Recent developments on the world political stage have brought the destructive potential of electromagnetic pulses (EMP) to the fore, and people seem to have internalized the threat posed by a single thermonuclear weapon. It’s common knowledge that one bomb deployed at a high enough altitude can cause a rapid and powerful pulse of electrical and magnetic fields capable of destroying everything electrical on the ground below, sending civilization back to the 1800s in the blink of an eye.
Things are rarely as simple as the media portray, of course, and this is especially true when a phenomenon with complex physics is involved. But even in the early days of the Atomic Age, the destructive potential of EMP was understood, and allowances for it were made in designing strategic systems. Nowhere else was EMP more of a threat than to the complex web of communication systems linking far-flung strategic assets with central command and control apparatus. In the United States, one of the many hardened communications networks was dubbed the Groundwave Emergency Network, or GWEN, and the story of its fairly rapid rise and fall is an interesting case study in how nations mount technical responses to threats, both real and perceived.
Reliability Through Physics
GWEN began as a patch for a perceived gap in the communications network connecting the country’s strategic nuclear assets — primarily the launch control centers (LCC) of the ballistic missile launch facilities — to the National Command Authority, which is basically the president. Like all strategic communications systems, GWEN was designed to incorporate best practices for surviving the electromagnetic effects of an EMP. But GWEN had another mission.
Groundwave propagation is the tendency of certain radio waves to hug the surface and follow the curvature of the earth and is an exception to the general rule that radio waves only travel in straight lines. The earth acts as a conductor below 5 MHz, so radio waves traveling along the surface of the earth induce currents. The induced currents slow down propagation near the surface, curving the wavefront down as it spreads out. There is considerable attenuation of the signal, of course, and careful consideration has to be given to antenna design and construction. But when properly engineered, ground wave propagation systems can be very effective at over-the-horizon communications that do not rely on the ionosphere.
Groundwave propagation requires long wavelengths to work, so GWEN operated in the low frequency (LF) band from 150 to 175 kHz, well below the commercial AM radio medium frequency (MF) band from 530 to 1700 kHz.
GWEN was envisioned as a wide area network of LF relay nodes about 150 to 200 miles apart. Each GWEN relay node communicated to input-output nodes, which were generally located at Air Force bases and other such facilities. The relay nodes were to take command and control messages from the IO nodes and propagate them over the entire network until they reached receive-only nodes, typically the LCC bases. GWEN encoded messages on the LF signals using minimum-shift keying at a data rate of 1200 bps. Messages were encrypted, of course.
Only about 58 of the planned 240 GWEN stations were built between 1982 and the early 1990s, when the program was shut down. GWEN was mostly a victim of Congress, who were unwilling to fund what they perceived to be a Cold War relic after the fall of the Soviet Union. There was also a certain amount of NIMBY-ism with regard to future GWEN sites; with the increasingly popular perception that everything from power lines to cell towers were capable of causing profound biological effects, the prospects of having a powerful radio transmitter that would also be a possible war target in the neighborhood was more than enough reason to mothball the program.
By that time, GWEN’s technology was certainly looking a little long in the tooth anyway, with the rise of the Internet and the proliferation of satellite communications. This may prove shortsighted, though; while there’s certainly a lot of redundancy built into today’s strategic communication systems, there’s something to be said for a simple and robust system that uses basic physical principle like GWEN did.