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.
Testing an EPIRB is subject to rules.
Excerpt from the document:
EPIRBs are subject to possibly the most demanding requirements of any shipborne equipment. Despite prolonged periods of continuous exposure to extreme weather conditions, with minimal maintenance attention, they are required to be ready to work without flaw, first time, in an emergency.
EPIRB equipment design has developed to the point where exceptionally high effectiveness and reliability have become the norm, but such a demanding level of reliability can only be assured over a long period of time by a program of testing and maintenance which, although it need not be frequent or expensive, must be rigorously applied and conducted.
To continue reading see the pdf below: