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 Transmitter and the PLBs - Personal Locator Beacon is 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 and ELTs are automatically activated.

Since July 2008 it became mandatory on international GA flights to carry the new 406 MHz beacon as the old style 121.5 MHz will be 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 date 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!

Aircraft distress beacons

COSPAS-SARSAT System Overview

Emergency Locator systems make use of the Cospas-Sarsat satellite system, see image. This system detects and locates distress beacons operating at 121.5 MHz and 406 MHz. The 406 MHz system is composed of: 406 MHz radiobeacons 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 built-in GPS are usually called GPIRBs, for GPS Position-Indicating Radio Beacon or Global Position-Indicating Radio Beacon.

EPIRB
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 EPRIBs are manually activated. Keep this one in your flight bag or on your belt so you can activate it when neccessary. 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. Testing an EPIRB is subject to rules.

EPIRBs 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.

ELT
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 militairy aircraft. Detection was primarily by overfying aircraft (within range) but these were unable to locate the signal. To do this satellites were used, but there were still limitations, see this comparison of 121 and 406 beacons.

The new 406 MHz TSO-126 ELT will be the only type of ELT detected by Cospas-Sarsat after February 1, 2009. These types are being 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 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.

PLB
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.

Location method

When a beacon is activated, either automatically or manually, its 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 and rescueing authority. SAR will then locate the position of the beacon and commences recovery. It usually takes about a minute to forward data once it has been received by a satellite.

Precision
Registered GPS equipped beacons have a precision of 100 meters everywhere on the earth. 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 world-wide 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.

Traditional, unregistered ELTs (and especially the cheapest models) transmit a warble on 121.5 MHz. Anonymously. Satellites can detect them over only 60% of the earth and will require up to 6 hours for notification, must be in view of beacon and groundstation at the same time, polar and south-hemisphere coverage is poor, subject to interference from other electronic and electrical systems, confirmation of the signal is by second satellite pass which could take some 4 hours, 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.