ELT-Emergency Locator Transmitter

Introduction:

ICAO defines an Emergency locator transmitter (ELT) as equipment which broadcasts distinctive signals on designated frequencies and, depending on application, may be automatically activated by impact or be manually activated. 

Distress radio beacons, also known as emergency beacons, PLB (Personal Locator Beacon), ELT (Emergency Locator Transmitter) or EPIRB (Emergency Position-Indicating Radio Beacon), are tracking transmitters which aid in the detection and location of boats, aircraft, and people in distress.When manually activated, or automatically activated upon immersion, such beacons send out a distress signals. The signals are monitored worldwide and the location of the distress is detected by non-geostationary satellite, and can be located by some combination of GPS trilateration  and Doppler triangulation.

Purpose:

The basic purpose of a distress radio beacon is to help rescuers find survivors within the so-called "golden day"(the first 24 hours following a traumatic event) during which the majority of survivors can usually be saved.

Different types of ELTs are currently in use. There are approximately 170,000 of the older generation 121.5 MHz ELTs in service. Unfortunately, these have proven to be highly ineffective. They have a 97% false alarm rate, activate properly in only 12% of crashes, and provide no identification data. In order to fix this problem 406 MHz ELTs were developed to work specifically with the Cospas-Sarsat system. These ELTs dramatically reduce the false alert impact on SAR resources, have a higher accident survivability success rate, and decrease the time required to reach accident victims by an average of 6 hours

Operation:

The following is the process through which a transmission usually gets processed:

1.    The transmitter is activated, either automatically in a crash or after sinking, or manually by survivors of an emergency situation.

2.    At least one satellite picks up the beacon's transmission.

3.    The satellites transfer the beacon's signal to their respective ground control stations.

4.    The ground stations process the signals and forwards the data, including approximate location, to a national authority.

5.    The national authority forwards the data to a rescue authority

6.    The rescue authority uses its own receiving equipment afterwards to locate the beacon and commence its own rescue or recovery operations.

Once the satellite data is in, it takes less than a minute to forward the data to any signatory nation.

How Different ELTs Activates:

An ELT may take any of the following forms:

§  Automatic fixed ELT (ELT(AF)). An automatically activated ELT which is permanently attached to an aircraft.

§  Automatic portable ELT (ELT(AP)). An automatically activated ELT which is rigidly attached to an aircraft but readily removable from the aircraft.

§  Automatic deployable ELT (ELT(AD)). An ELT which is rigidly attached to an aircraft and which is automatically deployed and activated by impact, and, in some cases, also by hydrostatic sensors. Manual deployment capability is also provided.

§  Survival ELT (ELT(S)). An ELT which is removable from an aircraft, stowed so as to facilitate its ready use in an emergency, and manually activated by survivors.     

How ELT Works: 

A simple Block diagram of Type-W ELT is shown

The supply is connected by means of a water switch.The unit shown only provides outputs at VHF ( 1 2 1 .5 MHz and 243 MHz). These two frequencies are harmonically related which makes it possible to generate the 243 MHz signal using a frequency doubler stage.

Cospas-Sarsat Satellites:

The International Cospas-Sarsat Programme is a satellite-based search and rescue (SAR) distress alert detection and information distribution system, established by Canada,France, US, and the former Soviet Union in 1979. It is best known as the system that detects and locates emergency beacons activated by aircraft, ships and back country hikers in distress.

The system uses satellites and ground stations to detect and locate signals from ELT operating at frequencies of 121.5 MHz, 243 MHz and/or 406 MHz. The system provides worldwide support to organizations responsible for air, sea or ground SAR operations.

 The basic configuration of the Cospas–Sarsat system features: 

• ELT that transmit VHF and/or UHF signals in case of emergency 
• Instruments on board geostationary and loworbiting satellites detecting signals transmitted by the ELT
• Local user terminals (LUT), which receive and process signals transmitted via the satellite downlink to generate distress alerts
• Mission control centres (MCC) which receive alerts from LUTs and send them to a Rescue coordination centre (RCC) 
• Search and rescue (SAR) units.  

Reference:

http://www.skybrary.aero/index.php/Emergency_Locator_Transmitter_%28ELT%29