ELT-EMERGENCY LOCATER TRANSMITTER full report
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A TECHNICAL PAPER PRESENTATION ON
MEDURI SHALEM RAJ
Introduction to the ELT
Types of ELT
General block diagram
Internal structure of ELT
Role of GPS in ELT
Location by Doppler
Actual system operation
Broad specifications of ELT
Classification of the ELTs as per standards
Advantages of ELT
Leading manufacturers of ELT
The Emergency Locator Transmitter (ELT) is used to emit an emergency signal on 121.5 MHz, 243MHz and 406.025 MHz in the event of an aircraft crash. This signal will be picked up by the Search and Rescue Satellites (SARSAT) and processed. SARSAT then sends the information about the crash and location of the crash to the owner of the aircraft.
This ELT has to withstand very stringent environment conditions. The system is available from number of reputed manufacturers at a cost of less than $5000
ELTs are emergency transmitters that are carried aboard most general aviation aircraft in the U.S. In the event of an aircraft accident, these devices are designed to transmit a distress signal on 121.5, 243.0-megahertz frequencies (and for newer ELTs, on 406 MHz).
This transmission is enabled automatically in the event of aircraft crash using G switch or by manual activation. With the use of ELTs the search and rescue teams can easily pin point the exact location of downed Aircraft.
Types of ELTs:
Two frequency Beacon systems: - Transmits distress signal on 121.5MHz & 243 MHz when the unit is activated by the force of crash (using G-switch) or by cockpit mounted remote switch. Accuracy of this system is limited to appx. 20 Kmâ„¢s.The major disadvantage of the system is there is no specific information about the owner or the aircraft.
406 MHz Beacon system: - operates on the latest Search and Rescue (SAR) frequency 406.025 MHz in addition to the conventional 121.5MHz and 243 MHz. With this 406 MHz ELT accuracy can be as high as 500 meters. This ELT system transmits digital message about the aircraft serial number, nationality, tail number, operator designator etc. This data will be received by Search and Rescue Satellites (SARSAT) and sends back to Ground Station. This information is decoded by ground computers and sends to Owner of the aircraft regarding the location .This can also be interfaced to Navigation System (GPS/ Loran etc) by which it will transmit last received Latitude and longitude data. This system facilitates an aural monitor to alert the pilot if it is accidentally activated. This unit does not take supply from aircraft. Internal battery is included in this unit.
GENERAL BLOCK DIAGRAM:-
The system consists of the following items
1) Main ELT unit
2) Remote control panel for manual activation and indication of Transmission
4) Coaxial cable for interconnections
INTERNAL STRUCTRE OF ELT:-
ROLE OF GPS IN ELT:
The Emergency Locator Transmitter (ELT) is used to emit an emergency signal regarding the crashed air craft. In order to provide the global positioning for the search and rescue (SAR). We require a system to provide the longitudinal and latitudinal positions which is named as GPS.
CAN YOU IMAGINE THE ELT WITH OUT GPS?
Location by Doppler
When the beacon has no GPS receiver, the system locates the beacon from its Doppler shift as received by the quickly-moving satellites. Basically, the frequency received varies depending on the speed of the beacon relative to the satellite. The amount of Doppler is proportional to the range and bearing to the satellite. The instant the beacon's Doppler shift changes from high to low indicate the time when the bearing from the beacon to the satellite's ground track is 90 degrees. The side of the satellite track is determined because the rate of change of the Doppler is faster when the Earth is turning towards the satellite track.
In order to handle multiple simultaneous beacons, modern 406 MHz beacons transmit in bursts, and remain silent for a few seconds. This also conserves transmitter power.
The Russians developed the original system, and its success drove the desire to develop the improved 406 MHz system. The original system is a brilliant adaptation to the low quality beacons, originally designed to aid air searches. It uses just a simple, lightweight transponder on the satellite, with no digital recorders or other complexities. Ground stations listen to each satellite as long as it is above the horizon. Doppler shift is used to locate the beacon(s). Multiple beacons are separated when a computer program performs a Fourier transform on the signal. Also, two satellite passes per beacon are used. This eliminates false alarms by using two measurements to verify the beacon's location from two different bearings. This prevents false alarms from VHF channels that affect a single satellite. Regrettably, the second satellite pass almost doubles the average time before notification of the rescuing authority. However the notification time is much less than a day.
WORKING: When an aircraft is crashed the ELT which is present in that aircraft transmits some distress signals to the satellite. The ELT system is activated by the G-switch (gravity switch).The acceleration of the flying aircraft is equal to the acceleration due to gravity. During this stage the switch is in OFF state and ELT does not transmit any signals. When the air craft is falling the acceleration is exceeds acceleration due to gravity which tends to change the state of the switch i.e, the switch becomes ON and the ELT transmits distress signals regarding position and owner of the aircraft to the satellites. Satellite sends these signals to the LOCAL USER TERMINALS (LUTs) .LUTs tracks these signals and recovers them. These perform bit error checking and correction and send the corrected signals to the mission control centers. Mission control centers match signals coming from the same source also improves accuracy by merging. It filters the redundant data and identifies the identifies the appropriate recepient.Finally it transmits alert messages to the search and rescue (SAR) team.
Broad Specifications of ELT:-
The transmitted Peak Effective Radiated Power (PERP) shall be not less than 75 mW at each carrier frequency (121.5 MHz /243 MHz).
Minimum duty cycle 33% and maximum duty cycle of 66%
Amplitude modulation factor shall be at least 0.85
ON power of 0.75 sec followed by OFF period of 1.5 Sec gives satisfactory results.
406 MHz ELT transmission is 450 mSec digital pulse every 50 Sec
In 406 MHz ELT mode , 121.5 /243 MHz should be selected to automatically cover the 49.5 Sec lapse time between two digital pulses
406 MHz Transmitted power shall be 5W +/- 2dB (35 dBm to 39 dBm)
Classification of the ELTs as per standards:-
Sl No Parameter FAA TSO C91 FAA TSO C91A FAA TSO C126
1 Environmental conditions Minimal Severe More stringent
2 Indication of Tx
Pilots intervention Not available
3 Transmitting frequencies 121.5 MHz
243 MHz 121.5 MHz
243 MHz 121.5 MHz
4 Appx Accuracy 20 Kms 20 Kms 500 mtrs
Advantages of 406 MHz system over Traditional 121.5 / 243 MHz system:-
1) Search And Rescue (SAR) organization do not respond as quickly to a 121.5 MHz ELT alerts as they do to a 406 MHz alert . Because of the large number of 121.5 MHz ELT false alerts, the common practice is to wait for either a confirmation of an alert by additional satellite passes or through confirmation of an overdue aircraft or similar notification.
2) By the year 2009, the international COSPAS-SARSAT satellite system will no longer provide satellite based monitoring of the 121.5 MHZ /243 MHz frequency signals. After the date of the satellite termination , in 2009 , 121.5 MHz signals transmitted from ELTs operating on the lower frequency will only be detected by ground based receivers such as local airport facilities or air traffic control facilities or by overlaying aircraft
3) 406 MHz transmission signal is almost instantaneously detected by geo- stationary satellites. This signal can be coded with following information
a) serialized number
b) aircraft operator designator with serialized number
c) aircraft nationality and identification marking (tail number)
4) In addition to the above information 406 MHz ELT can transmit an aircraftâ„¢s last known position in terms of latitude and longitude.
5). Accuracy can be up to 500mtrs as compared to 20 Kms with traditional 121.5 MHz /243 MHz ELTs
5) 406 MHz ELT transmission is having world wide coverage
6) 406 MHz ELT transmission will have an immediate search and rescue response.
Leading manufacturers of ELT:-
ARTEX (A Chilton group company)
Artex G406-1 ELT , Part No: ATX455-5005
Artex G406-2 ELT , Part No: ATX 455-5004
Model: KANNAD 406ATP
Model: KANNAD 406 AF
Model: TECHTEST 503ELT
Model: DFIRS with 406MHzELT
Â¢Position reporting and identification beacon can serve dual roles
Â¢Tracking of aircraft at lower cost than mode-S transponder
Â¢Beacon can be on whenever aircraft moves
Â¢All transmitted data can be encrypted
In order to transmit the signals regarding crashed aircrafts or drowned ships, the transmitter have to withstand under very stringent environment conditions.Hence, ELT features excellent in this area, in my view ELT is evergreen and developments in this area are a better option
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