- Written by Brian Keahl
- Category: Emergency Communications
- Published: 13 March 2015
- Hits: 1709
ELTs, or Emergency Locator Transmitters, are Search and Rescue devices utilized in aviation and marine environments. Other similar technologies include EPIRBs (Emergency Position Indicating Radio Beacons) and PLBs (Personal Locator Beacons).
I've covered emergency radio frequencies in a previous presentation entitled “International Distress Frequencies”, but much of that also includes maritime and as the title implies, international radio communications protocols. In this area the most likely distress signal will be aviation related.
Aircraft use one of two types of Search and Rescue (SAR) distress systems: Legacy “analog” ELTs or the new 406Mhz digital ELTs.
The new 406Mhz ELTs are satellite monitored and can, optionally, transmit GPS information. Satellites can localize the signal to a small geographical area utilizing Doppler Shift, although these ELTs can also transmit GPS data.
These new units, because of the digital encoding, seldom generate false signal detects (although falsely triggered ELTs are still an issue). The benefit of this system is rescue personnel can be dispatched to the location of the incident, with little need for direction finding.
The legacy ELTs are strictly beacons, communicating no GPS data, and must be located utilizing direction finding equipment. Additionally, as of February 1st, 2009, these units are no longer monitored by SARSAT (Search And Rescue Satellite Aided Tracking ) or COSPAS (Russian language equivalent to SARSAT) satellite detection systems due to the fact they use modulated AM and the satellites often misidentify simple electrical interference as a signal.
However, hundreds of thousands of aircraft were produced prior to the implementation of the 406Mhz ELT and other emergency location technology and the FAA still qualifies the legacy ELT equipment as adequate for General Aviation. While it is strongly recommended that pilots upgrade their aircraft, a very high number of these legacy systems are still in service. Additionally, the new 406Mhz ELTs also incorporate a 121.5Mhz transmitter to assist in the final leg of the SAR activity.
Airports, aviation maintenance & air traffic control facilities, and commercial aircraft are encouraged to monitor the legacy ELT frequencies as a means of detecting downed aircraft.The frequency these legacy systems broadcast on is 121.5Mhz and 243.0Mhz, although 121.5Mhz is almost universal in North America.
You might expect that this frequency would utilize FM modulation, but the units transmit on AM, modulated with a “warbling” tone. Here's an example of what the ELT sounds like: https://www.youtube.com/watch?v=I_AMzBuoLs0.
Virtually all scanners and many amateur radios will allow monitoring of these frequencies in AM mode, and I usually have 121.5Mhz programmed into any unit I have that allows it. Those that operate scanners might want to include this in scanned frequencies as well. In the event the signal is picked up, a call to 911 is all that is necessary.
Because these legacy ELTs do not utilize GPS and satellites no longer monitor them as well as the fact the new units still incorporate a 121.5Mhz beacon, there remains a need for those who can direction-find these units, and “Fox Hunting” is a great way to sharpen those skills and have fun at the same time.
For those that do not know what “Fox Hunting” is, it's a “game” where a radio transmitter, which broadcasts and ID periodically, is hidden and participants attempt to find it by using hand-held receivers, directional antennas, and other equipment. A simple HT or scanner is the minimum needed to participate.