Group 1 EAM/Claude Draft
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This subject of this article has no known official name. This article covers a subject which has not been named and/or discussed in unclassified military documents. Group 1 EAM/Claude Draft is a theoretical or proposed name. |
Group 1 EAMs (also called Group 1 messages) are those EAMs that begin with what HFGCS monitors have designated as Group 1 prefixes. A Group 1 prefix is a two character code identified through consistent behavioral patterns observed across all messages with that prefix: every message is invariably 30 characters long, no message ever includes addressee designations, and the prefix rotates on a multi-week schedule before being replaced by a new Group 1 prefix. Messages with Group 1 prefixes constitute the most common type of EAM on the High Frequency Global Communications System (HFGCS), representing approximately two-thirds of all EAM traffic and exhibiting absolute structural uniformity that makes them readily identifiable through pattern analysis of monitoring data.
The Group classification system derives from observed message behavior rather than official designation. When examining EAM traffice, monitors note that for certain periods of time, some prefixes will exclusively appear in 30 character messages with no addressees. Those messages can be identified as EAMs with Group 1 prefixes, and therefore categorized as Group 1 EAMs. This identification methodology relies on recognizing structural patterns rather than accessing message content, making Group 1 the most straightforward category to identify despite providing no insight into operational function.
Defining Characteristics
Group 1 messages are invariably 30 characters long. This is not just a general tendency but appears to be an absolute rule; excepting disregarded messages, there is no documented transmission of a Group 1 message that is anything other than 30 characters[Note 1]. The messages consist of a 2 character prefix followed by 28 alphanumeric characters using both letters (A-Z) and numbers (0-9) in what appears to be a random sequence.
Like Group 3 and 4 EAMs, Group 1 messages are never designated "FOR [CALLSIGN]." Unlike Group 2 messages, which frequently specify intended recipients, Group 1 messages are broadcast without any indication of who should receive them. This absence of addressees is consistent across all observed Group 1 transmissions.
Prevalence
Group 1 messages are by far the most common type of message broadcast over the HFGCS. During a period of comprehensive from June 2023 through February 2024[Note 2], 2,309 of the 3,405 documented EAM transmissions (68%) were Group 1 messages. Similar patterns were observed in July 2025, where Group 1 messages comprised approximately 73% of all EAM traffic. Group 1 messages consistently constitute the majority of EAM traffic, typically representing approximately two-thirds of transmissions over extended observation periods.
Group 1 messages are also most frequently rebroadcast message type. Individual messages may be repeated dozens of times over several hours by both ground stations and airborne platforms, particularly E-6B aircraft. When E-6B aircraft select messages for rebroadcast over HF and VLF frequencies, Group 1 messages are preferentially included, often comprising the majority or entirety of E-6B rebroadcast traffic.
Prefix Rotation
Group 1 prefixes change on a schedule that appears regular but is not publicly documented. Each prefix typically remains active for 3-8 weeks before the network transitions to a new one. These rotations occur independently of other message groups and appear scheduled rather than reactive to operational events.
The following table shows the Group 1 prefix rotation sequence observed during continuous monitoring from June 2023 through February 2024:
| Prefix | Active From | Active Until | Days Active | Unique Messages |
|---|---|---|---|---|
| YL | June 02 2023 | July 06 2023 | 29 days | 132 |
| YT | July 06 2023 | August 10 2023 | 36 days | 189 |
| PJ | August 10 2023 | September 20 2023 | 42 days | 239 |
| CZ | September 20 2023 | November 11 2023 | 52 days | 248 |
| WR | November 11 2023 | January 07 2024 | 54 days | 251 |
| AS | January 08 2024 | February 09 2024 | 33 days | 192 |
Identification Methodology
Hearing a 30 character EAM with no addressee is a strong indicator of a Group 1 message, but it's not definitive. Group 2 messages can also be exactly 30 characters long, and while many Group 2 messages include addressees, some do not. While are, Group 3 messages that are 30 characters long have been documented. A 30 character Group 4 message has yet to be documented.
The critical distinction is that Group 1 messages are always 30 characters, while other groups only sometimes produce this length. Definitive identification requires observing prefix behavior over time: if every message with a given prefix is 30 character long and never has any addressees, it is almost certainly the current Group 1 prefix. E-6B rebroadcast patterns provide additional confirmation, as these aircraft heavily favor rebroadcasting Group 1 messages.
To identify the current Group 1 prefix, monitor HFGCS traffic for several hours or review a day's worth of recorded traffic. Note which prefixes are being used for 30 character messages without addressees, check if any prefix is being used exclusively for such messages, and observe rebroadcast patterns. Confirmation over multiple days ensures the prefix behavior remains consistent.
Example 1: September 01 2023
| HFGCS TIMECARD 230901 |
Four messages were broadcast on September 01 2023, using two prefixes: RM and PJ. The first analytical step involves examining the RM prefix, which appeared in two transmissions that day. The 13:11 UTC and 21:03 UTC messages beginning with RM are both 30 characters long, initially suggesting potential Group 1 characteristics. However, both messages specify addressees, which immediately disqualifies RM as a Group 1 prefix since Group 1 messages never include addressee designations. This combination of 30 character length with addressee specifications clearly identifies RM as a Group 2 prefix, demonstrating how Group 2 messages can sometimes share the same character count as Group 1 while maintaining their distinct addressing behavior.
The PJ prefix presents a contrasting pattern that aligns perfectly with Group 1 characteristics. The 21:32 UTC and 22:28 UTC messages beginning with PJ are both 30 characters long, matching the expected Group 1 length requirement. More significantly, neither message specifies an addressee, fulfilling the second critical criterion for Group 1 identification. This combination of consistent 30 character length and complete absence of addressee designations makes PJ the likely Group 1 prefix for this period.
Monitoring subsequent days (September 02, September 03) confirms that PJ continues to behave as a Group 1 prefix, while RM continues showing Group 2 characteristics with variable lengths and frequent addressee specifications. This is an ideal case, where clear separation between prefix behaviors makes identification straightforward.
Example 2: June 17 2024
| HFGCS TIMECARD 240617 |
Seven messages were broadcast on June 17 2024, using three prefixes: BJ, N3, and 5F. This example demonstrates a more complex identification scenario where multiple prefixes with different characteristics must be distinguished to isolate the Group 1 prefix. The BJ prefix appears in a single transmission at 07:13 UTC with a message length of 216 characters. This substantial length immediately disqualifies BJ from consideration as a Group 1 prefix, as Group 1 messages maintain an invariable 30 character format. The dramatic deviation from this length requirement eliminates any ambiguity about BJ's classification.
The N3 prefix presents a case similar to the RM prefix from the previous example, where 30 character length combines with addressee specification to indicate Group 2 classification. The 21:17 UTC message beginning N3 is exactly 30 characters long, matching the Group 1 structural requirement, but the message specifies an addressee (FLUKE97), which definitively identifies N3 as a Group 2 prefix. This again illustrates the critical distinction that while Group 1 messages are always 30 characters without addressees, Group 2 messages can sometimes match the 30 character length while maintaining their characteristic addressee designations.
The 5F prefix exhibits all the hallmarks of Group 1 classification and benefits from additional confirming evidence through rebroadcast patterns. The 21:46 UTC message beginning 5F is 30 characters long with no addressee specified, satisfying both primary Group 1 criteria. More significantly, this message is rebroadcast by an E6 aircraft using the callsign GUNBARREL, providing strong supporting evidence for 5F's Group 1 status since E-6B aircraft preferentially select Group 1 messages for rebroadcast operations. Additionally, 5F becomes the most frequently transmitted prefix that day, a pattern consistent with Group 1's typical dominance in daily EAM traffic. The combination of structural characteristics, rebroadcast selection, and transmission frequency makes 5F very likely the Group 1 prefix for this period. Monitoring over subsequent days would confirm 5F continues showing consistent Group 1 behavior. This case demonstrates how E6 rebroadcast patterns can help confirm Group 1 identification when multiple groups are active.
Example 3: June 16 2024
| HFGCS TIMECARD 240616 |
Thirteen messages were broadcast on June 16 2024, using three prefixes: N3, MK, and 5F. This example illustrates the complications that arise when prefix transitions occur, requiring correlation with adjacent monitoring periods to resolve ambiguous identification scenarios. The N3 prefix, which appeared in the previous example as a Group 2 prefix, maintains its characteristic pattern here. The 18:03 UTC message beginning N3 is 30 characters long and specifies an addressee (LARD91), confirming N3's consistent Group 2 behavior across both monitoring days.
The MK prefix initially appears to be the clear Group 1 prefix based on all available evidence from this day's monitoring. The 18:32 UTC message and multiple subsequent messages beginning with MK are all 30 characters long without exception, demonstrating the absolute length consistency that characterizes Group 1 messages. No messages using the MK prefix specify addressees, fulfilling the second critical Group 1 requirement. Messages with the MK prefix are rebroadcast by an E6 aircraft using the callsign EXAMINER, providing the characteristic rebroadcast pattern associated with Group 1 messages. Most compellingly, MK is the most common prefix for most of the day, exhibiting the transmission frequency dominance typical of Group 1 prefixes. Based solely on this day's monitoring data, the initial assessment identifies MK as the Group 1 prefix with high confidence.
However, the appearance of the 5F prefix late in the monitoring period introduces a complicating factor that challenges this straightforward identification. The 20:57 UTC message beginning 5F is 30 characters long with no addressee specified, exhibiting precisely the same Group 1 characteristics as the MK prefix. This presents a puzzle, as only one Group 1 prefix should be active at a time under the established understanding of prefix rotation patterns. The appearance of two prefixes (MK and 5F) both exhibiting Group 1 characteristics suggests either an anomaly in the classification system or a transition in progress.
Resolution of this ambiguity requires examining subsequent monitoring data, which reveals that this day captured a prefix transition in progress. The previous example analyzed in this methodology section happens to be the next day after this one (June 17 2024). Recall that there we had identified 5F as a Group 1 prefix; 5F had become the dominant prefix while MK had vanished entirely. This pattern indicates a prefix transition in progress, with 5F replacing MK as the new Group 1 prefix. The June 16 2024 monitoring captured the final hours of MK's active period and the initial appearance of its successor 5F, creating the temporary appearance of two concurrent Group 1 prefixes. Monitoring into June 18 and beyond would confirm that 5F replaced MK and remained the Group 1 prefix going forward.
This example demonstrates information limitations which can be inherent to incidental monitoring.
Relationship to Other Groups
Group 1 exists within a four-group taxonomy of Emergency Action Messages. Group 2 messages have variable lengths (30-163 characters) and often specify addressees. Group 3 messages may be shorter or longer, strongly clustered at 22, 27, 32, and 37 characters, and never include addressees. Group 4 messages are the longest (36-292 characters), frequently contain repeated character sequences, and also never specify addressees.
The four groups appear to operate independently; their prefix rotations don't usually appear to be coordinated, and all four groups can be active simultaneously with different prefixes. Group 1's overwhelming prevalence compared to Groups 3 and 4 may suggest that it serves a more fundamental or frequent operational purpose within the HFGCS architecture.
See also
Notes
- ↑ This is technically not true, but it's at a point that whenever a message using a Group 1 prefix is anything other than 30 characters, it appears to be because of either an HFGCS operator error or else some sort of transmission/reception issue.
- ↑ This monitoring period corresponds to Part 1 of the NEET INTEL DAILY TIMECARD PROJECT.