How a PLB works?

Background information on how a PLB sends a signal

Live as if you were to die tomorrow.
Learn as if you were to live forever. Mahatma Gandhi

Understanding how a PLB works will help you think through where and when to activate the device.

All distress beacons (PLBs, EPIRBs and ELTs) send signals on dedicated world-wide frequencies. The signals are received by the COSPAS SARSAT satellites, the international satellite system for search and rescue (SAR).

Image courtesy of http://beacons.amsa.gov.au/about/how-they-work.asp

Image courtesy of http://beacons.amsa.gov.au/about/how-they-work.asp

Using the sequence in the diagram above:

  1. A distress signal is transmitted from a PLB/EPIRB/ELT.
  2. The signal is received by the COSPAS SARSAT satellites.
  3. The satellites send the signal to the nearest receiving stations on earth known as Local User Terminals (LUTs).
  4. A Local User Terminal forwards the message to a Mission Control Centre (MCC).
  5. When a signal is received and identified to be a distress call from Australia the Rescue Coordination Centre (RCC) in Canberra is alerted.

The appropriate emergency service for the distress call is then activated. In the case of bushwalkers this is a land rescue and therefore the Police is the responsible authority. The Police would activate emergency services as required to locate and rescue the walker in distress – usually a rescue helicopter.

In the Australian search and rescue region there are three LUTs located at Albany (WA), Bundaberg (QLD) and Wellington (NZ) that are controlled by the MCC located within the Australian Joint Rescue Coordination Centre (JRCC) in Canberra.

For a video presentation of the rescue process please check out this youtube video by ACR Artex.

Locating the PLB How the signal is located

(Information adapted from http://www.sarsat.noaa.gov/faq%202.html)

A satellite in a geostationary orbit is at an altitude of 22,300 miles (35,890 kilometers). The satellite moves in a circular orbit in the equatorial plane around the Earth at the same speed that the Earth rotates. Because of this, it appears to remain over a fixed point on the Earth’s surface. This position is ideal for making uninterrupted observations of the weather or environmental conditions in a given area. This same principle allows it to monitor for 406 MHz distress beacons. However, satellites in geostationary orbit cannot see the polar regions of the world.

A low earth/polar orbit allow the satellites to observe the entire Earth’s surface as it rotates beneath it. Most of these orbits are at an altitude of 500 miles (800 km) and take about 100 minutes to revolve around the earth. The sun-synchronous orbit is a special case of a polar orbit with inclination of 98.7 degrees, that precesses at exactly the required rate (~ 1 degree per day) to remain in the same local time plane as the Earth rotates around the sun. Satellites in polar orbit provide emergency beacon users with global coverage (including the Polar Regions).

There are three ways that a PLB can be located by authorities:

  1. By the device transmitting the GPS location.
  2. Satellites calculating location using Doppler effect.
  3. The use of the homing signal by aircraft or ground crew.

If a 406 MHz emergency beacon is equipped with a Global Position System (GPS) receiver, the digital message transmitted by the beacon can contain the GPS-generated position of the beacon. This is a common feature of modern PLB devices. Once the signal has been picked up by satellites, emergency services can dispatch to the exact location quickly.

In the absence of GPS coordinates being transmitted, Low Earth Orbiting Search and Rescue (LEOSAR) satellites can compute a location for a 406 MHz emergency beacon using a method called “Doppler shift”. When the beacon is moving toward or away from the satellite track due to the Earth’s rotation, the frequency shift induced by that motion can be used to compute location. Computing a location using Doppler shift requires the satellite to be moving, that is, the satellite cannot be in a geostationary orbit (GEOSAR).

Because their geostationary orbit does not provide a relative motion between a distress beacon and a GEOSAR satellite, there is no opportunity to use the Doppler effect to calculate the location of a beacon. Therefore, the GEOSAR satellites only can relay a beacon’s distress message. The LEOSAR satellites also provide global coverage, including the Polar Regions, for 406 MHz emergency beacon detection.

Many 406 MHz emergency beacon are also equipped with a 121.5 MHz ‘homing’ frequency, what aircraft overhead can use to locate the device. This is a separate signal to the 406 MHz distress signal, and can also be used by land crew to locate the device.

Timing How long does it take to be rescued?

Prepare for a long wait….

The time it takes for search and rescue personnel to reach you depends on a number of factors, including the weather, terrain and accessibility of your location. The more remote the location of the distress incident, the longer the response time. Whether your PLB is registered and GPS equipped also play a very important role.

Checklist after activation Dos and Don’ts

Do stay with the PLB as this is the location that the emergency services will use to locate the casualty. Wandering off attempting to find your way home will only delay rescue.

Don’t turn off the PLB, even if you no longer need rescuing. If the signal fails, the rescue team may assume your PLB battery has expired and commence a blind search. If you triggered the PLB by mistake or no longer need assistance do your best to contact emergency service and let them know what happened, they will direct you to switch it off.

Do leave the PLB on until directed to turn off by the emergency services. If you have not been located yet then leave it on until the battery goes flat.

In a group
If you are in a group, do not activate more than one beacon at a time. When the first beacon’s battery goes flat then turn it off and activate the second one. You will not get a stronger signal by activating two beacons at the same time – the transmissions actually interfere with one another and make it harder to find you. The battery life of a PLB is more than 24 hours, it is unlikely that rescue will take that long to arrive.

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