Radio Apocalypse: HFGCS, The Backup Plan for Doomsday [Hackaday]

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To the extent that you have an opinion on something like high-frequency (HF) radio, you probably associate it with amateur radio operators, hunched over their gear late at night as they try to make contact with a random stranger across the globe to talk about the fact that they’re both doing the same thing at the same time. In a world where you can reach out to almost anyone else in an instant using flashy apps on the Internet, HF radio’s reputation as somewhat old and fuddy is well-earned.

Like the general population, modern militaries have largely switched to digital networks and satellite links, using them to coordinate and command their strategic forces on a global level. But while military nets are designed to be resilient to attack, there’s only so much damage they can absorb before becoming degraded to the point of uselessness. A backup plan makes good military sense, and the properties of radio waves between 3 MHz and 30 MHz, especially the ability to bounce off the ionosphere, make HF radio a perfect fit.

The United States Strategic Forces Command, essentially the people who “push the button” that starts a Very Bad Day, built their backup plan around the unique properties of HF radio. Its current incarnation is called the High-Frequency Global Communications System, or HFGCS. As the hams like to say, “When all else fails, there’s radio,” and HFGCS takes advantage of that to make sure the end of the world can be conducted in an orderly fashion.

Bombs Away LeMay

The US Air Force has a long history radio, dating back to when airplanes were little more than wood and canvas contraptions. Radio, especially HF radio, played a huge role in prosecuting World War II, changing the face of warfare forever. As the Cold War years set in and strategic forces became increasingly important, HF radio systems continued to play a role. One of the biggest boosters of HF radio for coordinating strategic air forces was none other than General Curtis LeMay, who as an enthusiastic amateur radio operator well knew the power of HF radio to communicate long distances, particularly using single-sideband (SSB) modulation.

Despite this history, HFGCS itself is relatively new. It only came onto the scene in 1992, when post-Cold War military restructuring combined two earlier Air Force HF networks into the Global High-Frequency System. GHFS would undergo equipment upgrades in 2002 and get an extra letter in its rearranged acronym, becoming HFGCS. While HFGCS may have started out as the Air Force’s baby, its design is open and flexible enough that it can be used by Air Force, Army, and Navy assets anywhere in the world around the clock.

The primary fixed infrastructure of HFGCS is a network of thirteen ground stations scattered across the United States and its territories as well as allied countries around the world. The HFGCS ground stations are linked together through a combination of landlines and satellite stations to act as a unified network. Almost all of the stations on the network are “lights out” stations that are controlled remotely. The primary control point for the entire system is located at Andrews Air Force Base outside of Washington, DC, with a backup location deep in the interior of the continent at Offutt AFB in Omaha, Nebraska. Each of these two stations is manned around the clock and can control the entire network.

It’s obviously difficult to get a lot of technical detail on what sort of gear is being used at each HFGCS station, but there’s one aspect of the system that’s hard to keep from public scrutiny: the antennas. The Offutt AFB transmitter station provides a pretty good look at things, sitting as it does in the middle of a cornfield off a public road in Elkhorn, Nebraska. There sprouts a sprawling farm of directional and omnidirectional antennas, including a collection of massive AS-3482/GRC log periodic arrays. These giants have twin towers that support a rotating platform with three support booms for the radiation array. A balun at the base matches the antenna to the feedline, which is a 50-ohm hardline coax measuring a whopping 3-1/8″ (80 mm) diameter. HFGCS stations also have receive capability, of course, but given the 25,000-watt power rating on these antennas, the receivers are generally not located with the transmitters. In the case of the Offutt AFB station, the receivers are located 28 miles (45 km) away outside of Scribner, Nebraska.