ELT Technology
The Cospas-Sarsat System provides distress alert and location information to search and rescue (SAR) services throughout the world for maritime, aviation and land users in distress. The System is comprised of:
- satellites in Low-altitude Earth orbit (LEOSAR) and Geostationary orbit (GEOSAR) that process and / or relay signals transmitted by distress beacons;
- ground receiving stations called local users terminals (LUTs) which process the satellite signals to locate the beacon; and
- mission control centres (MCCs) that provide the distress alert information to SAR Services.
The Cospas-Sarsat System supports two types of distress beacons: old analogue technology beacons that transmit at 121.5 MHz and newer generation digital beacons that operate at 406 MHz..
|
ELT: Emergency Locator Transmitter
PLB: Personal Locator Beacon
EPIRB: Emergency Position
Indicating Radio Beacon
LUT: Local Users Terminal
MCC: Mission Control Center |
Two Frequency TSO C91a Beacons:
Once a unit has been activated either by the force of a crash or by a cockpit remote switch the beacon transmits on two frequencies, the Civilian band 121.5 MHz and the military band 243.0 MHz. The signal is received by a satellite and relayed to a ground receiving station where the location of the aircraft is determined by using the doppler effect. Due to the instability of the frequency, accuracy can only be determined to within 20 miles of the crash site. One major disadvantage to this technology is that there is no specific information in regards to the owner or the aircraft. This is where 406 MHz technology becomes valuable.
406 MHz TSO C126 Beacons:
The 406 MHz transmitters send out a 5 watt signal every 50 seconds for 520 milliseconds. This transmission contains the serial numbers of the beacon, the manufacturer code of the beacon and the country code allowing search and rescue to decode the owner's name, address, telephone numbers and type of aircraft from a database. This allows immediate contact of the aircraft owner to determine if the aircraft is in flight and the alert is real or if the ELT went off inadvertently. TSO C91a two frequency beacons do not have this capability and the signals received are often based on inadvertent activations requiring extensive search and rescue resources to identify. Again, the location of the crash sight is calculated by using the Doppler effect, which narrows the position of the aircraft to within two miles due to the much stronger signal.
A new type of satellites is now in service to track the 406 MHz distress beacons. Geostationary satellites are fixed above the equator and cover large areas of the earth. The main advantages of the 406 transmitter system are as follows:
-
-
The transmitter is identified by serial number and craft I.D.
-
The capability of uploading the "position fix" in the long message format
-
Total worldwide coverage
-
Immediate search and rescue response
FAA Requirments for Aircraft
There are three different classifications of ELTs that are present today. The first, and the oldest are the FAA, TSO C91 beacons. (Technical Standard Order) These beacons represent the oldest technology in ELTs, and are being replaced by the new TSO C91a beacons. All general aviation aircraft that are operating within the United States are required to carry a C91a beacon. The difference between the two TSOs is the fact the the latter can withstand much more severe abuse in a crash, and it utilizes a remote switch in the cockpit, which enables the pilot to either activate or reset the ELT. Any aircraft that currently has a TSO C91 ELT installed in it must replace it with a C91A beacon if the old ELT is deemed unrepairable. The most advanced type of beacons available today are the FAA TSO C126 beacons. These units must withstand even more stringent abuse than the TSO 91a beacons. The main difference between these technologies is the fact that these units operate on three frequencies (one with an extremely powerful signal) rather than two. which makes for a more precise location calculation. All of these factors increase the chance for survivability of a crash. |
