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!
Aviation uses a special beacon transmitting on two frequencies: 121.5 MHz and 406 MHz, it is installed in the tail section of the aircraft. This is supposed to arrive last on the scene of the accident and has the greatest chance on survival.
One of the first radio beacons developed were the ELTs and subsequently required by aviation law to be installed on almost all aircraft. They used frequency 121.5 MHz and 243 MHz allocated for military aircraft. Detection was primarily by overflying aircraft (within range) but these were unable to locate the signal with great precision. For this satellites were used, but there were still major limitations, see this comparison of 121 and 406 beacons.
The new 406 MHz TSO-126 ELTs will be the only type capable of being detected by COSPAS-SARSAT as of February 1, 2009. The following types are phased out: TSO-C91 - 121.5 / 243 MHz unregistered - have not been permitted for new installations since June 21, 1995 and TSO-C91a - 121.5 / 243 MHz unregistered - was the replacing standard; almost all current aviation ELTs are of this type.
Unfortunately, these old ELTs have sadly proven to be highly ineffective for SAR services. They have a 97% false alarm rate, activate properly in only 12% of crashes, and provide no identification data. To remedy this situation, 406 MHz ELTs were developed to work specifically with the COSPAS-SARSAT satellite system.
When a beacon is activated, either automatically or manually, the transmissions will be picked up by satellites. These will relay the signal for processing to ground stations. The processed data will be forwarded to a national rescuing authority. SAR will then locate the position of the beacon and commences recovery. It usually takes about a minute or so to forward data once it has been received by a satellite.
Registered GPS equipped beacons have a precision of 100 meters everywhere. During transmission its serial number is included so authorities can look up the owner and phone numbers for notification of family. This usually takes about five minutes. SAR operations will start soon after this time has elapsed.
Registered beacons without a GPS have worldwide coverage, are locatable within 2 km (12.5 km2 search area). Notification of rescuers and family is within 2 hours maximum (46 minutes on average) and facilitates a serial number to look up phone numbers. This process may take up to two hours because it has to use moving satellites to locate the beacon. Doppler is used to locate the beacon and this process takes some time.
Traditionally, unregistered ELTs (and especially the cheapest models) transmit just a warble on 121.5 MHz. Anonymously. Satellites can detect them only over 60% of the Earth and will require up to 6 hours for notification, they must also be in view of the activated beacon and ground stations at the same time; polar and southern hemisphere coverage is pretty poor and they are subject to interference from other electronic and electrical systems. Confirmation of the signal is done by a second satellite pass, which could take some 4 hours and locatable within minimum 20 km radius (search area of 1200 km2).
So, if you ever needed a reason to buy a new 406 beacon, this is the one!