How Does An EPIRB Work?

An EPIRB (that stands for – Emergency Position Indicating Radio Beacon) is designed to activate search and rescue services (SAR) in an emergency at sea and provide sufficient data that is necessary to conduct the rescue operation.  

EPIRBs At a Glance:
1️⃣ The system provides a quick and accurate location of the incident, enabling the most appropriately situated rescue services to be tasked.

2️⃣ EPIRBs are part of a family of rescue locating devices, including Maritime EPIRBs (Emergency Position Indicating Radio Beacons), Aviation ELTs (Emergency Locator Transmitters,) and Remote environment personal use PLBs (Personal Locator Beacons.)

3️⃣ EPIRBs are required to be carried on the following vessels.

4️⃣ Recreational boats that navigate over 50 miles offshore.

5️⃣ All fishing boats that operate 6 miles are required to install a “Category 1” EPIRB that activates automatically.

6️⃣ Charter boats that travel 12 miles or more or have space to carry 12 or more passengers must install a “Category 1” EPIRB that activates automatically.

7️⃣ Any vessel must comply with the Global Maritime Distress and Safety System (GMDSS.)

📡 Purpose Of An EPIRB 

There are two categories of EPIRBs:

  • Category One uses a hydrostatic switch that releases when it comes into contact with water and automatically starts the distress transmission.
  • Category two can only be activated manually by depressing a switch.

The system forms part of a rescue chain that starts with radio communications between the vessel and the relevant authorities.  

The use of the EPIRB is not the first link in the chain and will generally only be activated (manually or, in some cases, automatically) when the emergency situation prevents the use of the radio.

The distress signal emitted by the EPIRB includes the unique serial number (called a hex code) of the craft.

When cross-referenced with the computer database, it allows the rescuers to learn the following:

  • The type of beacon (EPIRB, ELT – Emergency location device found on aircraft, PLB – Personal Locator Beacons.)
  • The country code of the vessel’s registration.
  • The name of the vessel.
  • A description of the vessel type.
  • The vessel’s home port.
  • Other information that is useful to the rescuers.
  • The emergency contact phone details of the vessel that is in distress.

In addition to the hex codes, the transmission also contains either a 122-bit or 144-bit message containing a field (49-bit) that includes identification and position information (if fitted with a GPS.) 

This enables the rescue coordinators to contact the next of kin and inform them of the situation.

It also allows the rescue services to find out more information from the next of kin, including where the boat was heading, the rescue equipment onboard, and the number of passengers.

The rescuers can then activate the appropriate rescue resources with a more complete picture.

The EPIRB transmitter units are designed to float in water and continue transmitting the distress signal for 2 days (48 hours.)

🚨 How An EPIRB Sends Out A Distress Signal

The EPIRB device is required to transmit two signals as follows. These are just two types of marine emergency beacons.

Distress Signal On 406 MHz

The most powerful transmitter sends a signal at a frequency of 406 MHz. This signal is received by three satellite systems in different orbits.


The COSPAS-SARSAT satellite constellation is in a geostationary orbit above the equator.

Because a satellite is moving in line with the Earth as it rotates, it appears stationary when viewed from the Earth.

COSPAS -SARSAT satellites (GEO) are placed at 22,300 miles (35,800km.)

Medium Earth Orbiting (MEO)

These are placed at altitudes between 1,000 miles and 22,000 miles. A satellite that is positioned at 12,600 miles (20,200 km.) 

Since 2004 the COSPAS-SARSAT has been including emergency receivers as part of the payload on MEO satellites. These have included the following satellite systems.

  • The European Galileo Navigation Satellite Systems
  • The Russian GLONASS Navigation Satellite Systems.
  • The USA GPS Navigation Satellite Systems.
  • The Chinese BDS Satellite Systems, also known as “BeiDou.”
  • Weather satellites.

Some MEO satellite orbits are perfectly circular, while others have an elliptical shape called a “ball of yarn” orbit. 

These satellites are non-geostationary orbits and will orbit the Earth at least twice daily.

Because their rotation speed is different from the Earth’s, the Doppler effect can be used to calculate the location of the transmitting device.

Low Earth Polar Orbiting (LEO)

The LEO satellites are placed in a low earth polar orbit, which means they rotate in a north/south direction around the earth.

Because they move in a direction different from the earth’s rotation, these are called non-geostationary orbits. 

The benefit is that the system can calculate the location of the EPIRB by calculating the direction of the signal using the Doppler effect.

Location Signal Transmitting On 121.5mhz

A lower-powered location signal that is transmitted on 121.5MHz is sent out by the smaller transmitter and is used by the rescuers to locate the EPIRB with direction-finding equipment.

🔧 EPIRB’s Components And Technology

There are two categories of EPIRB.

Category One EPIRBs

Category One EPIRBs include a hydrostatic switch. When this device is exposed to water, it automatically releases and starts the unit transmitting, and the light flashes.

These devices need to be installed in a dry area where they will not be rained on or exposed to seawater, which will activate them erroneously.

Category One EPIRBs also have a switch that activates the unit manually.

Category Two EPIRBs

Category Two EPIRBs do not have a hydrostatic switch and can only be activated manually.

The Parts Of An EPIRB

EBIRPs contain the following components:

Radio Transmitter5-watt
Second, Lower-Powered Transmitter0.25-watt
Confirmation LEDPresent if the device is RLS enabled
Manual Activation SwitchFound on category 1 and 2 devices
Hydrostatic SwitchReleased when it comes into contact with water (Only Category I devices)
Protective HousingEquipped on Category One EPIRBs
AntennaStandard component
Integrated GPS SystemPresent in newer units
GPS AntennaPresent in newer units
12 Volt BatteryShould be replaced every 2 to 4 years
Test ButtonUsed to check that the EPIRB is operational and ready
Strobe LightStarts flashing when the unit is activated, used to assist the rescue services in seeing it
LanyardCan be used to attach the device to the life raft or Lifejacket

The transmitters send the distress signal on 406 MHz.

A location signal transmitting on 121.5MHz is also sent out by the smaller transmitter and is used by the rescuers to locate the EPIRB with radio direction finders (RDF.)

🛰️ How Is The Distress Signal Picked Up By Satellites 

The emergency location search and recovery system consists of several parts:

1️⃣ The EPIRB device that sends the distress signal.

2️⃣ The satellite network that receives the distress signal.

3️⃣ A Local User Terminal (LUT) located in 38 countries that process the signal.

4️⃣ Mission control centers (MCC) that distribute the signal to RCCs.

5️⃣ Rescue Co-ordinating Centers (RCC).

The EPIRB transmits a 5-watt distress signal on 406 MHz every 50 seconds for 48 hours.

The transmission includes a message that contains important data for the rescue services. If the unit has a GPS, it also includes the location data.

🌍 How Is The EPIRB’s Location Tracked By Using Satellites?

Two systems are employed by the system to obtain the location of the transmitting EPIRB:

Method One – GPS

If the unit is equipped with a GPS, the latitude and longitude are included in the transmitted message.

The GPS location is accurate to within 100 yards.

Method Two – Doppler Effect

When the distress message does not include GPS coordinates, the Local User Terminal (LUT) that receives the redirected message from the satellite can calculate the position to within 1 to 3 miles using the Doppler Effect.

Although a little more complex, this method essentially involves locating where the satellite was when the signal was first received and where it had moved to when it was the strongest. 

Using this data with the known height and speed of the satellite enables the LUT to triangulate the position of the transmitting device.

Activation Of An EPIRB

The following chain of events begins when an EPIRB is activated.

1️⃣ The EBIRP transmits the signal that is received by one, or many, satellite receivers.

2️⃣ On newer model EPIRBs that are RLS-Enabled, the satellite sends a confirming signal to the EPIRB confirming that the distress signal has been received.

3️⃣ This activates a confirming light on the EPIRB, comforting the survivors that their situation has been reported.

4️⃣ Simultaneously, the satellite forwards the distress signal to one of the 38 Local User Terminals (LUT) located around the world.

5️⃣ The LUT decodes and processes the signal, calculates the location, cross references the registration data from the database, and sends this to the nearest Mission control center (MCC).

6️⃣ The MCC matches other messages received from the different satellites and  LUTs, sorts the data geographically, and forwards the now useable distress message to the nearest national Rescue Coordination Center (RCC.)

7️⃣ The RCC proceeds with the rescue using whatever resources are needed.

8️⃣ When the Search and Rescue teams arrive at the search area, they use radio direction finders to locate the vessel using RDF equipment that receives the transmission on 121.5MHz.

9️⃣ The rescuers then look for the flashing light (at night) or the survivors as they follow the RDF “beam.”

🔋 Battery Life Of An EPIRB

To be called an EPIRB, the battery must be able to power the two transmitters, the RLS receiver and light, the GPS, and the flashing light for 48 hours.

How To Maintain And Replace The Battery 

The battery of most units needs replacing every 2 years.

Batteries can only be changed at an authorized service center.

When the unit is serviced, the following are actioned.

  • The batteries are replaced.
  • The water integrity of the unit is checked.
  • The transmitters are checked in a shielded environment (to prevent the signal from being received by a satellite.)

📶 Signal Range Of An EPIRB

The different orbit altitudes, types, and “shapes” ensure that there is complete coverage around the globe.

EPIRB Signals are received by satellites operating as far away as 22,300 miles (35,800km) above the Earth.

The signal will be in the range of one of the satellite constellations within at least 2 hours.

Factors That Affect EPIRB Signal Range EPIRB In Remote Locations

All EPIRB units must be powerful enough to transmit the distress signal regardless of sea states, weather, or cloud cover conditions.

While the COSPAS-SARSAT satellites cannot receive a signal within 20 degrees of the North and South Poles, the Low Earth Polar Orbiting (LEO) satellites pass over these areas every 100 minutes.

Essentially, once the unit is operating properly, its signal will be received by one or many satellites, irrespective of where it is and what the weather conditions are. 

🗝️ Key Takeaways

Whether they have an automatic or manual activation capability, these essential devices (known as EPIRBs) send a distress signal received by the satellite networks that put into motion a vast rescue network around the globe.

They can literally make the difference between survival or not.

Among the emergency equipment required on many vessels, EPIRBs are among the most important.

The other important emergency equipment should all be carried in the ditch bag of a vessel.

Now that you know how an EPIRB works, you might want to understand the differences between an EPIRB and a PLB.

I’m the founder and chief editor here at Sailing Savvy. I spent a decade working as a professional mariner and currently, I mix those experiences with digital publishing. Welcome, and I hope that we can be the hub you need for safe passage.