EAS messages are handled by specialized equipment called EAS encoder-decoders, or endecs. The decoder component receives and interprets EAS messages from at least two state-assigned stations, and the encoder component transmits relayed messages. Low-power stations are only required to have the decoder components, but all other stations must have both decoder and encoder components.
In the EAS system, messages are originated in four parts. The first part is the SAME header code, the most critical part of the EAS design. It is repeated three times, so that decoders can pick "best two out of three" for each byte, thereby eliminating most errors which can cause an activation to fail. The header code contains information about who originated the alert (President, state or local authorities, the National Weather Service, or the broadcaster), what the event is (tornado, flood, nuclear), the areas affected (up to 32 counties or states), how long the alert is valid for, the exact UTC time it was issued, and the identification of the station or other entity. (See SAME for a complete breakdown of the header.)
Each station must have installed an EAS decoder which then interprets these data bursts. Two of the three bursts must be found to be identical by the decoder for the message to be valid. The decoder then decides, based on its pre-programming, whether to ignore the message as not pertaining to the local area, or whether to relay it on the air. All EAN and EAT messages must be relayed immediately, and all required weekly and monthly tests (RWT and RMT) within 60 (formerly 15) minutes. For reliability, every station must monitor at least two other source stations, one of which must be designated a local primary.
The SAME header bursts are followed by an eight-second alert tone that carries no information. The tone is the same combination of two tones that the old Emergency Broadcast System used as its attention signal, shortened from the 20 seconds formerly required to activate the EBS. Like the old EBS, this tone is in turn followed by a voice message giving details of the alert.
The message is ended by three bursts of the AFSK "EOM", or end of message, which is the text NNNN, preceded each time by the binary 01010101 calibration.
All EAS equipment must be tested weekly. The RWT, or required weekly test, consists only of the three AFSK header bursts and the three AFSK EOM bursts with no voice, and are essentially internal tests of the system components. Required monthly tests (RMT) are transmitted with all four parts, with the voice (and text on TV) stating that it is only a test. The RMT must be transmitted in the daytime and at night in alternating months.
The number of events in the national system has recently grown by several increments, and is now forty-nine events. At first, they were almost all weather events with only one to three possible categories for civil emergencies, but several classes of non-weather emergencies have now been added. In most states, the Amber Alert System, for child abduction emergencies, has now been added as well.
Stations are required by law to keep full logs of all received and transmitted EAS messages, usually accomplished by a small receipt printer in the endec unit. Logs may also be kept electronnically inside the unit, as long as there is access to an external printer, sometimes via transfer to a personal computer.
EAS is designed to be useful for the entire public, not just those with SAME-capable equipment. However, several consumer-level radios do exist, especially weatheradio receivers, which are available to the public through both mail-order and retailers like Radio Shack, Circuit City, Best Buy, and several others. Other specialty receivers for FM radio are available only through mail-order, or in some places from federal, state, or local governments, especially where there is a potential hazard nearby such as a nuclear plant or chemical factory. These radios come pre-tuned to a station in each area that has agreed to provide this service to local emergency management officials and agencies, often with a direct link back to the plant's safety system or control room for instant activation should an evacuation or other emergency arise.
The ability to narrow messages down so that only the actual area in danger is alerted is extremely helpful in preventing false warnings, which was previously a major tune-out factor. Instead of sounding for all warnings within a station's area, SAME-decoder radios now sound only for the counties they are programmed for. When the alarm sounds, anyone with the radio knows that the danger is nearby and protective action should be taken. For this reason, the goal of the National Weather Service is that each home should have both a smoke detector and a SAME weatheradio.
Technical concept
EAS for consumers