ELTs must be attached to commercial aircraft. Typically, they are placed as far in the back of the plane as possible, such as on the tail wing. This positioning provides the safest area for complete retrieval of the transmitter if the airplane should crash.
ELTs use radio frequencies to emit their signals. Since the pilots are aware of the particular frequency the ELT uses, radio calls can be made on different frequencies to prevent interference. The ELT radio frequencies are 121.5, 243 and 406 megahertz.
As of February 1, 2009, authorities no longer monitor the 121.5-megahertz frequency. The 406-megahertz frequency is more reliable and functions better with the satellite technology available as of 2011.
