Emergency Action Message/Claude Draft

Emergency Action Messages (EAMs) are encoded alphanumeric messages broadcast via the High Frequency Global Communications System (HFGCS) that serve as the primary means of transmitting time-sensitive orders and information to U.S. strategic forces. EAMs are broadcast continuously across multiple high-frequency radio bands, transmitted by ground stations worldwide and rebroadcast by airborne platforms including E-6B Mercury aircraft.

EAMs consist of alphanumeric character sequences transmitted via voice by radio operators, with each message preceded by a two-character prefix and read twice for verification. The messages are encrypted, and their content is not publicly known. However, structural analysis of EAM traffic has revealed distinct categories of messages with different characteristics, transmission patterns, and apparent operational purposes.

Overview

EAMs are transmitted on the HFGCS primary frequencies of 4724, 8992, 11175, and 15016 kHz. Ground stations typically simulcast messages across multiple frequencies simultaneously, though exclusive single-frequency broadcasts also occur. Messages are transmitted by operators at control stations, with the Centralized Network Control Stations located at Joint Base Andrews and other facilities coordinating network operations.

The E-6B Mercury fleet, operated by Strategic Communications Wing 1, plays a critical role in the EAM system. According to official Navy documentation, the E-6B's primary mission is to "receive, verify and retransmit Emergency Action Messages (EAMs) to US strategic forces."

Message Structure

All EAMs follow a consistent broadcast format:

1. Preamble: The operator identifies the transmission type and may provide additional context
2. Prefix and message: A two-character prefix followed by the alphanumeric message content
3. First reading: The complete message is read character by character
4. Second reading: The message is repeated in its entirety for verification

Messages use alphanumeric characters (A-Z, 0-9) transmitted phonetically. The two-character prefix serves as a classification indicator, with different prefixes corresponding to different message categories. Prefixes rotate on schedules independent of one another, with each message category using a single active prefix at any given time.

Message Categories

Structural analysis of HFGCS traffic has identified four distinct categories of EAMs, differentiated by length patterns, structural characteristics, and addressee specifications. These categories are colloquially referred to as Groups 1 through 4.

Group 1 Messages

Group 1 Messages are the most common EAM type, constituting approximately 66% of all EAM traffic. They are characterized by absolute structural uniformity: every Group 1 Message is exactly 30 characters long, never includes an addressee designation, and uses a single active prefix across the entire HFGCS network at any given time.

Group 1 Messages are the most frequently rebroadcast message type. Individual messages are often repeated dozens of times over several hours by both ground stations and E-6B aircraft, with E-6B platforms showing strong preference for Group 1 content in their rebroadcast selections.

The Group 1 prefix rotates on a regular schedule, typically remaining active for 3-8 weeks before transitioning to a new prefix. These rotations occur independently of other message groups.

Group 2 Messages

Group 2 Messages constitute approximately 26% of EAM traffic and are characterized by variable message lengths and frequent addressee specifications. Group 2 Messages range from 30 to 163 characters, with the most common lengths being 30, 34, 35, 39, 51, and 55 characters.

Approximately 47% of Group 2 Messages are Directed EAMs, which include explicit "FOR [CALLSIGN]" specifications identifying intended recipients. These Directed EAMs are immediately identifiable during transmission and represent a distinct subcategory with documented callsign patterns including organizational designations (FORCE callsigns), geographic areas (REGION callsigns), E-6B-associated numbered callsigns, and tactical word-only callsigns.

The remaining 53% of Group 2 Messages are broadcast without addressee specifications and require observation of prefix behavior over time for reliable identification. Like Group 1, Group 2 uses a single active prefix at any given time, rotating on an independent schedule.

Group 3 Messages

Group 3 Messages constitute approximately 3% of EAM traffic and are characterized by specific length clustering and embedded repeated character sequences. The most common Group 3 lengths are 22, 27, 32, and 37 characters, with these four lengths accounting for approximately 60% of Group 3 traffic.

Approximately 40% of Group 3 Messages contain repeated character sequences of three or more characters embedded within the message structure. This high prevalence of internal repetition is nearly identical to Group 4 Messages and significantly exceeds Groups 1 and 2.

Group 3 Messages never include addressee specifications and show sporadic appearance patterns, with prefixes often appearing for only a few days before disappearing for extended periods. However, sustained periods of elevated Group 3 activity have been observed, suggesting operational factors affect transmission frequency.

Group 4 Messages

Group 4 Messages constitute approximately 3% of EAM traffic and are the longest and most structurally complex EAM category. Group 4 Messages range from 36 to 292 characters, with the most common lengths being 142, 120, 164, 68, 50, and 216 characters.

Approximately 60% of Group 4 Messages contain repeated single-character sequences of four or more characters (e.g., FFFF, XXXX, ZZZZ, RRRR), the highest prevalence of any repetition pattern across all EAM groups. Additionally, approximately 38% contain repeated multi-character sequences of three or more characters, similar to Group 3.

Group 4 Messages never include addressee specifications and show the highest transmission error rate of any EAM category due to their extended length and complexity. Group 4 is the only category that never produces 30-character messages.

Historical Context

Public information about the HFGCS has diminished significantly since the mid-2000s, despite the system's unclassified status. Larry Van Horn's 2006 documentation of the HFGCS remains widely cited, though portions have become outdated. Contemporary descriptions of the HFGCS appear in Department of Defense budget documents but are less accessible than historical sources.

The HFGCS formerly maintained a public website in the 1990s and the Air Force published articles about HFGCS operations through 2006, but such official disclosures have become increasingly rare.

Network Operations

The HFGCS ground station network consists of transmitter sites distributed globally to ensure redundant coverage. Ground stations are remotely controlled from Centralized Network Control Stations, with control facilities documented at Joint Base Andrews and Grand Forks AFB. Offutt AFB was previously identified as having or developing control capabilities, though operational status may have been affected by facility damage from flooding in 2019.

Ground stations typically simulcast messages across all four primary HFGCS frequencies simultaneously, though single-frequency transmissions also occur. Historical documentation suggests the HFGCS operates on predetermined frequency schedules, though current scheduling practices are not publicly documented.

E-6B aircraft serve dual roles as message receivers and rebroadcast platforms. When an E-6B rebroadcasts EAMs on HF frequencies, it selects messages from accumulated traffic, showing strong preference for Group 1 Messages. The same E-6B may simultaneously rebroadcast messages on VLF frequencies using its trailing wire antenna, extending coverage to submerged submarines.

Monitoring and Analysis

EAMs are monitored by radio enthusiasts, researchers, and open-source intelligence analysts worldwide. Online communities track EAM traffic, document message patterns, and maintain databases of observed transmissions. Projects such as the NEET INTEL DAILY TIMECARD PROJECT have compiled extensive records of EAM traffic over multi-year periods, enabling statistical analysis of message patterns, prefix rotations, and transmission frequencies.

Software-defined radio technology and web-based receivers have made HFGCS monitoring accessible to a global audience. Services like WebSDR allow real-time monitoring of HFGCS frequencies without specialized equipment, democratizing access to this aspect of military communications observation.

While the encrypted content of EAMs remains classified, structural analysis has revealed significant patterns in message formatting, categorization, and operational usage. The four-group taxonomy, prefix rotation schedules, addressee patterns, and embedded structural features represent observable characteristics that inform understanding of HFGCS architecture and usage patterns.

See Also

• EAM/Group 1 Prefix
• EAM/Group 2 Prefix
• EAM/Group 3 Prefix
• EAM/Group 4 Prefix
• Directed EAM
• High Frequency Global Communications System
• E-6B Mercury
• Strategic Communications Wing 1