3.

Frequently asked questions about personal distress beacons

Does an AIS MOB always alert the Coastguard?

It has no 406 MHz satellite link. A local alert may be received and relayed by vessels or suitably equipped shore stations. Do not rely on the same worldwide alerting route as a PLB.

Does my PLB replace the vessel’s EPIRB?

No. You carry the PLB on your person; the EPIRB forms part of the vessel’s equipment. A personal PLB does not fulfil an applicable EPIRB requirement.

Should I wait for RLS before doing anything else?

No. Follow your emergency procedure and the beacon’s manual. RLS does not provide an estimated arrival time or a voice connection.

How do I check my beacon?

Use only the test functions and intervals specified in your PLB or AIS MOB user manual. Check the battery expiry date, attachment, activation and registration details. Actual distress activation is not a practice test. Report an accidental alert to the Coastguard immediately.

A standard self-test and a GPS/GNSS test are not the same. ACR specifies a monthly self-test for the ResQLink View; GPS tests use more energy and are limited in number. Do not apply ‘test once a year’ as a general rule to all tests or models.

Sources: 6, 9, 10, 13 - ACR ResQLink View

Does every MOB beacon fit every lifejacket?

No. Having enough space is not sufficient. The beacon must be attachable in accordance with the manufacturer’s fitting instructions, must not impede activation or inflation of the lifejacket, and must continue to function correctly when the lifejacket inflates.

Larger beacons, such as the sMRT AU10 and certain Weatherdock easyRESCUE-PRO versions, require a suitable holder, mounting kit or lifejacket pocket for integrated installation. Placing a beacon loose between the bladder and outer cover may allow it to detach during inflation. The AU10 is also not inherently buoyant, so it may sink when the lifejacket activates if it is not correctly secured. Weatherdock easyRESCUE-PRO models are inherently buoyant, but can drift away if their connection to the wearer is lost. Correct attachment therefore remains essential.

Before installation or during servicing, have the compatibility of the exact lifejacket model, beacon and attachment checked. Could the beacon detach unintentionally, or is its operation impaired? Only use the combination once the installation has been corrected and checked.

Does a PLB work without mobile coverage?

Yes. A 406 MHz PLB sends its distress alert through the Cospas-Sarsat satellite system and does not require a mobile phone, Wi-Fi or a connected app. Follow the positioning instructions: the antenna and satellite reception area must remain unobstructed.

What is the difference between a PLB and a satellite messenger?

A PLB is designed for distress alerting. It cannot send everyday messages or let others track your journey. Depending on the model and service, a satellite messenger offers two-way messaging, tracking and SOS functionality. These services generally require an active subscription. The devices can complement each other: communication during your journey and a separate distress beacon for serious emergencies.

Do I still need a PLB if my phone has satellite SOS?

Satellite SOS can be a valuable addition, but it works differently from a PLB. Support depends on your phone model, location and service. During manual operation, you generally need to use the screen and follow instructions to establish a connection. Apple requires a clear view of the sky and horizon.

Consider whether you could operate your phone when wet, injured or with cold hands. A PLB has its own battery reserved for emergency use; accessibility and correct antenna positioning remain essential.