How Do PLBs Work

Personal Locator Beacons, commonly called PLBs are small handheld devices which, when activated will send an emergency message to Search and Rescue Forces if you get into an actual emergency situation.

PLBs are a fairly new addition the safety tool box which everyone who spends time outdoors, especially those who do risky activities away from civilization, should carry.

A Personal Locator Beacon when activated sends out an emergency signal which can be picked up by a worldwide satellite system, which then forwards the message to ground receivers and the appropriate Rescue Control Center (RCCS).

These RCCs will then dispatch Search and Rescue (SAR) Teams to assist you. PLBs should only be activated in an actual emergency situation as rescue workers will come looking for you.

The purpose of this article is to outline, in simple terms, just how it is that when you  push the button on your PLB, that a Search and Rescue (SAR) Team will come looking for you at your location.

The Personal Locator Beacon

With the push of a button, you are able to send a call for help to SAR Teams. What an amazing and potentially lifesaving ability. But how does this happen?

PLBs as well as their aviation (ELT) and Marine (EPIRB) counterparts, when activated send digital a signal on 406 Mega Hertz (MHz) to the SARSAT Search and Rescuer Satellite system. This 406MHZ encoded data burst can send various bits of information (depending on the type of beacon you have) which the SARSAT system picks up and decodes.

Personal Locator Beacons come in different varieties:

  • Very Basic PLBs which only have the 406MHz transmitter – These are the least expensive type
  • Some models have the 406MHz transmitter  plus a 121.5MHz “homing” transmitter – This allows rescuers to home in on your location once they are nearby
  • Some PLBs have an internal Global Positioning System (GPS) receiver – this allows the PLB to transmit your actual location with the distress message
  • The best models have all of these functions!
  • There are other options offered on some brands which while cool, do not affect the actual operation of the beacon in normal conditions. Options like – waterproofing, an internal strobe light, belt clips, etc.

The Satellite System

When you activate your PLB it sends out its signal on 406MHz (that is a frequency, like the different radio station frequencies on your car radio). There is an international system of satellites orbiting the earth listening for these signals.

The Search and Rescue Satellite System, or SARSAT System is a collection of receivers on various types of satellites. These satellites have many other devices onboard and perform many functions, but the only one we are concerned with is the SARSAT package.

I’m going to have to get a bit technical here on the different types of satellites, so bear with me this is important to how quickly someone will be able to assist you in an emergency. The speed of your rescue depends on both the type of PLB you’ve purchased and the type of satellite which picks up your signal. If you want to skip most of this section, make sure you read the KEY POINTS!

At the time I’m writing this there are 2 types of SAR Satellites, Low Earth Orbit and High Earth Orbit. A third, Mid Earth Orbit is in the works.

  • Low Earth Orbit (LEO) Satellites 
    • LEO Satellites orbit the earth in what is considered a “low orbit.”
    • LEO Satellites are in “polar orbits”, meaning they fly at a right angle to the earth’s equator, so on one orbit they fly northwards and then over the north pole and continue on southwards until they fly over the south pole and continue on northwards again.
    • As the LEO Satellite is orbiting south to north – north to south, the earth is spinning below the satellite. So the satellite flies over a different ground path on every orbit.
    • There are currently 6 LEO Satellites in the SARSAT System.
    • Because of their altitude LEO Satellites “view” a small area as the sweep over the earth.
    • LEO Satellites receive distress signals and transmit them to the ground stations if they are in view. They then store the message and transmit it to other ground stations as they pass over.
    • KEY POINT – Because LEO satellites move in relation to the earth below they can use Doppler Shift computations to determine the point on the earth from which a distress signal it is receiving is coming from. Simply, they have a way to determine where your PLB’s emergency signal is coming from.

 

  • High Earth Orbit (GEO) Satellites
    • High Earth Orbit satellites are placed in what is called Geostationary Orbits (called GEO for geostationary).
    • GEO Satellites sit at a height at which the speed they orbit keeps them always stationary over the same geographic point – Geo Stationary.
    • GEO Satellites orbit over and parallel to the equator.
    • There are currently 8 GEO Satellites in the SARSAT System, a 9th will come online soon. Only 4 are kept fully operational an one time.
    • Because of their altitude GEO Satellites “view” a very large area of the earth that that they are parked over which is always in view. This is why only 4 are needed.
    • GEO Satellites receive distress signals and immediately transmit them to the ground stations in view.
    • There is a GEO Satellite parked where it can view the entire United States.
    • KEY POINT – Because GEO satellites DO NOT move in relation to the earth below they CAN NOT use Doppler Shift computations to determine the point on the earth from which a distress signal it is receiving is coming from. Simply, they DO NOT have any way to determine where your PLB’s emergency signal is coming from.

 

How the PLB and the satellite work to together

As I mentioned before, the type of Personal Locator Beacon you buy could affect the speed at which rescue forces begin to head out to help you.

Why is this? Read on…

Remember there are different kinds of Personal Locator Beacons based on what features they have. The two that matter for this topic are PLBs without GPS capability and PLBs with GPS. And there are two kinds of SAR Satellites, LEO and GEO.

First, let’s discuss a PLB without GPS capability.

If you become injured or stranded in an emergency situation and activate your distress beacon without GPS the GEO satellite which covers your area will pick up the signal. But all a PLB without GPS sends is its identifier.

The GEO satellite has no way to determine where the PLB is located. So all rescue forces will know is that that beacon was activated but not where it is. Hopefully you’ve registered your PLB and some contact information is also sent with the distress message to help determine where you might be.

Without knowing your location there is nothing Search and Rescue forces can do!

They have to wait for the next part of the system, the LEO Satellites, to come into play. LEO satellites can determine your beacon’s location, but because of orbital mechanics it could be up to an hour and a half before one flies into a position where it can hear your PLB. Any response to your call for help could be delayed that long!

Also determining your location in this manner is not very accurate and it could take several passes from different satellites to get a valid usable location.

Now, let’s discuss a PLB with GPS capability.

In the same scenario, once you activate your distress beacon with GPS the GEO satellite which covers your area will pick up the signal. But this time your PLB sends not only its identification, it also sends your GPS location.

Rescue workers can then easily make a quick determine if you potentially are in distress. For instance, if you are located in a wilderness area, then you most likely are having an actual problem. But if you are located in a parking lot outside an Outdoor Man Store then you most likely turned your PLB on to “test it” without having read the directions.

A LEO satellite picking up your signal will also receive your GPS location.

So as you see having a Personal Locator Beacon with GPS capability can greatly improve your chances of being rescued in a timely manner. And let’s face facts, when you are hitting your emergency button on the PLB, asking for someone to help you, time most likely is of the essence.

This site recommends that you buy a PLB with GPS capability! Don’t try to save a few bucks, you life is worth much more!

Register your PLB

 

Check out the PLB Selection

 

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