ScanDiego.com

Every so often we hear of a search being conducted for ELT transponders around San Diego, whether it be from a possible downed aircraft or water craft.

If you don't know what an ELT is, basically it is a transmitter that is designed to transmit a signal on 121.5, 243.0 or 406.025 that is picked up by satellite and relayed to the appropriate agency. I know some marine ELT's are self actuating when they come in contact with salt water and are often mounted on the boat so that they will float free if the vessel flounders. I'm not entirely sure of self-actuating aircraft ELT's.

Aircraft users call them ELT's while on ships they're known as EPIRB's. Personal devices are PLT.

Emergency Locator Transmitters (ELT)
Emergency Position Indicating Radiobeacon (EPIRB)
Personal Location Transmitters (PLT)

So the scanning issue here is that you might want to tune one in and see if you can hear it. ABLE and ASTREA are often called into service to search for the downed vessel/craft and this is the best clue you will get to know to tune it in. Since their target is a sat, the signal is not very strong. I know the 406 devices are 5 watts, I'm unsure of the VHF models.

Here is a sample of what a ELT sounds like so you'll know if you find one.

http://www.scandiego.com/elt.wav

It sounds like an 80's video game.

The most modern ELT's have internal GPS units and they transmit that information with the signal, so those should sound different.

I also understand that ELT's designed for hunters and hikers are now coming on the market.

Here is a good primer on ELT techonology
http://www.artex.net/ELT_work.html

A USGS informational page on marine use:
http://www.navcen.uscg.gov/marcomms/gmdss/epirb.htm

Aircraft ELTs all operate on 121.5 MHz (civilian aircraft emergency) AND 243.0 MHz (military aircraft emergency) with a transmit power of about 50 to 100 milliwatts. 243.0 is the second harmonic of 121.5 so the same transmitter simulcasts on both freqs. (The transmit power on 243.0 is half that of 121.5). They are designed to go off on impact just like a vehicle airbag or they can be activated manually by a switch on top of the unit. When an ELT is activated it transmits continuously until someone manually shuts it off or the battery goes dead. ELTs transmit an obnoxious warble or chirping tone to draw attention but do not transmit any identifying data like Lo-Jacks. They can be a stand alone unit that just sits in the airplane in a special holder or they can be installed permanently with an outside antenna on the airplane. The impact activation is the main reason there are so many falses. If someone takes a unit out of his plane and sets it on a shelf without taking the battery out and the cat comes along and knocks it over...
Many CAP (Civil Air Patrol) planes are equipped with doppler direction finders (similar to Lo-Jack trackers) to pinpoint ELTs. Ground rescue crews use hand held direction finders.
I recently tracked an ELT signal to a house on Mount Helix. (No doubt a pilot's residence). The signal was so weak the battery was probably about gone. ASTREA had picked up the signal earlier that evening as they flew over the area. Since there was no evidence of a crash it was surmized to be a false, but BATMAN went after it anyway!

I don't know much about EPIRBs, but I'll share what I do know.
The original "Emergency Position Indicating Radio Beacons" operated on 156.8 MHz (maritime emergency VHF channel 16) but became such a false alarm nussiance they were moved to 406.0 MHz which is monitored by a satellite system. (I don't know if they transmit any identification or GPS data so someone help me on this one). The satellite determines the approximate position of the signal and notifies the U.S. Coast Guard.

If I had followed the links Wolfie posted first I'd had all my EPIRB questions answered.