EMERGENCY LOCATOR TRANSMITTER

ELTs are emergency transmitters that are carried aboard most general aviation aircraft 

In the event of an aircraft accident, these devices are designed to transmit a distress signal on 406 MHz . One of the reasons the Cospas-Sarsat system was developed was to provide a better receiving source for these signals. Another reason was to provide location data for each activation(something that overflying aircraft were unable to do).

The 406 MHz EPIRBs are divided into two categories. Category IEPIRBs are activated either manually or automatically. The automatic activation is triggered when the EPIRB is released from its bracket. Category I EPIRBs are housed in a special bracket equipped with a hydrostatic release. This mechanism releases the EPIRB at a water depth of 3-10 feet. The bouyant EPIRB then floats to the surface and begins transmitting. If you own a Category I EPIRB, it's very important that you mount it outside your vessel's cabin where it will be able to "float free" of the sinking vessel. 

Category II EPIRBs are manual activation only units. If you own one of these, it should be stored in the most accessible location on board where it can be quickly accessed in an emergency.