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EAM/Group 4 Prefix
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This draft was created using AI and requires human review. This article was generated by Claude, an AI assistant, and and should be reviewed for errors and unverified information. |
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This article uses a conjectural title. The name "EAM/Group 4 Prefix" is speculative. |
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| A chart quantifying EAM traffic over a 7 month period; Group 4 Messages in pink. (Click to view larger version) |
Group 4 Emergency Action Messages (Group 4 EAMs or Group 4 Messages) are the longest and most structurally complex category of EAMs broadcast on the High Frequency Global Communications System (HFGCS). Group 4 Messages are characterized by extended character counts, embedded repeated character sequences, and absence of addressee designations. Group 4 Messages constitute approximately 3% of total HFGCS traffic, making them equally rare as Group 3 Messages.
Group 4 Messages represent the highest information density of any EAM type, with lengths ranging from 36 to 292 characters. The prevalence of repeated character patterns and concentration at specific high character counts suggests these messages follow highly structured internal formatting rules.
Defining Characteristics
Group 4 Messages are distinguished by several consistent structural features:
Extended lengths: Group 4 Messages are consistently longer than other EAM groups. The most common lengths are 142, 120, 164, 68, 50, and 216 characters, with messages at 142 characters being most frequent (18% of Group 4 traffic). The documented range extends from 36 to 292 characters, though the vast majority fall between 50 and 216 characters.
Embedded repeated character sequences: Approximately 60% of Group 4 Messages contain repeated single-character sequences of four or more characters (e.g., FFFF, XXXX, RRRR, ZZZZ). These repetitions appear embedded throughout the message structure rather than at fixed positions. This is the highest prevalence of any repetition pattern across all EAM groups.
Common repeated patterns include:
FFFF (62 occurrences), 2222 (58), IIII (56), XXXX (53), LLLL (45), TTTT (43), CCCC (40), BBBB (40), OOOO (38), RRRR (37), VVVV (36), WWWW (34), 3333 (33), ZZZZ (32), HHHH (32), 7777 (32)
Examples of Group 4 Messages with repeated sequences:
`BA4GDLLNXPR7E6ZFU2XLYTRYGBF2LE44GXJSEYVS7ZZZZR6IWXGV635VPZN3D3KAAAAFIL4HEYN37BGOIOHFJRRRRQVFYFG2CR4JVUEUKGKKKKKTN2WHHZNDDC5TNYLN2LRQQHTX75Q5IE` (142 characters)
`BA5PZRQ3J42QLI2PYJIE2GW2Z5F6I6K6S3H2KKB2UVVVVRFHYTICTDOP3G5575GVVVVRN3RZFZFZCSTPDDMDCBBBBMSOJCWC3VWFSIPPAPIWWWWR7PT4NBXA` (120 characters)
`BAVKGYL7IR2XBICEYUJE2JL2Z5FQ6NCNN3UBBHUKRVVVVRFHYCTDCTUP7S5DUSJVVVVRN3RXZ2OCZSGMD7VM7BBBBMSOJ4CXVXZFHBPVHVIWWWWR75UINAETWXCSRLTCOAAAA4YKAZRERM` (142 characters)
Additionally, approximately 38% of Group 4 Messages contain repeated multi-character sequences of three or more characters, similar to the pattern observed in Group 3 Messages. This shared characteristic suggests both groups may employ related encoding or formatting schemes.
The function of these repeated sequences is unknown but may serve as delimiters, error detection codes, formatting markers, or structural indicators within the message architecture.
No addressees: Group 4 Messages never include "FOR [CALLSIGN]" specifications. Like Group 3 Messages, Group 4 is always broadcast without addressee designations.
Never 30 characters: Unlike Groups 1, 2, and 3, no documented Group 4 Message has been exactly 30 characters long. This absolute distinction is unique to Group 4 and serves as a reliable negative identifier—if a message is 30 characters, it is definitively not Group 4.
Minimum length: The shortest documented Group 4 Message is 36 characters, establishing a lower bound that distinguishes Group 4 from shorter message groups.
Prevalence
Group 4 Messages are rare, constituting approximately 3% of total HFGCS traffic. This places Group 4 at equal rarity with Group 3 Messages, and both groups combined represent only about 6% of total EAM broadcasts compared to Group 1's dominance at ~66% and Group 2's ~26%.
Group 4 activity appears more consistent than Group 3's sporadic pattern. While Group 3 prefixes often appear briefly then disappear for extended periods, Group 4 prefixes show more sustained usage, though still less frequent than the daily presence of Groups 1 and 2.
Length Distribution
Group 4 Messages show distinctive length clustering at specific high character counts:
Most common lengths:
- 142 characters: 18% of Group 4 traffic
- 120 characters: 8% of Group 4 traffic
- 164 characters: 7% of Group 4 traffic
- 68 characters: 6% of Group 4 traffic
- 50 characters: 10% of Group 4 traffic
- 216 characters: 5% of Group 4 traffic
Secondary common lengths:
- 166, 172, 92, 62, 246, 56, 108, 60 characters each appearing in 2-5% of Group 4 traffic
The concentration at 142 characters is particularly distinctive, with this length alone accounting for nearly one-fifth of all Group 4 Messages. The prevalence of lengths that are multiples of common block sizes (50, 60, 68, 120, 142) suggests these messages follow structured internal formatting with specific capacity requirements.
The longest documented Group 4 Message was 292 characters, representing an outlier in the overall distribution but demonstrating the upper bounds of Group 4 message capacity.
Identification Methodology
Group 4 Messages are among the easiest to identify due to their distinctive length profile and structural characteristics.
Length-based identification: Any message longer than 78 characters with no addressee is very likely Group 4, as Group 3 rarely exceeds this length. Messages at 142, 120, or 164 characters are almost certainly Group 4.
Repeated sequence detection: The presence of repeated 4+ character single-letter sequences (FFFF, XXXX, ZZZZ, etc.) strongly indicates Group 4. While Group 3 also contains repeated sequences, Group 4's prevalence (60% vs 40%) and tendency toward longer repetitions is distinctive.
Elimination process: Messages longer than 163 characters are definitively Group 4, as this exceeds the maximum documented length for Group 2 (163 characters) and Group 3 (78 characters).
Prefix consistency: A prefix can be identified as Group 4 if it is consistently used for messages in the 50-292 character range, never appears with addressee specifications, and frequently contains repeated character patterns.
The distinctive length profile makes Group 4 the most reliably identifiable group after Directed EAMs, which are immediately apparent through their addressee specifications.
Transmission Error Rate
Group 4 Messages show the highest rate of transmission errors and "DISREGARDED" broadcasts among all EAM groups. The combination of extended length and complex character patterns increases the likelihood of operator error during voice transmission.
Common error patterns include:
- Transmission aborted mid-message
- Operator disregards after realizing an error during first or second reading
- Signal loss during extended transmission
- Character misreads requiring correction or restart
This elevated error rate is an inevitable consequence of transmitting 100-200+ character alphanumeric sequences via voice communication and does not appear to indicate any operational deficiency.
Prefix Patterns
Group 4 prefix behavior shows more consistency than Group 3 but remains less frequent than Group 1 or Group 2.
Commonly observed Group 4 prefixes include:
JC (67 documented messages), MN (46), BH (42), JO (31), 6K (24), BW (23), PC (22), RO (21), BJ (21), BV (19), LO (18), ET (17), YP (14), BA (8), H5 (8)
During continuous monitoring from June 2023 through February 2024, Group 4 prefixes appeared intermittently but showed more sustained usage than Group 3 prefixes. For example, the 6K prefix was active across June-July 2023, and the JC prefix showed extended usage from July through September 2023.
Whether Group 4 operates on a rotation schedule similar to Groups 1 and 2 or represents demand-driven usage tied to specific operational requirements remains unclear. The more consistent appearance pattern compared to Group 3 suggests Group 4 may serve ongoing operational needs rather than exercise-specific or condition-dependent purposes.
Relationship to Other Groups
Group 4 exists within a four-group EAM taxonomy and shows particularly strong structural similarities to Group 3:
Structural similarity to Group 3: Both Group 3 (40%) and Group 4 (38%) show nearly identical rates of embedded repeated multi-character sequences (3+ characters). Additionally, Group 4's 60% rate of single-character repetition (4+ characters like FFFF) represents a related but more pronounced pattern. This shared emphasis on repeated sequences suggests both groups may employ related encoding or formatting schemes for specialized operational purposes.
Distinction from Group 3: The primary distinction is length. Group 3 Messages cluster at shorter lengths (22-48 characters typical, 78 maximum) while Group 4 Messages are consistently longer (50-216 characters typical, 292 maximum). This length distinction appears absolute, with no overlap in the typical operating ranges.
Contrast with Groups 1 and 2: Group 1 Messages are always 30 characters, never have addressees, constitute ~66% of traffic, and rarely contain repeated sequences (1%). Group 2 Messages have variable lengths (30-163 characters), frequently include addressees, constitute ~26% of traffic, and show moderate repetition rates (11%).
All four groups operate independently with separate prefix systems. The strong structural similarity between Groups 3 and 4, combined with their comparable rarity (~3% each) and shared absence of addressees, suggests these groups represent specialized communication categories distinct from the baseline Group 1 traffic and directed Group 2 communications.
Operational Context
The distinctive characteristics of Group 4 Messages—extended lengths, embedded repeated sequences, structured length clustering, and absence of addressees—suggest this group serves a specialized operational purpose requiring high information density.
The concentration at specific lengths (particularly 142, 120, and 164 characters) suggests standardized message templates with predetermined capacity requirements. The prevalence of repeated character sequences (60% with 4+ character single-letter repetitions, 38% with 3+ character multi-character sequences) may indicate formatting markers, error detection mechanisms, or structured data fields within the message architecture.
The higher error rate during transmission indicates that Group 4's extended length and complexity represent operational trade-offs—these messages carry more information but at the cost of increased transmission difficulty and potential for errors.
The shared structural characteristics with Group 3 (repeated sequences, specialized formatting) combined with the length distinction suggests Groups 3 and 4 may represent short-form and long-form variants of a related communication architecture serving similar operational purposes.
However, without access to message content or operational documentation, the specific purpose of Group 4 Messages remains speculative.