ELT

Acronym meaning: Emergency Locator Transmitter.

ELT's are devices carried by military aircraft, (some civilian as well,) that when activated by a sudden deceleration broadcast a continuous signal at 243.0 MHz and 121.5 MHz. This is designed to allow {SAR|search parties] and recovery teams to locate the downed aircraft faster in the event of a mishap or crash requiring outside assistance that rendered the radios unusable.

243.0 MHz and 121.5 (notice one is half of the other,) are commonly referred to as 'Guard' and are the most commonly used international distress frequencies.

These differ from the Underwater Acoustic Beacons carried by most aircraft in that they use RF and not sound as a primary means of transmission.
UAB's or 'pingers' (as they are more commonly referred) on the other hand, are not impact activated but by a plug dissolved by salt water. These dissipate after 4-10 days and then emit a pulse every second for several days following.

Author's Note: The pinger on a UAB is similar to the one's used on Exercise Torpedoes. We had one of these inadvertently start while onboard a ship during RIMPAC '98, you could hear the thing through two inches of steel and it was very annoying. It bears mentioning that on a Perry Class Frigate the torpedo magazine is just forward of the aviation maintenance shop, DET 8I was lucky enough to listen to this torp chirping for about three weeks.

To elaborate on Yurei's w/u, ELT's are not very big, and are of comparable size to a large shoe. Some are box-shaped, but the majority are cylinder-shaped.

As another clarification, the operation of ELT's on 121.5 MHz and 243 MHz will only be carried on for the next few years.

Currently made ELT's operate on 406 MHz, and SARSAT (Search and Rescue Satellites) is being changed to follow suit. Take note that 406 MHz is in the UHF area of the electromagnetic spectrum. The new ELT's are said to be much more reliable. In other words, the number of false positives (non-distress beacons) is reduced.

According to Cospas-Sarsat's (the Russian and US SARSAT's, respectively) plans, Russian satellites will no longer recieve 121.5/243 starting 2006, and the US satellites will follow suit in 2009.

I haven't heard one of the new ELT's in action, but I am told that the pings are much farther apart (I remember I was told they were 30 seconds apart, but it might be a bit more), as opposed to the continuous pinging of the current ELT's. If you have ever had any experience on a Civil Air Patrol SAR mission, you'd think that the new ELT's are harder to find, but I am told that they are really not, as a new strategy has been developed.

Some of the new ELT's come with a GPS function built in, which makes the job extremely easy for the search teams, but these cost considerably more (a normal ELT can cost about $200 US, so think about how much a GPS-augmented one would be).

Note that all three kinds of beacons are being affected. These are ELT's (Emergency Locator Transmitters), EPIRB's (Emergency Position-Indicating Radio Beacons), and PLB's (Personal Locator Beacons).

Sources: http://www2.acc.af.mil/afrcc// (Air Force Rescue Coordination Center), Personal experience and knowledge.

UPDATE: After doing a couple of searches for these new ELT's, I can tell you they are in some ways easier to find (hard to explain) and that the pings are about 50 seconds apart, so you better have a stopwatch ready if you want to track one of these suckers down.