EAM/Group 2 Prefix

A chart quantifying EAM traffic over a 7 month period; Group 2 Messages in orange. (Click to view larger version)

Group 2 Emergency Action Messages (Group 2 EAMs or Group 2 Messages) represent the most structurally diverse category of EAMs broadcast on the High Frequency Global Communications System (HFGCS). Group 2 Messages are characterized by variable message lengths and frequent use of specified addressees, though neither feature is universally present.

Group 2 Messages constitute approximately 26% of all EAM traffic. During comprehensive monitoring from June 2023 through February 2024, 2,967 Group 2 Messages were observed. Of these, approximately 47% included explicit addressee designations while 53% were broadcast without addressees.

Defining Characteristics

Group 2 Messages exhibit two primary distinguishing features, though neither is present in all Group 2 transmissions:

Variable length: Group 2 Messages range from 30 to 163 characters in documented observations. The most common lengths are 30, 34, 35, 39, 51, and 55 characters. This variability contrasts sharply with Group 1 messages, which are invariably 30 characters.

Addressee specifications: Approximately 47% of Group 2 Messages include "FOR [CALLSIGN]" designations identifying intended recipients. These Directed EAMs are immediately identifiable during transmission, as the operator announces the addressee before reading the message content. The remaining 53% of Group 2 Messages are broadcast without addressee specifications.

Like Group 1, Group 2 uses a single active prefix at any given time, with periodic rotations to new prefixes. The Group 2 prefix rotation schedule is independent of Group 1, with transitions occurring on different dates.

Prevalence

Group 2 Messages are less common than Group 1 but significantly more common than Group 3 or Group 4. During the comprehensive monitoring period from June 2023 through February 2024, Group 2 Messages constituted approximately 26% of all documented EAM traffic.

The distribution of Group 2 Messages shows some month-to-month variation. October 2024 showed unusually high Group 2 activity at 37% of total traffic, while November 2024 returned to more typical levels at 20%. These variations contrast with Group 1's more consistent dominance across monitoring periods.

Identification Methodology

Group 2 Messages can be identified through two distinct pathways depending on whether an addressee is specified.

Directed EAMs (With Addressee)

When a Group 2 message includes an addressee designation, identification is immediate and definitive. The operator announces "FOR [CALLSIGN]" during transmission, making the Group 2 classification unambiguous. These Directed EAMs represent approximately 47% of Group 2 traffic and are documented in detail in a separate article.

The presence of an addressee immediately identifies both the message as Group 2 and reveals the current Group 2 prefix. Once the Group 2 prefix is known, other messages using that prefix can be identified as Group 2 even if they lack addressee specifications.

Non-Directed Group 2 Messages

Group 2 Messages without addressee specifications present a more complex identification challenge. These messages are structurally similar to Group 1 messages and require observation of prefix behavior over time for reliable identification.

A prefix can be identified as Group 2 through several indicators:

Variable length usage: If a prefix is used for messages of different lengths (e.g., 30 characters, 34 characters, 51 characters), it is Group 2. Group 1 prefixes never vary in length.

Addressee correlation: If a prefix appears with addressees in some transmissions, all messages using that prefix are Group 2, regardless of whether individual messages specify addressees.

Prefix isolation: On most days, a single prefix dominates traffic with consistent 30-character messages and no addressees—this is Group 1. Other prefixes appearing less frequently or with variable characteristics are likely Group 2.

Multi-day consistency: Monitoring over 24-48 hours typically reveals clear patterns distinguishing Group 2 from Group 1 prefix behavior.

The ambiguity inherent in non-directed Group 2 Messages means that approximately 53% of Group 2 traffic cannot be definitively identified from a single transmission alone.

Prefix Rotation

Group 2 prefixes rotate on a schedule independent of Group 1. Like Group 1, only one Group 2 prefix is active at any given time, and transitions typically occur within a single day.

Observed Group 2 prefix transitions during monitoring periods include:

• January 26, 2023: VK prefix active
• September 10-21, 2023: QK to EG transition
• June 9, 2024: XS to N3 transition
• July 2025: NZ prefix active

The duration of Group 2 prefix activity appears similar to Group 1 (several weeks to months), though comprehensive documentation of Group 2 rotation schedules across extended periods remains incomplete.

Length Distribution

Group 2 Messages show a broader length distribution than any other EAM group. While 30-character messages are most common (representing approximately 50% of Group 2 traffic), substantial numbers of messages appear at other lengths.

Common Group 2 message lengths include 30, 34, 35, 39, 44, 51, and 55 characters. Less common but regularly observed lengths include 33, 37, 38, 40, 42, 43, 52, 53, 56, and 60 characters. This length variability is a defining characteristic of Group 2 and serves as a reliable identifier when observing prefix behavior over time. The concentration of messages at specific lengths (particularly 34, 35, 39, and 51 characters) suggests standardized message templates or formatting structures for different communication purposes.

Relationship to Directed EAMs

The approximately 47%/53% split between directed and non-directed Group 2 Messages suggests these may serve distinct operational purposes despite sharing the same prefix rotation system.

Directed EAMs are immediately identifiable and explicitly targeted to specific recipients, suggesting time-sensitive or mission-specific communication. Non-directed Group 2 Messages lack explicit targeting and may serve informational or coordination purposes for a broader audience.

However, both subtypes transition simultaneously when the Group 2 prefix rotates, indicating they are part of the same communication architecture rather than independent systems. The operational significance of this distinction remains unclear without access to message content or operational documentation.

Relationship to Other Groups

Group 2 exists within a four-group taxonomy of Emergency Action Messages:

Group 1 messages are always 30 characters, never include addressees, and constitute approximately 66% of traffic. Group 1's uniformity contrasts sharply with Group 2's variability.

Group 3 messages show strong length clustering at 22, 27, 32, and 37 characters, never include addressees, and constitute approximately 3% of traffic. Group 3's length patterns are distinct from Group 2's broader distribution.

Group 4 messages are the longest (36-292 characters), never include addressees, and constitute approximately 3% of traffic. Group 4's extended lengths and embedded repeated character patterns distinguish it from Group 2.

All four groups operate independently with separate prefix rotation schedules. Multiple groups can be active simultaneously with different prefixes, indicating a layered communication architecture serving different operational purposes.

See Also

• Emergency Action Messages
• Directed EAMs
• EAM/Group 1 Prefix
• EAM/Group 3 Prefix
• EAM/Group 4 Prefix
• High Frequency Global Communications System