EPIRBs come in two types. One type transmits an analog signal on the 121.5 MHz frequency; however, as of February 2009, satellite alerting with the Cospas-Sarsat satellite system is no longer available for this type of beacon. The other type of EPIRB transmits a digital identification code on 406 MHz and a low-power homing signal on 121.5 MHz.
Two categories of 406 MHz EPIRBs are available. Category I EPIRBs are activated either manually or automatically. To ensure automatic activation, a Category I EPIRB needs to be mounted outside a vessel's cabin where it can float free of the sinking vessel, because the automatic activation happens when the EPIRB is released from its bracket. Category II EPIRBs are only activated manually, so it's important to store one of these beacons in a location on your vessel where you can access it quickly in an emergency.
ELTs also come in 121.5 MHz and 406 MHz types. According to NOAA, although about 170,000 of the 121.5 MHz ELTs are in service, they have a 97 percent false alarm rate. Also, now that the Cospas-Sarsat satellite system no longer monitors 121.5 MHz, the only monitoring systems for these ELTs are ground-based facilities and aircraft in flight that have their VHF receivers tuned to 121.5.
PLBs are designed for an individual to carry and can only be activated manually. Although these beacons operate exclusively on 406 MHz, they also have a homing beacon that transmits on 121.5 MHz. This homing beacon lets rescue teams home in on a signal after the 406 MHz satellite system has led them to within about two to three miles of the signal.
Another type of beacon for personal use is the SPOT system, which operates on the GEOS satellite network, rather than the Cospas-Sarsat system that PLBs operate on. SPOTs use a higher frequency of 1610 MHz. The SPOT has a GPS, but does not transmit a 121.5 MHz homing signal. Unlike the PLB that can lead rescuers to within two to three miles of a signal, the SPOTs do not provide an approximate position and reply on the GPS for transmitting location information.
Another type of beacon, which does not transmit to satellites, is an avalanche beacon. Avalanche beacons transmit and receive on a frequency of 457 KHz, which transmits well through snow. Typically, the signals from avalanche beacons are received by similar units less than 100 meters away. Avalanche beacons come in three types: analog, digital and hybrid analog/digital.
