|USN E-6B Tacamo on the ramp at Offutt AFB, Nebraska. (US Navy photograph)|
This is an ongoing paper I have worked on for over three decades now on various high precedence messages passed by the U.S. military on their HF/VHF/UHF/EHF/Milsat frequency bands. I will continue to add to the thread as new material is found or changes are observed.
Most Unusual DoD Foxtrot/Skyking Message Yet!
On February 29, 2016, our good friend and European Ute monitor Ary Boender caught an unusual and first of its kind Foxtrot/Skyking message broadcast on the US DoD HFGCS communication network at 1548 UTC.
As noted in the sections below, Foxtrot aka Skyking messages is the highest precedence messages transmitted on the HFGCS network. They are aircraft advisory messages that we believe are intended for aircraft assets of the highest mission priority. This message is a bit unusual and was a departure from the normal format with no Skyking, message directed directly to Melon 68, included a diplomatic clearance and was transmitted in the blind.
In recent days we have had DoD recon assets flying in the Middle East out of Turkey in Syria observing the battlefield there. We believe that some of these unusual assets may have been using an odd ADS-B hex code and the MELON call sign. This message may have been important to pass along the Diplo clearance the aircraft needed to enter, say Turkish airspace.
Ary has shared that clip with us and allowed us to post it to our blog. You can hear that message by clicking on this link Audio Clip Here.
This message appears to confirm the circumstantial evidence that I have gathered over the last few years that Foxtrot messages do involve high-value aircraft assets such as recon aircraft. More hopefully soon - Chief.
Single Channel Transponder System (SCTS)
Found this little tidbit up at the FAS.org website. "The Single Channel Transponder System (SCTS) provides Emergency Action Message (EAM) and Force Direction Message (FDM) dissemination capability to command centers (USSTRATCOM, EUCOM, SHAPE, NMCC, USSPACECOM, and PACOM) and force elements for the control of strategic and non-strategic nuclear forces.
"The Single Channel Transponder System (SCTS) was fielded in 1985 as an interim system to provide jam, nuclear resistant communications to U.S. strategic forces. SCTS was to bridge the gap between the Air Force Satellite Communications (AFSATCOM) system and the new Milstar EHF satellite communications system. It provides SHF one-way satellite communications between the command centers and the nuclear-equipped ground forces through the fixed ICBM SHF Satellite Terminals (ISST) and the mobile Single Channel Transponder Receivers (SCTR). In addition, the SHF uplink from the command centers can be cross-banded through the satellite to UHF for reception by Dual Modem terminals located on bombers and tankers. The satellite element of SCTS uses the SCT package on the DSCS satellite system. The DSCS satellite is not planning to support strategic communications beyond 2003. Strategic ground systems will transition to the EHF spectrum. The ground terminals will be replaced by the Intercontinental Ballistic Missile (ICBM) EHF terminals and the Secure Mobile Anti-Jam Reliable Tactical Terminal (SMART-T).
"The system provides one of the survivable means of disseminating Emergency Action Messages (EAMs) and Force Direction Messages (FDMs) from the National Command Authorities (NCA) and the warfighting CINCs."
Besides EAMs there is another high precedence message that is passed on the HF-GCS, known within the monitoring community as a Foxtrot aka Skyking messages. Their purpose is a bit more obscure, but to date, they are the most important message passed on the HF-GCS network. We have strong indications that these are “NOT” part of the strategic message systems (such as EAMS) and they can and have bumped EAM messages traffic. That would indicate a higher message traffic precedence.
To validate that in two public Air Force instructions I found three specific references to Foxtrot aka Skyking messages. "The HFGCS missions are to operate and maintain HF Mystic Star C2 networks, disseminate Emergency Action Messages (EAM), provide air to ground phone patches, and ensure timely and accurate broadcasts of reconnaissance and aircraft advisories (italics is mine) for the President, Joint Chiefs of Staff, Combatant and Major Commands, allies, and other non-DoD users. . . . Perform all alerts/seizures for the entire Global System. IE: Emergency Action Messages and Aircraft Advisories." In another public AF instruction, they specifically referenced aircraft advisories as "FOX" messages. These messages use AF Form 3656 titled HF Radio Facility Foxtrot Message Blank.
Normally this message consists of 3-characters, a minute time stamp followed by a 2-character authentication code. This year (2015) we have seen occasional use of code words in place of the 3-characters ahead of the time stamp. We do not know what this change means but it is certainly something that bears watching.
Force Direction Messages (FDM)
Normally EAMs and another message type broadcast over the HFGCS Force Direction Messages (FDMs), close kin to the EAM, are indistinguishable to only those that have access to information to decode them. I recently came upon an interesting tidbit while doing an early morning Google search.
From an AFGSC Instruction, Re: Minuteman Tracking Capabilities: "Remote Targeting Description. MM has the capability to transfer Force Direction Message (FDM) targeting data received over the Strategic Automated Command and Control System (SACCS) directly into the Weapon System Control Element (WSCE) for input, generation, and Remote Data Change (RDC)."
And from another publicly posted document. "For decades, Air Force missileers have maintained hard-copy casebooks which document all target and execution plan cases . . . In the past, updating hard-copy casebooks usually meant cutting and pasting the contents of a Force Direction Message (FDM) . . ."
Wonder if the long variable string EAMs we hear occasionally on HFGCS are in reality Force Direction Messages (FDM)?
Major Change to EAM Broadcast Format - First Change in 17 Years
Around April 1, 2013, during the Spring 2013 Korean Nuclear Missile crisis there was something noteworthy on that occurred on the HFGCS as posted by Jeff Haverlah. The HFGCS system has been running many 30-character EAM strings since at least 0300z+/- and probably earlier, with all 30- character strings rebroadcast at the systems h+00/h+30 restoral windows. OFFUTT broadcast 30-character strings 4V75ZO; 4VDWVK; 4VY6Vv at 1100z; and, a single string (4V75ZO maybe)
at 1130z. As with any string greater than 28-characters all strings are sent with the character count statement. Unusual.
Yes, this is very unusual Jeff. I had an HF receiver parked on an HFGCS and it was interesting to not hear any 6/20/28c EAMs. I did monitor an RQ 30c/ RV 30c/ RQ 39c series that had the look and feel of the old 6/20/28 messages.
Normally we only see major changes to the EAM broadcast system at the beginning of DoDs fiscal year (i.e. October 1). So this makes us wonder since this change occurred around April 1, was this due to the Korean crisis (maybe a Defcon level increase which changed the EAM format), or is this a permanent change to the EAM broadcast system. Only time and additional monitoring will tell us for sure.
Monitoring Times Milcom Column December 2009By Larry Van Horn
Copyright 2009 by author and Monitoring Times magazine. This article may not be reprinted or reposted in any form, links only are ok back to this article.
"November Foxtrot India India Four Sierra, I say again, November Foxtrot India India Four Sierra, this is Andrews out."
So you just heard a transmission like the one above on 8992 kHz and it now has you scratching your head wondering what was it? Most likely it was an EAM or Emergency Action Message transmitted by the U.S. military.
It really doesn't seem that long ago that I first wrote about this topic here in the pages of MT. In reality, it has been over 15 years while I was still writing the Utility World column that I first discussed what an EAM was. Since that time the learning process has not stopped.
Dedicated monitors continue to intercept these unique high priority message and learn about how the military uses them in their operations. To bear fruit, this sort of effort takes a long time to monitor the broadcast, compile information, compare it to public information, analyze it and come to some sort of conclusions based on what has been compiled.
I really need to put in a caveat here for all you who prowl the Internet. I have seen a lot written about these military messages over my many years as a monitor and writer, and unfortunately, quite a bit of it is just plain junk. Fortunately for our radio hobby several monitors have dedicated a major portion of their listening hobby to the study of these broadcasts and have slowly, but surely uncovered some of the basic facts that surround the usage of these U.S. Department of Defense transmissions.
In this edition of MT's Milcom, I will publish some of the new information never before released within our radio hobby on these interesting DoD messages that we have found and put together.
But first, I'm going to wind the clock back a bit. Here is some of what we published in the past to catch you up regarding what we knew about EAMs transmissions 15 years ago.
So what is an EAM really?
From Jeff Haverlah's "What is an EAM?" available on the Monitoring Times website (Blog editor note: note also published below after this article):
From Strategic Command, Control, and Communication - Alternative Approaches For Modernization; John J Hare, Richard H. Davison, and Peter Tarpgaars; Congressional Budget Office (CBO), October 1981 –
Page 12: ". . . . Proper coding and formatting of EAMs are of crucial importance since nuclear forces are prepared to execute any messages they receive that meet rigid specifications. In addition to specific instructions contained in an EAM, proper coding provides the means by which a commander expresses his authority to release nuclear weapons and an officer controlling those weapons verifies that authority."
Page 44: "EAM: Though generally referring to a category of urgent messages from commanders to deployed forces, EAM is often used as a short-hand expression for a specially coded nuclear attack directive."
From the May 1995 MT page 33, Monitoring Times Utility World column by this author: "Several issues ago (Dec 94) we talked about the U.S. military's Emergency Action Messages (EAM) broadcast. Here is an interesting explanation, taken from a U.S. Air Force manual, of what an EAM is.
"Joint Chiefs of Staff Emergency Action Messages (EAMs) contain key instructions or information from high-level authority and have predetermined formats (pro forma). Such messages are transmitted by various communications systems and normally carry FLASH precedence. They are vital messages of an extremely time-sensitive nature, and rapid processing is mandatory to obtain the fast reaction required by their content. Usage and handling procedures are of the highest classification and have been issued by the JCS only to those who have a need to know." (AFM-01-1-18, sub 3, amended 01 Jan 1990)."
Since that information was published I have uncovered a few more sources with information on EAM messages. A 2001 U.S. Army regulation issued at the Rock Island Arsenal gave a slightly different twist on EAMs from their service viewpoint.
"EAMs come in as FLASH or IMMEDIATE messages. Ordinarily, they provide notification of a change in Defense Readiness Condition (DEFCON) status, but they are also used to alert personnel of any emergency situation needing immediate action."
According to the instruction above an EAM may pertain to "a real emergency, a scheduled exercise, or a special test." So one of the important things that monitors need to keep in mind is that not "all" EAMs involve critical real-world events.
In a recent USAF Wing instruction, it indicated that an EAM could be used for emergencies, contingencies, and exercises as well. In this instruction, one possible result based on the unit receiving an EAM follows: "The 403 WG will notify the 81 TRW Command Post upon receipt of an emergency action message directing recall for the 403 WG."
In fact, during one of our recent sweeps of the Internet, we uncovered an unclassified document that clearly stated that TACAMO aircraft regularly receive "exercise equivalents of actual emergency action messages."
The Navy and EAMs
Every time I think of EAMs and the Navy, I always think about one of my favorite movie dramas – Crimson Tide. We certainly saw a lot of EAM traffic in that movie. How accurate was that portrayal compared to the real world? I really can't say for sure as I have never wanted to do the sub thing. Based on some information that I have uncovered, it may not be totally accurate and some license was definitely taken when compared with what actually happens regarding how a submarine finds out it has an EAM message when it is doing the underwater gig.
In the world of Navy SSBN submarines, they operate a bit differently when working with and using EAM transmissions. The Navy ELF communications system broadcast three-letter codes – as described by Douglas Waller in Big Red: Inside the Secret World of a Trident Nuclear Submarine – that didn’t seem to be authorization codes to launch nuclear weapons:
"If the sub had to operate more covertly, still another wire antenna could be reeled out for two thousand feet to receive extremely low frequency (ELF) signals that penetrated deep into the water. The ELF signal came in agonizingly slowly, so the message consisted of only three-letter codes. The shack had an inch-thick book in its safe that could translate each trigraph, often sent as a bell-ringer to order the sub to sail nearer the surface so it could pick up a lengthier message on another frequency."
At this point, if you're on the sub you would probably hear "Conn, Radio, receiving EAM."
The Emergency Action Message, according to Waller, contained much more information – sets of instructions identifying the war plan indicating the number of weapons and targets; date and time window for attack; combination to the safe containing the launch keys and an authentication code.
The Definitive Word?
But I think the best information comes from the top dogs in DoD, the Joint Chiefs of Staff (JCS). In a JCS instruction 5721.01D dated February 8, 2008, on Nuclear Command, Control, and Communications (NC3) Hybrid Solution (HS), they wrote:
"(1) The NC3 HS Emergency Action Message (EAM) architecture supports fixed and mobile EAM injectors and recipients and provides for EAM dissemination to time-critical (TC) and non-TC users...In addition to EAM dissemination, the NC3 HS provides transport for the general service (GENSER) traffic up to TOP SECRET OPLAN 8044...
"(2) EAMs are highly structured, authenticated messages primarily used in the C2 of nuclear forces. EAMs are disseminated over numerous survivable and non-survivable communication systems, including terrestrial and space systems. The NC3 HS is the principal means of dissemination of EAMs in a pre-attack environment. The NC3 HS is comprised of several existing systems including the Navy’s Nova, the Air Force’s Strategic Automated Command Control System, the Defense Improved Emergency Message Automatic Transmission System Replacement Command and Control Terminal, the DMS, and the Pentagon Telecommunications Center."
In another online publication published in 2008, the Nuclear Matters: A Practical Guide, Chapter 5 had these two statements:
"Emergency Action Message—Use Authorization Control. An Emergency Action Message (EAM) is the medium through which actions involving nuclear weapons are authorized. These messages are encrypted and sent to lower-echelon units for action. The messages have different formats and may require authentication with sealed authentication code cards depending on the intent of the message.
"National Military Command and Control System. The Joint Staff Director for Operations (J-3) operates the C2 system. EAMs are conveyed to the Combatant Commands through secure communications links."
But this may not be the whole story. In another publication we recently uncovered on the Internet it clearly indicated that the JCS may not be the only originator of EAM messages that we hear coming from our radio speakers. There are also STRAT Emergency Action Messages that are transmitted by Navy TACAMO aircraft. If a major command like the US Strategic Command can issue EAMs, who else also can originate and release their own EAM traffic into the various communications systems that DoD uses?
So how do they disseminate EAMs?
This depends on the branch of the military you are looking at. Radio hobbyists usually hear EAM broadcasts on the Joint Chiefs of Staff/Air Force operated HF-GCS network. But that is only a small tip of the iceberg, and there is a lot more that we never hear or see.
The following radiotelephone, radioteletype, and land-based systems/communications networks have been confirmed as carriers of DoD EAM broadcasts:
Landline based systems: Autodin/Nova, Strategic Automated Command and Control System (SACCS), Digital Remotely Programmable Conference Arrange (DRPCA), Defense Switching Network (DSN), Chairman, Joint Chiefs of Staff Alerting Network (CJCSAN), Strategic Emergency Action Telephone System (SEATS), DDN, and the Strategic Operational Conference System (SOCCS).
Other known communications networks: UHF-FDM Northstar GEP Network, CJCS HF Broadcast on CJCS HF network and its sub-nets, Survivable Mobile Command Center (SMCC) HF network, Worldwide TACAMO (WTAC), SMCC UHF LOS, CJCS VLF/LF, CJCS CINCNET, TACAMO Intranet, UHF ADIS, CJCS Data* (JCSDATA), CJCS Voice* (JCSVOICE), CJCS EAM Cross-banded Network* (JCSEAM-X), Missile Warning Teletype* (MWTTY), Mobile Ground System-Global Summary Message* (MGS-GSM), CTF-124 Voice, Strategic Force Management Network* (STRATFM), Strategic Report Back Submarine Network* (STRRBSUB), STRINTEL*, STRINTD-E*, STRRB-E, STRFD-E, National Command Authority Secure* (NCASEC), North Secure* (NORTHSEC), STRAT Secure* (STRATSEC), and Airborne Launch Control System (ALCS).
These networks can use VLF, HF and UHF frequency ranges, AFSATCOM transponders and Milstar EHF/AEHF satellite capability. In the list above an * indicates a Military Strategic and Tactical Relay (MILSTAR) satellite network.
Things may not stay the same.
Nothing in the nuclear mission world stays static, especially with the dissemination of EAM messages. Back in 2005, Rockwell Collins was selected by the U.S. Air Force as the prime contractor for Phase 2 of the Ground Element Minimum Essential Emergency Communications Network (MEECN) System (GEMS) program.
The GEMS program replaces Air Force and Navy fixed and deployable communications for bomber, tanker, reconnaissance and other alert communications facilities. When operational, this system will provide an upgraded networked infrastructure incorporating improved capabilities for aircrew alerting, message handling and the supporting communications links.
Improvements include updated Extremely High Frequency (EHF)/Advanced EHF (AEHF) satellite communications and redundant Very Low Frequency (VLF) communication paths for critical strategic message traffic. The improvements will eliminate obsolescence issues associated with the current aircrew alerting devices (pagers and klaxons), Ultra High Frequency (UHF) communications and Emergency Action Message (EAM) processing systems.
Rockwell Collins has been involved in the MEECN system since its inception and continues to provide key portions of the network for the U.S. military. As of this writing, we are still not sure if GEMS is up and running. There may be additional systems and networks that we still do not know about as we approach 2010.
Where to Hear an EAM.
So if you want to hear an EAM broadcast, where do you park your receiver or scanner?
The best place to monitor these transmissions is on HF when they are transmitted by stations working within the previously mentioned USAF HF-GCS network. Tune your receiver to one of the following frequencies (low at night, high during the day), switch on the upper sideband (USB) mode, and wait. Frequencies: 4724.0 6739.0 8992.0 11175.0 13200.0 15016.0 kHz
If you are lucky to be within line of sight range of one of the airborne units that transmit EAMs you might want to program 311.000 321.000 or 323.800 MHz (AM mode) into your scanner and wait for one to be transmitted.
Whether they are transmitted for exercise or contingency purposes, or an indication of something important happening in the real world, Emergency Action Messages are a fascinating aspect of the Milcom radio hobby to monitor and study.
Milcom Resource Guide
Nuclear Matters: A Practical Guide, Chapter 5 http://www.acq.osd.mil/ncbdp/nm/nmbook/chapters/ch5.htm
What does an EAM sound like? Check out this YouTube EAM Video: http://www.youtube.com/watch?v=fTZuJQ4xtE0
What is an EAM?By Jeff Haverlah
"EAM: Though generally referring to a category of urgent messages from commanders to deployed forces, EAM is often used as a short-hand expression for a specially coded nuclear attack directive."
From "Strategic Command, Control, and Communication - Alternative Approaches For Modernization"; John J Hare, Richard H. Davison, and Peter Tarpgaars; Congressional Budget Office (CBO); October 1981; Page 44:
"....Proper coding and formatting of EAMs is of crucial importance since nuclear forces are prepared to execute any messages they receive that meet rigid specifications. In addition to specific instructions contained in an EAM, proper coding provides the means by which a commander expresses his authority to release nuclear weapons and an officer controlling those weapons verifies that authority."
Ibid, Page 12:
“[In the December 94 issue of Monitoring Times] we talked about the U.S. military's Emergency Action Messages (EAM) broadcast. Here is an interesting explanation, taken from a U.S. Air Force manual, of what an EAM is.
"Joint Chiefs of Staff Emergency Action Messages (EAMs) contain key instructions or information from high-level authority and have predetermined formats (pro forma). Such messages are transmitted by various communications systems and normally carry FLASH precedence. They are vital messages of an extremely time-sensitive nature, and rapid processing is mandatory to obtain the fast reaction required by their content. Usage and handling procedures are of the highest classification and have been issued by the JCS only to those who have a need to know." (AFM-01-1-18, sub 3; amended 01 Jan 1990)."
From the May 1995 Monitoring Times; Utility World column by Larry Van Horn; page 33; section titled What are EAMs?"
From a shortwave utility hobbyist's standpoint, what are they? Park your HF receiver (set to upper sideband mode) on 15016.0 kHz, 13200.0 kHz, 11175.0 kHz (the most productive for day to day monitoring of the U.S. military on HF), 8992.0 kHz (ideal for monitoring during North American nights), 6739.0 kHz, 6712.0 kHz or 4724.0 kHz.
Eventually, you'll hear ground stations of the USAF's HF-GCS (led by ANDREWS or OFFUTT or MCCLELLAN, identified in the clear since 1992) broadcast one or more Emergency Action Message or EAM. You will hear a six-character alphanumeric string (known as the "preamble") read phonetically, repeated three times. This will be followed by the same 6-character string either by itself (as the entire message), or concatenated with additional alphanumeric characters to produce alpha-numeric strings that total 28 characters (the most common length; 30-characters prior to 01 Oct 2000; 26-characters prior to 01 Oct 1998), or 22 characters (20 character prior to 01 Oct 2000), or strings with character-counts that can extend into the hundreds of characters (with the available character set universe consisting of all 26-characters of the English alphabet plus the numerals two, three, four, five, six and seven; with extremely rare exceptions there are no zeros, ones, eights or nines heard in these strings).
[Eventually you might discover that this HF EAM activity is also heard on a group of HF frequencies that are known as the ZULU frequencies utilized by communication assets (both airborne and ground-based) of the JCS and U.S. Strategic Command (see the "Military Lists Area" column in any recent WUN newsletter for the known frequencies) and on HF frequencies utilized by the U.S. Navy during apparent exercises. However, since FY 2000 the ZULU frequencies have become much less active with daily connectivity communications to the point of silence.]
The above activity is heard daily, 7 days a week, 365 days a year, year after year.
The second group of coded messages heard on the GHFS (and the ones that produce the most comments from new listeners because of the "SKYKING do not answer" nature of the broadcasts) are the so-called FOXTROT broadcasts. These are heard only on the GHFS frequencies listed in the previous paragraph and take the form of the GHFS operator broadcasting a message that states "SKYKING, SKYKING. Do not answer. [3-element alpha-numeric group] [minutestamp] [time dependent two-character authenticator]" and repeated once.
These transmissions appear to be initiated by any of the GHFS ground stations except ASCENSION and HICKAM, with the initiating ground station prefacing the broadcast with a codeword that can consist of DECENT (or DESCENT), ENLIST, FAIRLY, EYESTRAIN (or sounds as), DEFROSTER, "ANY STATION" and maybe one more codeword recently reported. The codeword appears to determine which other GHFS ground stations are to "echo" the transmission. As an example, the DECENT transmission appears to apply only to CONUS stations while all the others appear to apply (with an occasional exception) to non-CONUS stations that fall outside an arc from Guam to Japan to Alaska to Greenland to the UK to the Azores.
These coded messages are said to be *only* for the positive control of ACC/AMC airborne forces detailed to the U.S. Strategic Command (USSTRATCOM). These messages appear to have a higher priority on the GHFS voice circuits than do the EAM transmissions as an EAM transmission will be terminated in mid string in order to immediately transmit these "SKYKING" DNA broadcasts.
04 Apr 2005 Update:The information in the above paragraph was written in 1998 and will remain unaltered in the interest of "history”; however, unlike the subtly changing information in the earlier EAM paragraph, there are some notable changes in the FOXTROT broadcasts since 1998.
Prior to 1992, all FOXTROT broadcasts were initiated with aliased echo rotation callups similar to that described above.
For a short time in the second half of 1992 (which represents the activation of the USSTRATCOM following the deactivation of Strategic Air Command) the echo rotation call-ups were "in the clear."
From maybe late in 1992 (or early 1993 lost in the fog of memory) the GHFS went back to aliased callup echo rotations as described in the above paragraph.
In the 20th century, the GHFS ceased to exist, replaced with the HF-GCS.
In the immediate post 11 September 2001 time period, during the run-up to the purging of the Taliban from Afghanistan, the echo rotations appeared to go through a transformation. They were no longer aliased and at least two new stations were added to the echo rotation call up: DIEGO GARCIA and for a brief period CYPRUS FLIGHT WATCH. After this conflict, ALL requests for echo rotation disappeared from the FOXTROT broadcasts, and it remains that way into CY 2005.
Detailed information can be found from the following sources:Hobby sources
Ary Boender's "Numbers & Oddities" column in the July 1995 (FOXTROT broadcasts) and August 1995 ("EAMs") WUN Newsletters (both newsletters are now "archived" at the WUN web site to save drive space - wunv1n7.zip and wunv1n8.zip) - a short overview of how these broadcasts manifest themselves on various HF frequencies.
The December 1994 issue of Monitoring Times containing the "Utility World" column of Larry Van Horn, titled "US Air Force Global High Frequency (HF) System." A concise overview of the GHFS, and the traffic contained on the GHFS.
The September 1995 issue of Monitoring Times containing the "Utility World" column of Larry Van Horn, titled "What's the meaning behind the messages." An overview of the "message" traffic heard on the GHFS and the NIGHTWATCH net.
"The Aeronautical Communications Handbook - HF Edition" by Robert E. Evans; 1989 (and out of print, I believe); pages 7.11-7.13. Written while the Strategic Air Command was still in existence, so most of the information is out of date in its details, but apparently not in its overview of the EAMs.
There are no known public sources for detailed descriptions of these strings, but there are a number of books and papers published that cover this topic in broad strokes (and, which I suspect are in many ways greatly out of date - I've found nothing that covers the post 1992 strategic world in a way that is as detailed as they cover the pre 1992 world. It may be too early to do so, as it is probably still in transition). Some examples follow:
"The Logic of Accidental Nuclear War" by Bruce G. Blair; The Brookings Institution; 1993 (still in print); ISBN 0-8157-0983-8 (paper). There are numerous additional sources listed within the extensive "notes" section of this book. The notes section also contains detailed information that covers the uses of these messages.
"Strategic Command and Control - Redefining the Nuclear Threat by Bruce G. Blair; The Brookings Institution; 1985 (still in print as of the middle of 1996); ISBN 0-8157-0982-X (hardbound). Much of this information covers what is now the foundation for today's strategic world, but I suspect that many of the specific details covered in the book are now greatly altered, maybe beyond recognition (such as the integration of the USN into much of the then SAC-centric activity in his book.)
"Global Zero Alert for Nuclear Forces" by Bruce G. Blair; The Brookings Institution; 1995 (May or so, and still in print); ISBN 0-8157-0941-2. The only "book" (it's actually an "occasional paper" of 108 pages) in this group that can be said to be up-to-date in the post-1992 strategic world. On EAMs, in particular, see pages 59-60.
"Guarding the Guardians: Civilian Control of Nuclear Weapons in the United States" by Peter Douglas Feaver; Cornell University Press; 1992 (out of print, unfortunately). Mostly covers PAL (Permissive Action Link) locks but has an overview of EAMs throughout the book.
"Dark Sun - The Making of the Hydrogen Bomb" by Richard Rhodes; Simon and Schuster; 1995 (still in print); ISBN 0-684-80400-X. Page 573: "..SAC routinely transmitted DefCon increases as unclassified messages until 1972." Question: what happened after 1972? - "SAC routinely transmitted DefCon increases as classified messages"?
The 10 May 1976 issue of "Aviation Week and Space Technology", devoted almost entirely to the Strategic Air Command. Written during the "Alpha Net" days of OFFUTT / BARKSDALE / WESTOVER / MARCH but has information that might still apply in some aspects 20 years later (probably modified.)
The Winter 1996 (Volume 27) issue of "World Air Power Journal" devoted to the B-52H with a long article beginning on page 54 written by Robert F. Dorr and Brian C. Rogers. See page 89 for a description of the receipt and authentication of an emergency war order onboard an airborne B-52H.
"The Hunt for Red October" by Tom Clancy; Naval Institute Press; 1984 (still in print); ISBN 0-87021-285-0. Pages 65-66 (hardcover) for EAM information; and page 68 (hardcover) for "traffic analysis" fans.
"Arc Light" by Eric L. Harry; Simon and Schuster (his editor was also Rhodes' editor on "Dark Sun"); 1994 (Aug); ISBN 0-671-88048-9. The author gets to engage the Midnight Express (see the "Logic..." book) and run his SIOP, in a probable pre-92 way though.
04 April 2005 update to information published on 02 September 1998 and 30 March 2005
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