Radio Apocalypse: The GWEN System

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. Continue reading “Radio Apocalypse: The GWEN System”

Sferics, Whistlers, and the Dawn Chorus: Listening to Earth Music on VLF

We live in an electromagnetic soup, bombarded by wavelengths from DC to daylight and beyond. A lot of it is of our own making, especially further up the spectrum where wavelengths are short enough for the bandwidth needed for things like WiFi and cell phones. But long before humans figured out how to make their own electromagnetic ripples, the Earth was singing songs at the low end of the spectrum. The very low frequency (VLF) band abounds with interesting natural emissions, and listening to these Earth sounds can be quite a treat.

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Sorry US; Europeans Listen to Space with GRAVES

In Europe, the GRAVES radar station beams a signal on 143.050 MHz almost straight up to detect and track satellites and space junk. That means you will generally not hear any signal from the station. However, [DK8OK] shows how you can–if you are in Europe–listen for reflections from the powerful radar. The reflections can come from airplanes, meteors, or spacecraft. You can see a video from [way1888] showing the result of the recent Perseid meteor shower.

Using a software-defined radio receiver, [DK8OK] tunes slightly off frequency and waits for reflections to appear in the waterfall. In addition to observing the signal, it is possible to process the audio to create more details.

Why is there a giant vertical radar transmitter in the middle of France? The transmitter uses a phased array to send a signal over a 45-degree swath of the sky at a time. It makes six total steps every 19.2 seconds. A receiver several hundred miles away listens for reflections.

Even the moon reflects the signal when it is in the radar’s path. If you are interested in a moon bounce, you may be able to build a station to hear the reflections without being in Europe.

Of course, if you can transmit yourself, you might want to bounce your own signal off airplanes. If you want to do it old school, you could emulate [Zoltán Bay].

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40-Acre HAARP Rides Again, And They Want You To Listen

News comes to us this week that the famous HAARP antenna array is to be brought back into service for experiments by the University of Alaska. Built in the 1990s for the US Air Force’s High Frequency Active Auroral Research Program, the array is a 40-acre site containing a phased array of 180 HF antennas and their associated high power transmitters. Its purpose it to  conduct research on charged particles in the upper atmosphere, but that hasn’t stopped an array of bizarre conspiracy theories being built around its existence.

The Air Force gave up the site to the university a few years ago, and it is their work that is about to recommence. They will be looking at the effects of charged particles on satellite-to-ground communications, as well as over-the-horizon communications and visible observations of the resulting airglow. If you live in Alaska you may be able to see the experiments in your skies, but residents elsewhere should be able to follow them with an HF radio. It’s even reported that they are seeking reports from SWLs (Short Wave Listeners). Frequencies and times will be announced on the @UAFGI Twitter account. Perhaps canny radio amateurs will join in the fun, after all it’s not often that the exact time and place of an aurora is known in advance.

Tinfoil hat wearers will no doubt have many entertaining things to say about this event, but for the rest of us it’s an opportunity for a grandstand seat on some cutting-edge atmospheric research. We’ve reported in the past on another piece of upper atmosphere research, a plan to seed it with plasma from cubesats, and for those of you that follow our Retrotechtacular series we’ve also featured a vintage look at over-the-horizon radar.

HAARP antenna array picture: Michael Kleiman, US Air Force [Public domain], via Wikimedia Commons.

Lost Moon Found: The Satellite That Came Back to Life

The late 1950s and early 1960s were a tumultuous time in world history. The Cold War between the East and the West was in full-swing, driving the new fields of nuclear weapons and space exploration and giving the period its dual monikers of “Atomic Age” and “Space Age.”

Changes in these fields often went hand in glove, with developments in one requiring responses in the other. In 1958, the US conducted nuclear tests in the Pacific that effectively destroyed the ionosphere over the test site and shut down high-frequency communications to places like Hawaii and New Zealand. The strategic implications of this were clear, and the US began looking for ways for the military to reduce its reliance on HF communications and ionospheric skip by using space-based assets to communicate at much higher frequencies.

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Bombing The Sky For The Sake Of Radio

If you are familiar with radio propagation you’ll know that radio waves do not naturally bend around the earth. Like light and indeed all electromagnetic radiation if they are given a free space they will travel in a straight line.

At very high frequencies this means that in normal circumstances once a receiver moves over the horizon from a transmitter that’s it, you’re out of range and there can be no communication. But at lower frequencies this is not the case. As you move through the lower end of the VHF into the HF (Short Wave) portion of the spectrum and below, the radio signal routinely travels far further than the horizon, and at the lower HF frequencies it starts to reach other continents, even as far as the other side of the world.

Of course, we haven’t changed the Laws Of Physics. Mr. Scott’s famous maxim still stands. Radio waves at these frequencies are being reflected, from ionised portions of the atmosphere and from the ground, sometimes in multiple “hops”. The science of this mechanism has been the subject of over a hundred years of exploration and will no doubt be for hundreds more, for the atmosphere is an unreliable boiling soup of gasses rather than a predictable mirror for your radio waves.

Radio amateurs have turned pushing the atmosphere to its limits into a fine art, but what if you would prefer to be able to rely on it? The US military has an interest in reliable HF communications as well as in evening out the effects of solar wind on the ionisation of the atmosphere, and has announced a research program involving bombing the upper atmosphere with plasma launched from cubesats. Metal ions will be created from both chemical reactions and by small explosions, and their results on the atmosphere will be studied.

Of course, this isn’t the first time the upper atmosphere has been ionised in military experiments. Both the USA and the USSR exploded nuclear weapons  at these altitudes before the cessation of atmospheric nuclear testing, and more recently have directed high power radio waves with the aim of ionising the upper atmosphere. You may have heard of the USA’s HAARP project in Alaska, but Russia’s Sura Ionospheric Heating Facility near Nizhniy Novgorod has been used for similar work. It remains to be seen whether these latest experiments will meet with success, but we’re sure they won’t be the last of their kind.

We’ve looked at radio propagation in the past with this handy primer, and we’ve also featured a military use of atmospheric reflection with over-the-horizon radar.

Fishbowl Starfish Prime upper atmosphere nuclear test image via Los Alamos National Laboratory. As an image created by an officer or employee of the United States government as part of their official duties this image is in the public domain.

TV Going the Distance: Propagation

It has to be hard to be a kid interested in radio these days. When I was a kid, there was a lot of interesting things on shortwave. There wasn’t any cable TV (at least, not where I lived) so it was easy to hack antennas and try to pull in weak TV and broadcast stations. The TV stations were especially interesting.

It was one thing for me to build a dish antenna to pick up Star Trek from a station just barely out of range. But sometimes you’d get some really distant TV station. The world’s record is the reception of a BBC TV station in Australia (a distance of 10,800 miles). That’s extreme, but even from my childhood home near New Orleans, I’ve personally picked up TV stations from as far away as New Mexico. Have you ever wondered how that’s possible?

Radio signals behave differently depending on their frequency. The TV frequencies used in the old analog signals were VHF signals (well, the channels between 2 and 13 in the United States, anyway). In general, those signals usually travel through the air, but don’t bounce off any part of the atmosphere. So if you aren’t in a line of sight with the transmitter, you can’t see the broadcast. The other problem is that local stations tend to drown out weak distant stations. A TV DXer (ham lingo for someone trying to hear distant signals) has to wait for local stations to go silent or listen on frequencies where there are no local stations.

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