One of the missions that the Civil Air Patrol carries out frequently is to search for aircraft Emergency Locater Transmitters(ELTs), marine Emergency Position Indicating Radio Beacons (EPIRBs) and Personal Locating Beacons (PLBs) using specialized radio direction-finding (DF) equipment. In the event of an aircraft/boating mishap or other emergency, these devices are designed to transmit a distress signal on 121.5, 243.0, and (for newer EPIRBS) 406 MHz. While Civil Air Patrol units are called upon daily to search for these beacons, the majority of the searches turn out to be "non-distress," in that they involve inadvertent, non-emergency activation of these beacons. These non-distress activations are a drain on valuable resources and may also mask an actual distress signal.
ELTs may be activated manually by the pilot or by a "g-force" or impact force generated by an aircraft mishap. They can also be activated by a hard landing, corroded or old batteries, or careless handling.
EPIRBs are designed to float out of their holders and remain on the water's surface should a boat sink. Weighted to turn upside down; they are then activated by a mercury switch. Of course, they may also be activated manually. Inadvertent activation may be caused by corroded or old batteries or careless handling - removing an EPIRB from its holder and setting it sideways or right side up (with the antenna facing up) will also activate the mercury switch.
PLBs are portable units that operate much the same as ELTs or EPIRBs. These beacons are designed to be carried by an individual person instead of on a boat or aircraft. Unlike ELTs and some EPIRBs, they can only be activated manually and operate exclusively on 406 MHz. And like ELTs and EPIRBs all PLBs also have a built-in, low-power homing beacon that transmits on 121.5 MHz. This allows rescue forces to home in on a beacon once the 406 MHz satellite system has gotten them "in the ballpark" (about 2-3 miles).
Some newer PLBs also allow GPS units to be integrated into the distress signal.This GPS-encoded position dramatically improves the location accuracy down to the 100-meter level thats roughly the size of a football field!