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.
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