Those iconic words, coupled with the tell-tale "brraaaaap... brraaaaap...brraaaaap" data tones, are widely known as the EMERGENCY ALERT SYSTEM, or EAS. Like its predecessors the Emergency Broadcast System and CONELRAD (CONtrol of ELectromagnetic RADiation), EAS is - at its core - a means by which the US Government can disseminate emergency information to the public at large in a short amount of time.
EAS might seem a little quaint in the era of tweets, smartphones and 24-hour cable news networks. But its massive scope makes it such a powerful system; there are more than 13,000 broadcast AM and FM stations (plus TV and cable) and all of them are part of EAS.
EAS uses a "daisy chain" system amongst all those stations to spread alerts. Every station has a special "ENDEC" (ENcoder/DECoder) device that can interpret EAS data tones, similar to a fax machine. They have regular radios connected to the ENDEC that just listen for those data tones to be broadcast by the "Local Primary" (LP) stations. Usually big signals in larger markets, for Rhode Island it's WWLI 105.1FM (local stations must "monitor" 105.1 and any other AM/FM station...including weather radio...of their choice). The LP stations then listen for data tones from the "Primary Entry Point" (PEP) stations, which are over 30 of the biggest stations, strategically spread out across the country. And the PEP stations have special connections to the Federal Emergency Management Agency (FEMA), who in turn has a special connection to the White House. This system covers over 95% of the entire USA populace in the fifty states and major US territories.
Since a "chain" is only as strong as its weakest leak, there are other paths: the NOAA Weather Radio system, XM/Sirius Satellite Radio, FEMA's new CAP (Common Access Protocol) internet-based system, and even NPR's own Public Radio Satellite System! In addition, each state's Governor's Office, State Police, and Emergency Management Agency also have the ability to use EAS in their states for civil alerts.
While EAS has never been actually used for its primary purpose of the President speaking on a national scale, state and local alerts see everyday use: weather alerts for tornados and other fast-moving severe storms, or child abduction emergencies (aka AMBER Alerts), chemical spills, and other civil emergencies. EAS is very flexible in scale so it's ideal for alerting relatively small communities as well as entire cities, states or regions.
Speaking of never being used nationally, several industry experts have asked why EAS wasn't used on September 11th 2001, when the entire country was riveted by the attacks on the Pentagon and the World Trade Center? The answer is that EAS works best as a "heads-up," a means of getting peoples' attention and telling them to turn to established news sources for more info. In that case, by the time the White House had information to disseminate on EAS, the entire news media enterprise was already devoting 24/7 coverage it. Public awareness was already very high - EAS would've been redundant.
So now when you hear our weekly test of our EAS system, and now you'll know a little more about the massive infrastructure behind it!