Specific Area Message Encoding - Functionality

Functionality

In the SAME system, messages are constructed in four parts, the first and last of which are digital. The first part is a header message, which is transmitted 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 is an AFSK data burst, with each individual bit lasting 1920 μs (1.92 ms) each, giving a bit rate of 5205⁄₆ bits per second. A mark bit is four complete cycles of a sine wave, translating to a mark frequency of 20831⁄₃ Hz, and a space bit is three complete sine wave cycles, making the space frequency 1562.5 Hz.

The data is encoded in 7-bit ASCII but uses all 8 bits, with no parity bit and no stop bit ("8-N-0"). The least-significant bit of each byte is transmitted first, including the preamble.

The text of the header code is a fixed format of ZCZC-org-eee(-pssccc {up to 31 })+tttt-jjjhhmm-llllllll:

1. A preamble of binary 10101011 (0xAB in hex) repeated sixteen times, used for "receiver calibration" (i.e., clock synchronization), then the letters ZCZC as an attention to the decoder
2. org: Originator code; programmed per unit when put into operation
   • EAN – Emergency Action Notification Network
     • President or other authorized national officials. Shall be used starting November 9, 2011 for the national test
   • PEP – Primary Entry Point Station
     • President or other authorized national officials
   • CIV – Civil authorities
     • i.e. Governor, state/local emergency management, local police/fire officials
   • WXR – National Weather Service (or Environment Canada.)
     • Any weather-related alert
   • EAS – EAS Participant
     • Formerly Broadcast Station or Cable System.
     • Broadcasters. Generally only used with test messages.
3. eee: Event code; programmed at time of event
4. pssccc: Location codes (up to 31 locations); programmed at time of event
   • In the United States, the first digit (p) is zero if the entire county or area is included in the warning, otherwise, it is a non-zero number depending on the location of the emergency.
   • In the United States, the remaining five digits are the FIPS state code (ss) and FIPS county code (ccc). The entire state may be specified by using county number 000.
   • In Canada, all six digits specify the Canadian Location Code, which corresponds to a specific forecast region as used by the Meteorological Service of Canada. All forecast region numbers are six digits with the first digit always zero.
5. tttt: Purge time of the alert event (from exact time of issue)
   • In the format hhmm, using 15 minute increments up to one hour, using 30 minute increments up to six hours, and using hourly increments beyond six hours. Weekly and monthly tests sometimes have a 12 hour or greater purge time to assure users have an ample opportunity to verify reception of the test event messages; however; 15 minutes is more common, especially on NOAA Weather Radio's tests.
   • For short term events (like a tornado) this value could be set to 0000, which will purge the warning after the message has been received. However, this is not typical, and FCC guidelines normally suggest a minimum 15 minute purge time.
   • The purge time is not intended to coincide with the actual end of the event. Longer events that may not end for days (like hurricanes) may have a purge time of only a few hours. That an event message has been purged does not indicate or imply that the threat has passed.
6. jjjhhmm: Exact time of issue, in UTC, without time zone adjustments
   • jjj is the Ordinal date day of the year, with leading zeros
   • hhmm is 24-hour hours and minutes, in UTC, with leading zeros
7. Eight-character station callsign identification, with "/" used instead of "–" (such as the first eight letters of a cable headend's location, WABC/FM for WABC-FM, or KLOX/NWS for a weather radio station programmed from Los Angeles).

Each field of the header code is terminated by a dash character; individual pssccc location numbers are also separated by dashes, with a plus (+) separating the last location from the purge time that follows it.