EAM/Group 1 Prefix

A chart quantifying EAM traffic over a 7 month period; Group 1 EAMs in green. (Click to view larger version)

Group 1 Emergency Action Messages (Group 1 EAMs or Group 1 Messages) are the most common type of EAM broadcast on the High Frequency Global Communications System (HFGCS). They are characterized by absolute structural uniformity: every Group 1 Message is exactly 30 characters long, consists of a 2-character prefix followed by 28 alphanumeric characters, and never includes an addressee designation.

This uniformity makes Group 1 EAMs simple to identify. While dominating HFGCS traffic, constituting approximately two-thirds of all EAM transmissions, their lack of distinguishing features provides no insight into their content or operational function.

Defining Characteristics

Group 1 EAMs are invariably 30 characters long. This is not a tendency but an absolute rule—no properly transmitted Group 1 Message has ever deviated from this length. Shorter transmissions that appear in monitoring logs are transmission errors where operators aborted the message mid-broadcast, typically marked as "DISREGARDED." 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.

Group 1 EAMs are never designated "FOR [CALLSIGN]." Unlike Group 2 Messages, which frequently specify intended recipients, Group 1 EAMs 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 EAMs are by far the most common type of message broadcast over the HFGCS. During a period of comprehensive from June 2023 through February 2024, 2,309 of the 3,405 documented EAM transmissions (68%) were Group 1 EAMs. Similar patterns were observed in July 2025, where Group 1 EAMs comprised approximately 73% of all EAM traffic. Group 1 EAMs consistently constitute the majority of EAM traffic, typically representing approximately two-thirds of transmissions over extended observation periods.

Group 1 EAMs 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 EAMs 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 EAMs 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 EAMs.

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

Load video YouTube YouTube might collect personal data. Privacy Policy Continue Dismiss

HFGCS TIMECARD 230901

Four messages were broadcast on September 01 2023, using two prefixes: RM and PJ.

Analysis of RM prefix:

• The 13:11 UTC and 21:03 UTC messages beginning with RM are both 30 characters long
• Both messages specify addressees
• Conclusion: RM is clearly a Group 2 prefix

Analysis of PJ prefix:

• The 21:32 UTC and 22:28 UTC messages beginning with PJ are both 30 characters long
• Neither message specifies an addressee
• Conclusion: PJ is likely the Group 1 prefix

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

Load video YouTube YouTube might collect personal data. Privacy Policy Continue Dismiss

HFGCS TIMECARD 240617

Seven messages were broadcast on June 17 2024, using three prefixes: BJ, N3, and 5F.

Analysis of BJ prefix:

• The 07:13 UTC message beginning BJ is 216 characters long
• Conclusion: BJ cannot be Group 1 due to length

Analysis of N3 prefix:

• The 21:17 UTC message beginning N3 is 30 characters long
• The message specifies an addressee (FLUKE97)
• Conclusion: N3 is clearly Group 2

Analysis of 5F prefix:

• The 21:46 UTC message beginning 5F is 30 characters long
• No addressee is specified
• The message is rebroadcast by an E6 (GUNBARREL)
• 5F becomes the most frequently transmitted prefix that day
• Conclusion: 5F is very likely the Group 1 prefix

The E6 rebroadcasts provide strong supporting evidence for 5F's as the Group 1 prefix, as these aircraft preferentially rebroadcast Group 1 EAMs. 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

Load video YouTube YouTube might collect personal data. Privacy Policy Continue Dismiss

HFGCS TIMECARD 240616

Thirteen messages were broadcast on June 16 2024, using three prefixes: N3, MK, and 5F.

Analysis of N3 prefix:

• The 18:03 UTC message beginning N3 is 30 characters long
• The message specifies an addressee (LARD91)
• Conclusion: N3 is Group 2

Analysis of MK prefix:

• The 18:32 UTC message and multiple subsequent messages beginning with MK are all 30 characters long
• No messages specify addressees
• Messages are rebroadcast by an E6 (EXAMINER)
• MK is the most common prefix for most of the day
• Initial assessment: MK appears to be the Group 1 prefix

Analysis of 5F prefix:

• The 20:57 UTC message beginning 5F is 30 characters long
• No addressee is specified
• Problem: This also exhibits Group 1 characteristics

The appearance of two prefixes (MK and 5F) both exhibiting Group 1 characteristics presents a puzzle, as only one Group 1 prefix should be active at a time. The previous example we looked at happens to be the next day after this one. 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. Monitoring into June 18 and beyond would confirm that 5F replaced MK and remained the Group 1 prefix going forward.

This example demonstrates why incidental monitoring may not always provide definitive answers.

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

• Emergency Action Messages
• EAM/Group 2 Prefix
• EAM/Group 3 Prefix
• EAM/Group 4 Prefix
• High Frequency Global Communications System