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 its mandatory to carry the new 406 MHz beacon on international GA flights. 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.
In short: These new beacons take the search out of the search and rescue!
Emergency Locator systems make use of the COSPAS-SARSAT satellite system, see the image. This system detects and locates distress beacons operating at 121.5 MHz (update: not anymore as of Feb 2009) and 406 MHz.
The 406 MHz system is composed of: 406 MHz radio beacons carried aboard ships (EPIRBs), aircraft (ELTs), or used as personal locator beacons (PLBs); polar orbiting satellites in low Earth orbit from the LEOSAR system and geostationary satellites from the GEOSAR system; and the associated Local User Terminal (LUT) for the respective satellite systems (referred to as LEOLUTs for the LEOSAR system and GEOLUTs for the GEOSAR system).
EPIRB beacons with builtin GPS are usually called GPIRBs, for GPS Position Indicating Radio Beacon or Global Position Indicating Radio Beacon.
EPIRBS are mainly used in maritime applications and there are two models. One transmits an analog signal on 121.5 MHz. The other transmits a digital identification code on 406 MHz and a low power "homing" signal on 121.5 MHz. The 121.5 MHz model EPIRB is being fased out.
There are two categories EPIRBs: CAT I are manually or automatically activated. This is triggered when the beacon is released from its bracket on the vessel, done when the vessel is sinking at a certain depth. This EPIRB is buoyant and will surface and start transmitting.
CAT II EPIRBs are manually activated. Keep this one in your flight bag or on your belt so you can activate it when necessary. The signal from these beacons will be instantly detected by the geostationary satellites. It is thus possible that a false alarm is set and detected immediately by the rescue center. Keep in mind that testing an EPIRB is subject to rules.
EPIRBs also need to be registered. This will make it easier to locate the owner in case of a false alarm before dispatching unneeded SAR aircraft and vessels. Which would result in unnecessary cost to the owner should this happen.
Personal Locator Beacons (PLB) operate like ELTs or EPIRBs, they sometimes have a GPS receiver built in and this position is transmitted when the beacon is activated. PLB can only be operated manually (in contrary to ELTs and some EPIRBs) and transmit this location with high power on 406 MHz and have a low power homing signal on 121.5 MHz.
These PLBs are not so expensive as some EPIRBs and can be used in aircraft, boats and when hiking. They also must be registered. If only Steve Fossett would have had one of these, he would have been found almost the next day when he crashed in Sep 2007.
This personal tracker can be used by hikers, bikers and pilots alike to let everyone know where you are. It sends its location through a message via a satellite which can be used in Google Maps to pinpoint the location of the device.
It has an OK button which sends its location to a number of different mobile phones to let them know that you are ok. The help button can be used to send a message that you need assistance where the emergency button sends a distress signal to a public emergency responder, the GEOS search and rescue service.
Note: you will need a subscription service for this to operate.