Basically there are three types of emergency locator beacons in use today to transmit distress signals: the maritime EPIRBs - Emergency Position Indicator Radio Beacon. The aviation community uses ELTs - Emergency Locator Transmitters, the PLBs - Personal Locator Beacons are used for land-based applications. Although Cat II EPIRBs and PLBs can be used by the pilot of an aircraft as these are manually activated beacons whereas the CAT I EPIRBs are housed in a special bracket, aviation ELTs are automatically activated and manually tested.
As of July 2008 it became mandatory on international GA flights to carry the new 406 MHz beacon as the old style 121.5 MHz were fased out per Feb, 2009. The reason being that 121.5 MHz beacons are not reliable (2 out of 1000 signals are for real), they transmit anonymously, have very low accuracy (15 - 25 km), unable to transmit digital data and thus need Doppler detection to locate the beacon and have no GPS capability.
Comparison of 121 and 406 beacons.
Excerpt from the document:
121.5 MHz: Only about 1 in 5 alerts come from beacons. Satellites cannot discern beacon signals from many non-beacon sources. Beacons transmit anonymously with no unique identifier. Non-beacon interferers have included ATM machines, pizza ovens, and stadium scoreboards!
Fewer than 2 in 1000 alerts and 2 in 100 composite alerts are actual distress. Since 121.5 MHz beacons transmit anonymously, the only way to ascertain the situation is to dispatch resources to investigate -- a costly disadvantage.
High false alert rate makes first-alert launch unfeasible. Absent independent distress information means RCCs must wait for additional alert information.
To continue reading see the pdf below: