406 beacons provide an enhanced ability for search and rescue personnel to locate and bring aid to persons in distress.
The USA’s Federal Communications Commission (FCC) issued a rule prohibiting the certification, manufacture, importation and sale of Emergency Locator Transmitters (ELTs) that operate only on 121.5 MHz.
The rule allows for the continuing operation of existing units.
The international Cospas-Sarsat search-and-rescue program stopped monitoring 121.5 MHz in 2009 due to reliability and false alert concerns.
Along with other Cospas-Sarsat participants, the National Oceanic and Atmospheric Administration, U.S. Coast Guard, U.S. Air Force and NASA have been advising users to switch to 406 MHz ELT units for nearly a decade.
Read more here:
https://www.avweb.com/avwebflash/news/Sale-and-Certification-of-1215-MHz-ELTs-Prohibited-232010-1.html