minuteman missile communications

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Speaking of annoying, this is kind of a long and dry one. I started looking into something called HICS after visiting a historic site, posted about it a bit on Mastodon, realized that there just wasn't a lot of good historic information about it in general. And I felt like if I was going to talk about communications in Minuteman missile fields, I also had to cover how they would get their war orders.

Look on the bright side: it's got pictures!

Minuteman Missiles

Since the early days of the Cold War, the United States has maintained a nuclear triad: independent capabilities to deliver nuclear weapons from land, sea, and air. The sea and air components are straightforward, consisting of submarines carrying submarine-launched ballistic missiles (SLBMs) and long-range strategic bombers. The land leg is more often forgotten: the intercontinental ballistic missiles, or ICBMs.

The US ICBM arsenal currently consists of 400 Minuteman III missiles emplaced throughout the midwest: the Air Force's 90th Missile Wing, 150 missiles, Wyoming Nebraska, and Colorado; the 91st Missile Wing, 150 missiles, in North Dakota; and the 341st Missile Wing, 100 missiles, in Montana. Historically, there were as many as 1,000 active Minuteman missiles, to say nothing of retired missile programs like Titan and Atlas. At least three Minuteman missile facilities are now historic sites open to the public, a somewhat incongruous experience considering their broad similarity to the facilities still in active use. Many others are abandoned, typically in various states of permanent destruction to satisfy treaty obligations. Fifty Minuteman IIIs of the 341st are currently held in an inactive "reserve" state, out of service but ready for future emplacement, a notable situation given the strict limits Cold War treaties place on stockpiled ICBMs.

Minuteman employed a significantly different launch configuration from earlier ICBM programs. Large facilities were difficult to protect from a first strike; distance was the only effective protection from increasingly accurate Soviet weapons. Scattered single facilities were difficult and costly to staff. Minuteman selected a compromise point: clusters of ten independent Launch Facilities (LFs), spaced miles apart and called a "flight," are remotely monitored and operated from a single Missile Alert Facility (MAF). Groups of four to five missile flights constitute a squadron, and about four squadrons compose a wing, which is supported by an Air Force Base.

In each Missile Alert Facility, a Missile Combat Crew Commander (MCCC) and Deputy Missile Combat Crew Commander (DMCCC) lock themselves into an underground capsule called a Launch Control Center (LCC) for each 24 hour watch. Originally designed by Boeing, the LCC resembles the interior of an aircraft more than a building, fitting its crew of Air Force officers. The LCC is an isolated, self-contained system with all of the equipment needed to monitor, configure, and launch the missiles. A surface building above contains a security control center and quarters for the security force, responsible not only for the MAF but also for the ten LFs under its supervision. Still, the surface building is both powerless over and largely unneeded by the LCC beneath it. In the event of nuclear war, it was assumed, surface structures in these sparsely populated but strategically critical parts of the country would be wiped cleanly away from the earth. Only the hardened infrastructure would remain: the LCCs, the LFs, and communications infrastructure.

Minuteman missile combat crews had more duties than just to wait. They performed remote tests on the missiles, launch, and control equipment; they monitored alarm systems that reported malfunctions and remotely supervised the work of maintenance crews; and they monitored the security systems that protected the unmanned LFs, authorizing access and dispatching security forces on the surface to any unknown intrusion. Still, their primary responsibility, the one for which we all know them, is the disposition of Emergency War Orders (EWO).

In fine Air Force tradition, actions that may very well presage the end of the world as we know it are presented in the form of a checklist. EWOs are authenticated against codes and secrets. Two keys, just like in the movies, are inserted. The missiles are enabled by remote command. Targeting information is transmitted to the missiles. The keys are turned, the launch code sent, and the rest is automated, under the control of the missiles themselves. The missile crew are miles away, so the Real Thing, the Shit Hitting the Fan, must feel pretty anticlimactic. The only apocalyptic horsemen they'll see are a series of indicator lights on the MCCC's console: LCH CMD. LCH IN PROC. MISSILE AWAY.

The realities of ICBM operation are fascinating and incline one towards drama. Somehow the work of submarine and bomber crews seems more ordinary; they are at least "out there," in or near enemy territory. Missile crews are sealed in a very small room buried below a small building in a corner cut out of a farm field near, but not too near, to a highway for logistical convenience. They are entirely dependent on electronic communications, not only to receive their orders, but even to use their weapons. They have the original email job: since 1962, they have served primarily to send and receive messages, mostly by text.

With the rather purple introduction complete, I am going to talk about this communications technology. But first, just a little more preface.

Most of the reduction of the Minuteman force has been a direct response to treaties, which imposed progressively lower caps on the nuclear stockpile. Some reductions were more of a historical accident. In the 1980s, political considerations lead to a decision to "temporarily" deploy the Peacekeeper missile, with 10 MIRV (multiple independent reentry vehicle) warheads, to a set of fifty silos of the 90th Missile Wing's 400th Missile Squadron, in Wyoming. The Peacekeeper, fielded late in the Cold War, was a profoundly controversial program. The fifty Peacekeepers retrofitted into Minuteman silos would be the only ever installed, and their temporary homes became permanent. Most of the missile's warheads were removed for compliance with Start II treaty, which Russia never ratified and the United States withdrew from. Still, for cost-savings reasons, the odd-duck Peacekeeper program was terminated. The last Peacekeeper missiles were retired in 2005.

I got it into my head to write a detailed description of the missile field communications system because of my visit to QUEBEC-01, one of the five MAFs associated with these Peacekeeper missiles, and now a Wyoming State Historical Site. For that reason, I will most closely describe the communications system as installed in the 90th Missile Wing, the remainder of which is still active today. Minuteman missile fields were built over a period of years by different contractors and have since been through multiple modernization programs. Each change has introduced inconsistencies. While I will point out some of the more interesting variations between Minuteman installations, this is best taken as a description of the "average" Minuteman squadron, one that is typical of the others but does not exactly exist.

Although I am not exactly aiming for academic rigor, this information is based mostly on documents available through the Defense Technical Information Center, which include both original documentation from the Minuteman program and more recent documents related to modernization programs, proposed changes, and the retirement of many Minuteman facilities. I have supplemented those documents with recollections by former Air Force personnel when available, and as always, I welcome any corrections or additional information. One of the pleasures of writing about military history is the tendency of veterans to reach out to me with corrections and stories; I apologize that I am not always good about getting back to people, particularly phone calls.

The Minuteman III, a fairly direct evolution of the original Minuteman design, is still in active service. Many detailed materials about the Minuteman program are probably currently classified, most of the others are formerly classified and thus have not consistently made their way to archives. Certain basic questions remain frustratingly unanswered. That's just how it goes.

I'm also going to try really hard not to be too annoying with the acronyms, but it's not easy.

Personnel

While there were originally three-person crews, Minuteman LCCs have had two crew members for many decades, the MCCC and DMCCC. The MCCC is superior to the DMCCC, but the nature of missile operations and such a small crew mean that their roles are somewhat more complex than commander and deputy. Missile crews operate according to the "two-person concept," a general prohibition on any person working alone. This rule is intended to improve safety, reduce mistakes, and most importantly, mitigate the risk of an unauthorized launch. There are additional safeguards against unauthorized launch in the Minuteman system which will be discussed later. Both the MCCC and the DMCCC are required to initiate a launch.

The MCCC sits at a console that is focused around monitoring and control of the launch facilities. They have ready access to procedures and documentation. The DMCCC sits at a separate console that is focused on communications. Their chair slides on rails, allowing them to access the equipment racks and teleprinters to the sides of the DMCCC position. The DMCCC is primarily responsible for communications, so we are most interested in the equipment under their control.

Missile Alert Facilities were designed with a goal of self-containment for survivability. Most communications equipment is within the LCC itself, readily available to the DMCCC so that they can at least diagnose problems, if not make a repair. To this end, DMCCCs receive significant training on technical details of the communications and computer systems.

Should a problem occur outside of the LCC, the MCC would request assistance from the Air Force Base. Several Air Force ratings had expertise in communications equipment, ranging from communications technicians that would investigate problems within the LF to cable splicers that would repair damage in the outside plant.

The Minuteman program is somewhat unusual in the extensive construction of long-distance communications equipment by the Air Force. AT&T's role in the missile fields was surprisingly limited; most communications followed routes fully under the control of the Air Force.

External Communications

We can generally divide Minuteman communications systems into two categories: external and internal. External communications systems are primarily used by the Missile Combat Crew (MCC) to receive orders, including Emergency War Orders authorizing the use of nuclear weapons. Internal communications systems are used within the missile field, primarily to allow the MCC to communicate with the launch facilities under their control. Some details blur the lines: for example, there are communications systems which allow the MCC to contact their Air Force Base, where support facilities and maintenance crews are found. I will consider these internal systems, but you could argue for the opposite.

The external communications systems available to Minuteman crews have varied over time. Perhaps the most exotic was the Survivable Low Frequency Communications System (SLFCS), based on the LF equipment used by the Navy for communications with submarines. Missile facilities are not underwater, but nuclear detonations cause significant disruption to the atmosphere that greatly interferes with radio propagation in the HF range. LF communications are expected to be less affected in a nuclear combat environment. SLFCS specifically operated between 14kHz and 60kHz. Some, but not all, Minuteman MAFs were equipped with a magnetic loop antenna, about 6' in diameter, buried shallowly underground.

All MAFs were equipped with HF antennas, although they were decommissioned in the 1980s. The HF antennas are described as hardened, but it is not feasible to truly harden an HF antenna. They must be fairly large, and HF does not penetrate the ground well, making it impractical to bury them. Instead, hardened HF antennas are perhaps better described as "hidden" HF antennas. The typical design is a monopole that stores in a long, narrow silo underground, awaiting post-attack deployment. MAFs had two separate hardened HF antennas, one for transmit, and one for receive.

The receive antenna was the most critical, as it would be needed to receive war orders over the High Frequency Global Communications System (HFGCS). HFGCS is one of the primary ways that an Emergency War Order would be distributed to Air Force units including both missiles and bombers. The hardened HF receive antenna assembly actually included six 160' monopoles: one was extended for normal use, but in the event of nuclear attack, the five others were stored telescoped in a silo about 30' deep and could be deployed by a small explosive charge. There were, in the parlance apparently used by the Air Force, five "reloads."

The HF transmit antenna, being less important in an attack scenario, had only one replacement. A "soft" HF transmit antenna of conventional design was in normal use but backed up by a single 120' hardened antenna stored in a separate silo. A 50' radius buried ground plane surrounded the hardened antenna.

Near the MAF surface building, a small, white metal cone protrudes from the ground. The cone consists of a huge cast steel blast deflector with a depression in the center, which is covered by a fiberglass cone. The cone houses a compact UHF antenna. This antenna, a 1970s upgrade, can receive war orders via several satellite systems or directly from an aircraft such as the E-6. The E-6 airborne command post can serve in various roles, including as a "Looking Glass" airborne command post (taking control of nuclear forces in the event of a loss of ground-based command posts) or a "TACAMO" Take Charge and Move Out relay, transmitting a war order from elsewhere to forces in the field.

UHF communications are essentially line of sight, especially with the use of a partially in-ground hardened antenna. In a scenario with extensive loss of communications infrastructure, particularly with ASAT warfare disabling military communications satellites, an E-6 or another similarly equipped aircraft would fly over Minuteman missile fields and deliver an emergency war order directly to each LCC.

Complimenting this capability, in-service Minuteman launch facilities have themselves been equipped with a similar UHF antenna. Looking Glass and TACAMO aircraft actually have Air Force missileers on board who serve as an Airborne Launch Control Center (ALCC). In the event of a loss of most of the LCCs in a missile field, an ALCC can issue launch instructions directly to each LF without any need for the regular missile crews or internal communications infrastructure.

During the early '90s, a super high frequency (SHF) small satellite terminal was installed at each active MAF. It is housed in a small, white radome at the top of a pole near the surface building. SHF is widely used by modern military satellite systems, such as the Air Force's Wideband Global SATCOM.

Each of these antennas is connected by buried conduits to radio equipment in the LCC's radio racks. At the DMCCC's position, the telephone console allows the DMCCC to talk or listen on each radio by selecting it with a pushbutton. Over time, the radios were also attached to more modern digital systems. Depending on the year, teleprinters or computer displays would receive text messages via the UHF and SHF radio systems.

The external radio systems were actually all backup or secondary. The primary means of nuclear C2 within the Strategic Air Command and, later, Global Strike Command, has long been a digital computer network. While Minuteman installations began with just a teleprinter to receive orders via leased telephone line, the late '60s saw the introduction of the Strategic Automated Command and Control System (SACCS), which was itself replaced by the Strategic Air Command Digital Network (SACDIN). A small (for the era) computer in a rack to the right of the DMCCC's station allowed two-way messaging with SAC headquarters over leased telephone lines. Reportedly, these and other Bell System telephone lines to LCCs were carried by buried telephone cable to small hardened exchange buildings serving each missile field. I have not yet researched this topic closely.

In Peacekeeper LCCs as well as Minuteman LCCs from the '70s to the '90s, the specific computer used for this purpose was called the Command Data Buffer (CDB). The CDB was connected to both SACCS/SACDIN and the internal communications network, in order to accurately relay targeting information to the missiles. This will be discussed later in the context of rapid retargeting. In the 1990s, the REACT system was installed for a similar purpose.

Quebec-01 LCC was equipped with a teleprinter with a selector between SACDIN and AFSAT (UHF satellite) receivers. I'm not sure why the teleprinter was retained after the CDB upgrade, very possibly just for redundancy.

Somewhere between external and internal, each LCC had access to two dial telephone lines. These connected directly to the PSTN. Some circumstantial evidence leads me to think these dial lines were shared with the surface building, which probably explains the need for two. The dial lines were mostly used to contact support crews at the Air Force Base for routine maintenance issues.

HICS Digital Communications

I was most interested in the internal communications systems, which have garnered less historical documentation than the external links. In most cases, there is only really one: the Hardened Intersite Cable System, HICS.

HICS consists of multipair pressurized telephone cables trenched between Minuteman facilities. HICS carries digital traffic for C2, and many Air Force documents use the term "HICS" exclusively to mean the digital channel, but the same cables carried multiple voice pairs. Let's consider the digital capability first, though.

HICS, as originally installed, operated at 1.3Kbps. Details on the actual encoding are hard to come by, but given the time period I assume it was generally similar to the AFSK schemes used by other early telephone data links. The punch line, of course, is that the 1.3Kbps stuck---based on some Air Force journal articles on options for upgrades, it seems that contemporary Minuteman III fields still communicate over HICS at 1.3Kbps. Remember that when we get to retargeting.

The topology of the digital HICS network is rather interesting. It was designed for redundancy and reliability, but prior to most of our modern understanding of computer networking. There's a mix of a few different ideas.

One of the things I'm not completely confident of is the size of the collision domain within HICS, or how much of the cable network was a common bus. From reading between the lines of some different reports and considering the overall design, I'm fairly confident that the entire digital HICS network was a single shared bus within each flight, and I think it is likely that it was a shared bus within each squadron. This bus would be tens of miles long with multiple branches, a challenging electrical situation that perhaps explains why the Air Force has repeatedly found it to be infeasible to make HICS faster.

Thanks to minutemanmissile.com for this image of the Warren AFB/90th Missile Wing HICS map, which is far more legible than the photo I had taken.

Each of the LCCs, denoted in the map by open rectangles, is connected to four "loops" of HICS cable. The legs of adjacent loops to the LCC are shared, though, so it's perhaps easier to describe this way: each LCC is surrounded by a ring of HICS cable, to which it is connected by four legs spread roughly 90 degrees apart. This design gives a fair amount of redundancy, a break anywhere in the ring or even breaks of more than one of the LCC's legs would still leave a working path.

This latter scenario was probably one of the designer's greatest concerns, as the LCCs would be obvious targets for inbound nuclear attacks. The cable layout provides four-times redundancy on the cables to the LCC, but no redundancy at all on the cables to individual LFs. That tells you a lot about their threat modeling. Facilities were spaced far enough apart that a precision strike on an LF would probably disable only that single LF; a precision strike on an LCC, though, could potentially disable ten LFs at once. As the accuracy and power of nuclear warheads improved, it seemed more likely that a first strike would succeed in disabling at least some LCCs. A lot of the complexity of HICS is intended to account for that possibility.

Each of the LFs is connected to the ring via a single leg. In some cases, multiple LFs are along the same leg. These are often the same long runs that connect the rings of two different LCCs together. In general, each LCC ring is connected to two of its neighbors, although sometimes it will instead have two redundant connections to a single neighbor. Based on the map and situation on the ground, these inter-flight connections don't seem to have required any active equipment, only a splice case. That supports the theory that an entire squadron was a shared bus, although it's possible that a separate pair to the "foreign" LCC ring would home-run to the LCC to allow separately sending messages to either. The network doesn't seem to have that kind of selective routing capability, though, so I find it unlikely.

Digital messages do seem to have been packetized, and were distributed through the network on a "flood fill" basis. That is, every active node on the digital network repeated every message it received. You might wonder about flow control and the avoidance of cycles; only a very primitive method was used. Each node, after transmitting a message, would "lock out" the cable it was transmitted on for a long enough period for the node on the other end to finish repeating the message.

This explanation is a little more difficult to understand, though, when applied to the actual layout of the HICS system. What exactly constitutes a node? You will note that the map distinctly shows intersquadron connections, with both thicker lines and open circles where they connect to an LCC ring. My theory is that these intersquadron connections where the only places where active repeating of messages was required. Whether or not repeating messages between squadrons was selective is unclear. Did a message from an LCC to one of its nearby LFs get repeated across squadrons to the opposite end of the field? If repeating had originally been completely non-selective, I suspect that was changed as part of the work done to facilitate retargeting.

We can infer certain things about the HICS network from the equipment in Quebec-01. For example, HICS must have had a fair number of active repeaters. along a path.

Inside of the LCC, we find something like a tiny long-distance telephone test desk. A rack includes pressurization alarms for five cables (we know of four legs to the LCC ring, is the fifth perhaps a cable to the local telephone exchange?), and a fault isolation panel. When a cable seemed to have been lost, the DMCCC could use this panel to locate the problem along the cable. This probably relied on a loopback test feature of the repeaters, but I'm not sure exactly the operating principle. Further down in the rack is what appears to be a cable power supply.

Repeaters were definitely installed inside the LCCs, but the number of selections on this test panel makes me suspect that there were also in-line repeaters on the cable, perhaps taking power from that power supply. This is entirely speculative, but A repeaters may have been located along the LCC ring and B repeaters on legs, making the two-knob selector arrangement useful to test a specific repeater on a specific leg.

In the equipment side of the LCC, where the generator and chiller are located, we also find the terminations of the HICS cables. Note the mostly empty rack that would have held repeater equipment, and the air dryer and flow gauges for cable pressurization.

Finally, I should talk a bit about the exception to all of this: the 321st Missile Wing, in North Dakota, was built later than the 90th and 91st and by a different contractor. Sylvania, not Boeing, won the bid to build the LCCs and LFs. Much of the equipment is the same, but Sylvania did inject a few of their own ideas, and one of them was radio redundancy for HICS. The 321st apparently had a simplified HICS topology; I'm not sure of the details but I would guess that they may not have provided the four redundant cables to each LCC.

To make up for it, each LCC and LF in the 321st is equipped with a large, buried antenna, a grid-like arrangement of crossing dipoles that took up an area similar to the sewage lagoons outside of the fence. These antennas made up a medium-frequency, ground-wave communications network that could be used as an alternative to HICS. The 321sts redundant radio system, apparently called "Deuce" at the time, could be viewed as a precursor to the later nationwide GWEN radio C2 network. It seems to have carried the same digital messages as HICS, and the DMCCC had selectors to choose whether messages would be sent by cable or radio.

HICS Voice Communications

Now, let's take a close look at the DMCCC's communications console, which tells us a lot about the voice capabilities.

They get a lot of buttons! There appear to be two separate busses, I'm not completely sure of the significance of that layout. I know from an airman's anecdote that the DMCCCs could conference together LF phones and the dial telephone lines, and sometimes did so that maintenance crews stuck at an LF overnight could make apologies to their families. This makes me think that it is not a matter of "one selection per bus," but rather probably indicates that lines can only be joined within a bus. The logic behind that design is not clear to me.

Anyway, let's see if we know enough to explain all of these buttons. Some are easy: for the speaker and handset, there are selectors each of the radios. The "LF Lines" correspond to each of the ten LFs, numbered 2-11 since the LCC is numbered as site 1. We see the two dial lines, regular telephone lines provided at each site, and they are even labeled with their phone numbers. The five-digit notation dates this hand-written addition to the 2L-5N era, which probably persisted unusually late in rural Wyoming.

The rest of the buttons correspond to specific pairs that would emerge in different places in the HICS network. The "SCC" button likely allows communications with the security command center in the surface building, just up the elevator from the LCC. The "LCC" button I am less sure of; perhaps it was a party line of other LCCs in the squadron? The "LCC Ring" selections must correspond to the four HICS cable rings extending from the LCC, but I'm not sure which devices would be found on those pairs. They may be "order wires," available in the splice boxes and as jacks at sites and normally used only by maintenance crews working on cables outside of the LFs.

The EWO buttons are interesting. EWO is, of course, Emergency War Order, but in the context of Minuteman was also the term used for party lines connecting the Air Force Base to the LCCs. These could be used, of course, as a redundant way to deliver EWOs, as well as for general communications across the missile field. There are two for redundancy: one was routed via AT&T infrastructure, following a cable from the LCC to a telephone exchange. The other was routed via HICS. I am not sure why only one merits a "RNG" button, that could apply ringing voltage to get the attention of other stations.

I am assuming, by the magic of speculation, that the "OPR" button on the left of each bus probably selected which bus the headset was connected to. There is also a dial, for use with the dial lines.

These voice connections within the field were of critical importance because of Minuteman's strict security posture. The unattended LFs were equipped with intrusion alarms for physical security, initially a bistatic radar system more similar to that used at Titan, and later a DSP-based monostatic radar system called the IMPSS. Any personnel or, reportedly, large rabbits approaching an LF would cause an alarm, and security forces were dispatched to investigate unless the intruder used a HICS voice circuit to authenticate themselves to the LCC. The process of "penetrating" a secure LF could take a missile crew thirty minutes or more, and involved multiple calls to the LCC as different alarms were triggered.

HICS Outside Plant

Old aerial images and historical documents from the Air Force give us some insight into the construction of HICS. HICS cables were installed in open trenches and then covered, rather than placed directly with a vibratory plow as would become common later. Splices were done in large holes with scotch-lok connectors and cast iron splice casings. Over time, many of the splice casings had to be replaced due to premature corrosion, and different materials were tried before settling on brass. A cathodic protection system was installed as a permanent solution to the problem.

I have done my best to trace some of the HICS cables along their routes. The holes used for splicing are sometimes visible as scars, but it does not appear that any manholes were installed; instead splices were made near RoW markers and will have to be excavated for repairs. The lack of manholes suggests that there may not be active equipment along the cable routes, I'm not really sure.

The RoW markers used by the Air Force are substantially similar to the style used by AT&T at the time. They are round wooden posts, about 6' tall, with metal bands around the top. Unlike AT&T, the Air Force used white bands, and it seems that there are always two. Shorter markers are used in some places, I suspect where there are splice cases not near road crossings. Where the cables cross roads, the Air Force usually installed gates with sturdy metal posts in the roadside fences. Sometimes these gates are the easiest evidence to find in aerial photos.

One of my biggest questions is about the inter-squadron relays. The map depicts them as nodes, but they aren't located at LFs or other facilities. I wondered if there might be active equipment, but I found one of the locations where in inter-squadron cable takes off from an LCC ring and there is no indication of even a manhole. In case you might be interested, here is a KML with the cable tracks I have worked out so far.

Cross-Flight Communications

HICS served primarily to allow an LCC to communicate within the ten missiles in its flight. However, the entire squadron and then the entire missile wing were interconnected, and Minuteman took advantage of this capability for several purposes.

First, a specific LCC in each squadron was designated as the Squadron Command Post (SCP). The SCP was capable of sending launch orders to any missile within the squadron, and of countermanding launch orders issued by any of the squadron's LCCs. This provided a measure of protection against a destroyed or compromised LCC.

Over time, the security of Minuteman missiles was further enhanced by the addition of a "vote to launch" system. Minuteman missiles can only be launched if launch orders are sent by at least two LCCs, requiring a total of four individuals.

In some Minuteman fields (and all current fields), a Wing command post serves in a role similar to the SCP but across the entire wing. It provides one central point where the entire wing's missile inventory can be monitored and, if necessary, controlled.

Alarms

Besides missile C2 and voice communications, one of the main functions of the HICS was the reporting of alarms within unattended LFs to the LCC. Some alarms, particularly related to the missile itself, would be sent by the missile guidance computer over the HICS digital network. These alarms would be printed by the teleprinter below the DMCCC's desk, along with confirmations of commands received and other routine traffic.

There was also a second, dedicated alarm system called the Voice Reporting Status Assembly or VRSA. VRSA seems to have relied on its own pairs in the HICS cables, and resembled the simple alarm reporting telephones coming into use by AT&T. The DMCCC could select an LF and press a button to send a tone, which triggered a device at the LF to "read back" any status alarms via voice recordings. At the time this almost certainly involved some interesting magnetic tape equipment, but I haven't found much information on the LF end of the VRSA. Toggle switches on the VRSA console allowed the DMCCC to reset the alarm device in the LF, clearing any recorded alarms that weren't active.

The VRSA was an upgrade over the original Minuteman installations, which used a very similar panel to send a safe/arm tone to the LFs as part of the launch process. Since part of standard testing practice was for DMCCCs to flip the toggle switch for an LF to remove the "safe" tone and arm the site, it was a fairly obvious evolution to have the site report any faults in response to a tone. Reportedly, the VRSA panels were the original safe/arm panels with modifications.

Retargeting

When Minuteman was originally installed, each missile's targeting data was loaded from tape using equipment in the LF. To retarget a missile, a maintenance crew had to travel to the site, access it, run the new target tape through equipment in the LF that sent the data to the guidance computer, and complete a recalibration of the inertial reference platform in the missile. This was something like a 12-hour process overall, and retargeting a squadron would take weeks.

Fixed targets were practical when the "enemy" was self-evidently the Soviet Union and any attack would be all-out. Over the 70-year lifespan of the Minuteman program, though, the geopolitical and military environment has changed. There are now other nuclear adversaries, and their military assets are increasingly mobile. The biggest challenge to Minuteman's targeting was the Soviet Union's development of road-mobile ICBMs like the RT-21. To eliminate the USSR's nuclear capability, we would have to fire on these mobile systems wherever they were located. Aerial and satellite surveillance could be surprisingly effective in keeping track of these large, slow-moving TELs, but the Minuteman missiles could not be retargeted to keep up with that intelligence.

In response, a series of enhancements were made (often as part of the Minuteman II program) to introduce "rapid retargeting." Rapid retargeting allowed the missiles to be retargeted from within the LCC. During the 1970s, a computer system called the Command Data Buffer (CDB) was installed in each LCC. The CDB could receive targeting parameters from SAC and then transmit them to the LFs. It was theoretically possible to retarget missiles shortly before launch. In practice, the "shortly" wasn't very achievable

The HICS network was capable of 1.3Kbps, and because of the "flood" design of the network, that was essentially 1.3Kbps of total capacity across a single collision domain. In other words, only 1.3Kbps of total traffic could be handled, with far less available from point to point when the network is under heavy use. Further, enhancements to the Minuteman system added cryptographic authentication of messages over HICS and, later, encryption of the messages themselves. The added overhead of the cryptographic system further reduced network capacity.

Retargeting a squadron of Minuteman missiles via the CDB took over 20 hours. Retargeting a single missile could take 30 minutes.

CDB represented a major step forward in Minuteman C2, particularly with its real-time messaging capability. Retargeting was still a severely limited capability, though.

During the 1990s, active Minuteman sites were upgraded to the Rapid Execution and Combat Targeting System, or REACT. More than just an upgrade for retargeting, REACT brought a completely new control system that significantly changed the layout of LCCs. Instead of sitting at opposite ends of the tube, REACT put the MCCC and DMCCC directly alongside each other and centralized almost all control functionality onto computer displays.

It also further refined retargeting: retargeting an entire squadron now takes only ten hours. More radically, though, a single missile can be retargeted in only a couple of minutes, making it feasible to retarget a missile just before firing in a limited attack scenario.

The future of HICS

While both over budget and behind schedule, the Sentinel program is expected to replace the Minuteman missiles. Sentinel will likely be an in-place upgrade, installing new missiles and control systems in the existing Minuteman silos. It has been clear for decades now that HICS isn't capable of meeting modern expectations, so Sentinel will include a complete replacement.

Various options including DSL over HICS cables and radio were considered, but the current plan is to trench new fiber-optic cables across the launch fields. They're less interesting, but fiber optic cables have both capacity and reliability advantages over telephone cables, and could easily remain in service for the life of the Sentinel program.