Several different types of ELT are in current use.
These include the older (and simpler) units that
produce a modulated RF carrier on one or both of
the two spot VHF frequencies used for distress
beacons (121.5 MHz and its second harmonic
243.0 MHz). Note that the former frequency is
specified for civil aviation use whilst the latter is
sometimes referred to as the military aviation
distress frequency. Simultaneous transmission on
the two frequencies (121.5 MHz and 243.0 MHz)
is easily possible and only requires a frequency
doubler and dual-band output stage.