Chapter 8

ELT

Emergency Locator Transmitter

ELT History

In 1972, a law was passed requiring aircraft to carry a radio beacon for search and rescue.

This beacon operates on 121.5 MHZ, VHFAnd 243 MHZ VHF (Military).

New, Improved ELTIn 1995, tighter regulations called for better maintenance, and improved design.

Now, ELT is monitored by satellites, and signals are sent to ground stations.

The satellites measure the “Doppler shift” in the signal to locate the beacon (accurate to within 1-3 miles).GPS is being introduced, and it’s even more accurate than the “Doppler shift” method (accurate to within 300 feet).

2 countries’ satellites monitor ELT: SARSAT, operated by NOAA (US) and COSPAS by Russia.

Textbook page 51

http://www.physics.ucla.edu/demoweb/demomanual/acoustics/effects_of_sound/doppler_shift.html

Doppler Shift

Cospas-Sarsat System

There are 2 types of satellites in the Cospas-Sarsat System: LEO & GEO.

LEO (Low Earth Orbit), moves quickly across the sky and circle over the poles however will not pick up distress calls if they aren’t in the line-of-sight.

GEO (Geostationary Earth Orbit) are “stationary” above the earth at the equator.

Cospas-Sarsat Diagram

Textbook page 55

406 ELT Registration

In order to prevent false alarms, each 406 MHZ ELT must be registered.The data collected is: Aircraft Type, Ownership, Telephone Number, Home Base, etc.This data allows for confirmation of the distress call.

406 MHZ ELT Block Diagram

Textbook page 53

ELT for Fleet Operators

• Fleet Operators (such as airlines) need to be able to move the ELT components between aircraft.

• Therefore, they would need a way to reprogram the tail number code.

• The “dongle” solves this problem.

• The “dongle” is a hardware key that automatically reprograms the code, it remains with the aircraft and the ELT is removed if needed elsewhere.

ELT Components• Here is the Artex G406-2 ELT

system.• When a crash activates the ELT

system, through a G-Switch, a varying audio tone is transmitted.

• The ELT must be mounted in alignment of the direction of flight for the G-Switch to function correctly.

• The ELT can be activated manually by a switch in the flight deck.

• The antenna’s are selected according to the aircraft’s speed, the whip for slower aircraft and the rod for high-speed aircraft (> 350 KT).

Textbook page 52

Review Q&AChapter 8 ELT

8.1 What 3 radio frequencies are sent out by an ELT during a distress call?Answer: 121.5, 243 and 406 MHZ8.2 Why must an ELT be mounted in line with the direction of flight?Answer: In order for the G-Switch to function properly.8.3 Name the satellites that pick up and relay ELT signals.Answer: LEO (low earth orbit) and GEO (geosynchronous earth orbit)8.4 Name one method satellites use to locate a downed aircraft transmitting an ELT signal.Answer: Doppler Shift or GPS8.5 Where do satellites relay the location of downed aircraft?Answer: To a Ground Station8.6 What is the most accurate method for identifying the location of an ELT signal, as used in the 406 MHZ system?Answer: The GPS Method8.7 What is the main benefit of registering ELT’s giving aircraft ID, and ownership?Answer: To confirm that an actual emergency exists before dispatching search and rescue teams.8.8 How accurately can searchers locate a 406 MHZ ELT coupled to a GPS source?Answer: To within 300 feet.