Marine and aeronautical <a href="https://en.wikipedia.org/wiki/emergency_locator_beacon" class="extiw" title="w:emergency locator beacon">emergency locator beacon</a>.  Carried on ships and aircraft, in the event of a crash or ship sinking the battery powered <a href="https://en.wikipedia.org/wiki/transmitter" class="extiw" title="w:transmitter">transmitter</a> is activated by water and transmits a periodic radio signal which is used by <a href="https://en.wikipedia.org/wiki/search_and_rescue" class="extiw" title="w:search and rescue">search and rescue</a> teams with <a href="https://en.wikipedia.org/wiki/radio_direction_finding" class="extiw" title="w:radio direction finding">RDF receivers</a> to find the emergency.   Transmits on 121.5 MHz and the COSPAS/SARSAT emergency frequency of 406.0 MHz
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