The world’s militaries have always been at the forefront of communications technology. From trumpets and drums to signal flags and semaphores, anything that allows a military commander to relay orders to troops in the field quickly or call for reinforcements was quickly seized upon and optimized. So once radio was invented, it’s little wonder how quickly military commanders capitalized on it for field communications.

Radiotelegraph systems began showing up as early as the First World War, but World War II was the first real radio war, with every belligerent taking full advantage of the latest radio technology. Chief among these developments was the ability of signals in the high-frequency (HF) bands to reflect off the ionosphere and propagate around the world, an important capability when prosecuting a global war.

But not long after, in the less kinetic but equally dangerous Cold War period, military planners began to see the need to move more information around than HF radio could support while still being able to do it over the horizon. What they needed was the higher bandwidth of the higher frequencies, but to somehow bend the signals around the curvature of the Earth. What they came up with was a fascinating application of practical physics: meteor burst communications.

Blame It on Shannon

In practical terms, a radio signal that can carry enough information to be useful for digital communications while still being able to propagate long distances is a bit of a paradox. You can thank Claude Shannon for that, after he developed the idea of channel capacity from the earlier work of Harry Nyquist and Ralph Hartley. The resulting Hartley-Shannon Theorem states that the bit rate of a channel in a noisy environment is directly related to the bandwidth of the channel. In other words, the more data you want to stuff down a channel, the higher the frequency needs to be.

Unfortunately, that runs afoul of the physics of ionospheric propagation. Thanks to the physics of the interaction between radio waves and the charged particles between about 50 km and 600 km above the ground, the maximum frequency that can be reflected back toward the ground is about 30 MHz, which is the upper end of the HF band. Beyond that is the very-high frequency (VHF) band from 30 MHz to 300 MHz, which has enough bandwidth for an effective data channel but to which the ionosphere is essentially transparent.

Luckily, the ionosphere isn’t the only thing capable of redirecting radio waves. Back in the 1920s, Japanese physicist Hantaro Nagaoka observed that the ionospheric propagation of shortwave radio signals would change a bit during periods of high meteoric activity. That discovery largely remained dormant until after World War II, when researchers picked up on Nagoka’s work and looked into the mechanism behind his observations.

Every day, the Earth sweeps up a huge number of meteoroids; estimates range from a million to ten billion. Most of those are very small, on the order of a few nanograms, with a few good-sized chunks in the tens of kilograms range mixed in. But the ones that end up being most interesting for communications purposes are the particles in the milligram range, in part because there are about 100 million such collisions on average every day, but also because they tend to vaporize in the E-level of the ionosphere, between 80 and 120 km above the surface. The air at that altitude is dense enough to turn the incoming cosmic debris into a long, skinny trail of ions, but thin enough that the free electrons take a while to recombine into neutral atoms. It’s a short time — anywhere between 500 milliseconds to a few seconds — but it’s long enough to be useful.
A meteor trail from the annual Perseid shower, which peaks in early August. This is probably a bit larger than the optimum for MBC, but beautiful nonetheless. Source: John Flannery, CC BY-ND 2.0.
The other aspect of meteor trails formed at these altitudes that makes them useful for communications is their relative reflectivity. The E-layer of the ionosphere normally has on the order of 10⁷ electrons per cubic meter, a density that tends to refract radio waves below about 20 MHz. But meteor trails at this altitude can have densities as high as 10¹¹ to 10¹² electrons/m³. This makes the trails highly reflective to radio waves, especially at the higher frequencies of the VHF band.

In addition to the short-lived nature of meteor trails, daily and seasonal variations in the number of meteors complicate their utility for communications. The rotation of the Earth on its axis accounts for the diurnal variation, which tends to peak around dawn local time every day as the planet’s rotation and orbit are going in the same direction and the number of collisions increases. Seasonal variations occur because of the tilt of Earth’s axis relative to the plane of the ecliptic, where most meteoroids are concentrated. More collisions occur when the Earth’s axis is pointed in the direction of travel around the Sun, which is the second half of the year for the northern hemisphere.

Learning to Burst

Building a practical system that leverages these highly reflective but short-lived and variable mirrors in the sky isn’t easy, as shown by several post-war experimental systems. The first of these was attempted by the National Bureau of Standards in 1951. They set up a system between Cedar Rapids, Iowa, and Sterling, Virginia, a path length of about 1250 km. Originally built to study propagation phenomena such as forward scatter and sporadic E, the researchers noticed significant effects on their tests by meteor trails. This made them switch their focus to meteor trails, which caught the attention of the US Air Force. They were in the market for a four-channel continuous teletype link to their base in Thule, Greenland. They got it, but only just barely, thanks to the limited technology of the time. The NBS system also used the Iowa to Virginia system to study higher data rates by pointing highly directional rhombic antennas at each end of the connection at the same small patch of sky. They managed a whopping data rate of 3,200 bits per second with this system, but only for the second or so that a meteor trail happened to appear.

The successes and failures of the NBS system made it clear that a useful system based on meteor trails would need to operate in burst mode, to jam data through the link for as long as it existed and wait for the next one. The NBS tested a burst-mode system in 1958 that used the 50-MHz band and offered a full-duplex link at 2,400 bits per second. The system used magnetic tape loops to buffer data and transmitters at both ends of the link that operated continually to probe for a path. Whenever the receiver at one end detected a sufficiently strong probe signal from the other end, the transmitter would start sending data. The Canadians got in on the MBC action with their JANET system, which had a similar dedicated probing channel and tape buffer. In 1954 they established a full-duplex teletype link between Ottawa and Nova Scotia at 1,300 bits per second with an error rate of only 1.5%

In the late 1950s, Hughes developed a single-channel air-to-ground MBC system. This was a significant development since not only had the equipment gotten small enough to install on an airplane but also because it really refined the burst-mode technology. The ground stations in the Hughes system periodically transmitted a 100-bit interrogation signal to probe for a path to the aircraft. The receiver on the ground listened for an acknowledgement from the plane, which turned the channel around and allowed the airborne transmitter to send a 100-bit data burst. The system managed a respectable 2,400 bps data rate, but suffered greatly from ground-based interference for TV stations and automotive ignition noise.

The SHAPE of Things to Come

Supreme HQ Allied Powers Europe (SHAPE), NATO’s European headquarters in the mid-60s. The COMET meteor-bounce system kept NATO commanders in touch with member-nation HQs via teletype. Source: NATO
The first major MBC system fielded during the Cold War was the Communications by Meteor Trails system, or COMET. It was used by the North Atlantic Treaty Organization (NATO) to link its far-flung outposts in member nations with Supreme Headquarters Allied Powers Europe, or SHAPE, located in Belgium. COMET took cues from the Hughes system, especially its error detection and correction scheme. COMET was a robust and effective MBC system that provided between four and eight teletype circuits depending on daily and seasonal conditions, each handling 60 words per minute.

COMET was in continuous use from the mid-1960s until well after the official end of the Cold War. By that point, secure satellite communications were nowhere near as prohibitively expensive as they had been at the beginning of the Space Age, and MBC systems became less critical to NATO. They weren’t retired, though, and COMET actually still exists, although rebranded as “Compact Over-the-Horizon Mobile Expeditionary Terminal.” These man-portable systems don’t use MBC; rather, they use high-power UHF and microwave transmitters to scatter signals off the troposphere. A small amount of the signal is reflected back to the ground, where high-gain antennas pick up the vanishingly weak signals.

Although not directly related to Cold War communications, it’s worth noting that there was a very successful MBC system fielded in the civilian space in the United States: SNOTEL. We’ve covered this system in some depth already, but briefly, it’s a network of stations in the western part of the USA with the critical job of monitoring the snowpack. A commercial MBC system connected the solar-powered monitoring stations, often in remote and rugged locations, to two different central bases. Taking advantage of diurnal meteor variations, each morning the master station would send a polling signal out to every remote, which would then send back the previous day’s data once a return path was opened. The system could collect data from 180 remote sites in just 20 minutes. It operated successfully from the mid-1970s until just recently, when pervasive cell technology and cheap satellite modems made the system obsolete.

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