In the global game of nuclear brinksmanship, secrets are the coin of the realm. This was especially true during the Cold War, when each side fielded armies of spies to ferret out what the other guy was up to, what their capabilities were, and how they planned to put them into action should the time come. Vast amounts of blood and treasure were expended, and as distasteful as the whole thing may be, at least it kept armageddon at bay.

But secrets sometimes work at cross-purposes to one’s goals, especially when one of those goals is deterrence. The whole idea behind mutually assured destruction, or MAD, was the certain knowledge that swift retaliation would follow any attempt at a nuclear first strike. That meant each side had to have confidence in the deadliness of the other’s capabilities, not only in terms of their warheads and their delivery platforms, but also in the systems that controlled and directed their use. One tiny gap in the systems used to transmit launch orders could spell the difference between atomic annihilation and at least the semblance of peace.

During the height of the Cold War, the aptly named Survivable Low-Frequency Communication System was a key part of the United States’ nuclear deterrence. Along with GWEN, HFGCS, and ERCS, SLFCS was part of the alphabet soup of radio systems designed to make sure the bombs got dropped, one way or another.

Skipping the Skip

The hams have a saying: “When all else fails, there’s amateur radio.” It’s true, but it comes with a huge caveat, since hams rely on the ionosphere to bounce their high-frequency (HF) signals around the world. Without that layer of charged particles, their signals would just shoot off into space instead of traveling around the world.

For the most part, the ionosphere is a reliable partner in amateur radio’s long-distance communications networks, to the point that Cold War military planners incorporated HF links into their nuclear communications systems. But since at least the Operation Argus and Operation Hardtack tests in 1958, the United States had known about the effect of high-altitude nuclear explosions on the ionosphere. Further exploration of these effects through the Starfish Prime tests in 1962 revealed just how vulnerable the ionosphere is to direct attack, and how easy it would be to disrupt HF communications networks.

The vulnerability of the ionosphere to attack was very much in the minds of U.S. Air Force commanders during the initial design sessions that would eventually lead to SLFCS. They envisioned a system based on the propagation characteristics of the EM spectrum at lower frequencies, in the low-frequency (LF) and very-low-frequency (VLF) bands. While wavelengths in the HF part of the spectrum are usually measured in meters, LF and VLF waves are better measured in kilometers, ranging between 1 and 100 kilometers.

At these wavelengths, radio behaves very differently than they do further up the dial. For LF signals (30 to 300 kHz), the primary mode of propagation is via ground waves, in which signals induce currents in the Earth’s surface. These currents tend to hug the surface, bending with its curvature and propagating long distances. For VLF signals (3 to 30 kHz), Earth-ionosphere waveguide propagation dominates. Thanks to their enormous wavelengths, which are comparable to the typical altitude of the lowest, or D-layer, of the ionosphere, the waves “see” the space between the ground and the ionosphere as a waveguide, which forms a low-loss path that efficiently guides them around the globe.

Critically for the survivability aspect of SLFCS, both of these modes are relatively immune to the ionospheric effects of a nuclear blast. That’s true even for VLF, which would seem to rely on an undisturbed ionosphere to form the “roof” of the necessary waveguide, but the disruption caused by even a large blast is much smaller than their wavelengths, rendering any changes to the ionosphere mostly invisible to them.

Big Sticks

Despite the favorable propagation modes of LF and VLF for a communications system designed to survive a nuclear exchange, those long wavelengths pose some challenges. Chief among these is the physical size of the antennas necessary for these wavelengths. In general, antenna size is proportional to wavelength, which makes the antennas for LF and VLF quite large, at least on the transmitting side. For SLFCS, two transmission sites were used, one at Silver Creek, Nebraska, and another in the middle of the Mojave Desert in Hawes, California. Since ground wave propagation requires a vertically polarized signal, each of these sites had a guyed mast radiator antenna 1,226 feet (373 meters) tall.

While the masts and guy wire systems were as reinforced as possible, there’s only so much that can be done to make a structure like that resist a nuke. Still, these structures were rated for a “moderate” nuclear blast within a 10-mile (16-km) radius. That would seem to belie the “survivable” goal of the system, since even at the time SLFCS came online in the late 1960s, Soviet ICBM accuracy was well within that limit. But the paradox is resolved by the fact that SLFCS was intended only as a backup method of getting launch orders through to ICBM launch facilities, to be used to launch a counterattack after an initial exchange that hit other, more valuable targets (such as the missile silos themselves), leaving the ionosphere in tatters.

The other challenge of LF/VLF communications is the inherently low data transfer rates at these frequencies. LF and VLF signals only have perhaps a kilohertz to as few as a few hertz of bandwidth available, meaning that they can only encode data at the rate of a few tens of bits per second. Such low data rates preclude everything but the most basic modulation, such as frequency-shift keying (FSK) or its more spectrally efficient cousin, minimum-shift keying (MSK). SLFCS transmitters were also capable of sending plain old continuous wave (CW) modulation, allowing operators to bang out Morse messages in a pinch. When all else fails, indeed.

No matter which modulation method was used, the idea behind SLFCS was to trade communications speed and information density for absolute reliability under the worst possible conditions. To that end, SLFCS was only intended to transmit Emergency Action Messages (EAMs), brief alphanumeric strings that encoded specific instructions for missile commanders in their underground launch facilities.

Buried Loops

While the transmitting side of the SLFCS equation was paradoxically vulnerable, the receiving end of the equation was anything but. These missile alert facilities (MAFs), sprinkled across the upper Midwest, consisted of ten launch facilities with a single Minuteman III ICBM in an underground silo, along with one underground launch control center, or LCC. Above ground, the LCC sports a veritable antenna farm representing almost the entire RF spectrum, plus a few buried surprises, such as the very cool HFGCS antenna silos, which can explosively deploy any of six monopole antennas up from below ground to receive EAMs after the LCC has gotten its inevitable nuking.

The other subterranean radio surprise at LCCs is the buried SLFCS antenna. The buried antenna takes advantage of the induced Earth currents in ground wave propagation, and despite the general tendency for LF antennas to be large is actually quite compact. The antennas were a magnetic loop design, with miles of wire wrapped around circular semi-rigid forms about 1.5 meters in diameter. Each antenna consisted of two loops mounted orthogonally, giving the antenna a globe-like appearance. Each loop of the antenna was coated with resin to waterproof and stiffen the somewhat floppy structure a bit before burying it in a pit inside the LCC perimeter fence. Few examples of the antenna exist above ground today, since most were abandoned in place when SLFCS was decommissioned in the mid-1980s. One SLFCS antenna was recently recovered, though, and is currently on display at the Titan Missile Museum in Arizona.

Sign of the Times

Like many Cold War projects, the original scope of SLFCS was never fully realized. The earliest plans called for around 20 transmit/receive stations, plus airplanes equipped with trailing wire antennas over a mile long, and more than 300 receive-only sites across the United States and in allied countries. But by the time plans worked their way through the procurement process, technology had advanced enough that military planners were confident that they had the right mix of communications modes for the job. In the end, only the Nebraska and California transmit/receive sites were put into service, and even the airborne transmitters idea was shelved thanks to excessive drag caused by that long trailing wire. Still, the SLFCS towers and the buried loop antennas stayed in service until the mid-1980s, and the concept of LF and VLF as a robust backup for strategic comms lives on with the Air Force’s Minimum Essential Emergency Communications Network.