What happens when you press the button

A personal locator beacon (PLB) sends your identity and position to rescuers in four steps: it transmits on 406 MHz, a satellite picks up the signal, a ground station works out where you are, and a rescue coordination centre sends help. The whole chain can start within minutes of activation.

Understanding that chain helps you decide whether a PLB belongs in your safety kit, how to register it, and what it cannot do. If you are still comparing devices, start with our guide to PLB vs EPIRB vs SART vs AIS beacon.

What a PLB is and what is inside it

A PLB is a small, handheld emergency transmitter designed for one person. NOAA describes PLBs as small, personal units for hikers, paddlers and others in remote areas, activated manually and transmitting for at least 24 hours.

Most 406 MHz PLBs share the same core parts:

  • A 406 MHz transmitter that sends a short digital burst containing your beacon’s unique Hex ID.
  • A GNSS receiver on many models, which adds your position to the signal.
  • A 121.5 MHz homing signal that rescuers use for the final approach or final miles.
  • A battery sealed for years of standby, with the antenna and a strobe on many units.

You can browse marine-ready examples in our personal locator beacons and S.A.R.T.s category, including the sMRT Shield range.

From activation to rescue, step by step

Here is how the Cospas-Sarsat system handles your alert:

  1. You activate the beacon. Standard 406-only PLBs require manual activation, which is why we do not sell them. We exclusively recommend water-activated units (especially for the commercial market) that automatically begin transmitting a continuous radio signal on 406 MHz upon water contact.
  2. A satellite detects it. Search and rescue instruments on low-Earth, medium-Earth and geostationary satellites pick up the signal.
  3. A ground station processes it. The satellite relays the signal to a local user terminal, which decodes the data and calculates the beacon’s location.
  4. A mission control centre routes it. The centre matches the alert with others and forwards it to the right search and rescue authority, such as a national rescue coordination centre.
  5. The rescue centre responds. It checks the alert and, if it looks genuine, launches rescue assets. Because this satellite-assisted rescue process relies on the proximity of the local Coast Guard or SAR team, response times can take hours. This is why we highly recommend “Locally managed rescue” devices equipped with 121.5, AIS, or DSC technologies.

If your PLB is registered, its Hex ID links to your contact details and trip information, which helps responders act faster and rule out false alarms. Our guide on PLB registration and battery life shows how to register.

How rescuers pinpoint your position

A PLB gives rescuers two kinds of location help.

Satellite and GNSS position. The satellite system can calculate where the signal came from. A PLB with a built-in GNSS chip includes its own position in the alert, which NOAA says improves accuracy to about 100 metres. That is why a GNSS-equipped model is worth the extra cost.

Homing signal. Near the site, responders can home in on the beacon’s low-power analog signal, according to NOAA. This 121.5 MHz transmission lets a boat, helicopter or search aircraft steer toward you for the last stretch.

Return Link Service. Some beacons, such as the sMRT Shield, offer Return Link Service (RLS). It gives you a light or message confirming the alert was received and forwarded. NOAA notes this does not mean a rescue has launched, but it can reassure you that your signal got through. See the PLB and S.A.R.T. category for models with RLS.

Manual vs automatic activation, and life jacket integration

PLBs are manual-only, according to NOAA. You have to be conscious and able to deploy the antenna and press the button. Automatic activation belongs to other devices: Category I EPIRBs release and activate when a vessel sinks, and AIS MOB beacons switch on when they hit the water.

That is why many crews wear a personal beacon that activates itself:

  • An AIS MOB beacon with water activation alerts nearby vessels even if you are unconscious.
  • The easy2 AIS MOB is designed to sit inside an inflatable PFD, so inflation and activation work together.
  • A manual 406 MHz PLB can be stowed in a life jacket pocket so it travels with you.

For the full comparison, see PLB vs EPIRB vs SART vs AIS beacon.

What a PLB cannot do

A PLB is a one-way alert. Knowing its limits helps you plan around them.

  • It does not talk back. Without Return Link Service, you get no confirmation that help is coming.
  • It alerts a rescue centre, not nearby boats. The satellite chain takes time, and the vessel you fell from is not told automatically.
  • It needs a clear view of the sky. Satellites must see the signal, so heavy cover or a covered position can delay detection.
  • It must be registered and in date. An unregistered beacon slows verification, and an expired battery may not last 24 hours.

An AIS MOB beacon fills the first two gaps by alerting vessels in VHF range within seconds and letting your own boat steer to your position. Together, a 406 MHz PLB and an AIS beacon cover both the close-range and the global case.

Advantage of the sMRT 406 Shield

An integrated dual-frequency beacon combines the global reach of a 406 MHz satellite signal with the immediate, localized alerting of AIS and 121.5 MHz frequencies. This multi-layered approach ensures that while international search and rescue authorities receive your coordinates, nearby vessels are simultaneously alerted and can navigate directly to your position. Featuring automatic water activation, a combined unit guarantees an instant distress transmission even if you are incapacitated, eliminating the need to manually trigger an alert or attach multiple separate devices to your gear.

Do you need a PLB for boating?

If you boat offshore, sail short-handed, or go where other vessels are rare, a PLB is a strong addition to your kit, because its alert does not rely on anyone being nearby. Inshore boaters near busy waters may be well served by an AIS MOB beacon alone.

Regulations vary by vessel and flag, so confirm current carriage rules with the U.S. Coast Guard or your national authority. Whatever you choose, register the beacon and keep its battery in date. Compare models in our personal locator beacons and S.A.R.T.s category, or use our free marine safety consulting to build the right layers for your boat.

Frequently asked questions

How long does a PLB transmit after activation? At least 24 hours, according to NOAA. Check your model’s manual for its rated operating time.

Does a PLB need a subscription? No. 406 MHz beacons use the Cospas-Sarsat system, and registration is free through NOAA.

Can a PLB work without GPS? Yes. The satellites can still calculate a position from the signal, but a GNSS-equipped PLB gives a more precise one, around 100 metres per NOAA.

What is the difference between a PLB and an AIS MOB beacon? A PLB sends a satellite alert to a rescue centre. An AIS MOB beacon alerts vessels near you. See our beacon comparison.

Can I test my PLB? Follow the self-test procedure in the manual and never trigger a live alert. Self-tests also use battery, which we cover in PLB registration and battery life.

Who services PLBs? We are an authorised service centre for sMRT and WeatherDock PLBs. See our service and inspection page.

Know the chain before you need it

A PLB works by passing your signal from beacon to satellite to ground station to rescue centre, with your GNSS position and homing signal guiding the final approach. It is a powerful backstop when you are far from help.

Ready to equip your crew? Explore our personal locator beacons and S.A.R.T.s or book a free consultation.