The Federal Emergency Management Agency (FEMA) conducted its first nationwide test of the US Emergency Alert System on Wednesday which interrupts regular programming on TV and radio stations with an emergency tone and message.
It replaced the Emergency Broadcast System in 1997 and is primarily designed to allow the President to address the nation in the event of a national emergency.
However, owing to widespread news coverage in these situations, it hasn’t been used for this purpose before.
It’s more commonly used for information regarding imminent threats to public safety at a regional level, such as severe weather.
A network of 77 "Primary Entry Point" stations, in co-ordination with FEMA, distribute emergency information before, during and after major incidents/disasters to other broadcasters.
These stations are fitted with additional backup communications equipment and power generators allowing them to continue broadcasting.
Once an Emergency Action Notification is received, a "daisy chain" is formed as stations send the message to others. Each participant is required to monitor at least two others.
This video shows how EAS codecs respond to an EAN command:
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