Spectrogram of HM01. Thought to be operated by the Cuban Dirección General de Inteligencia, HM01 broadcasts a mixture of computer voice-read numbers and digital data transmissions, and is speculated to target agents in the United States and Latin America. All images courtesy of Lewis Bush

"Numbers stations" are the mysterious coded radio broadcasts that have been transmitting, in some cases, for decades across the world. The transmissions themselves have an eerie air, featuring at times clunky automated voices, at others quaintly dated human voices rambling streams of numbers that, at first, seem like ghostly gibberish. While they're the focus of myriad conspiracies and explanations, the most widely held theory about these stations is that they are a means by which intelligence agencies can communicate with assets around the world, who can receive these coded messages securely, using nothing more elaborate than a household radio.

While on the one hand they seem to be unnerving echoes of the Cold War, these messages are also indicators of the proximity of intelligence work to our everyday lives. Lewis Bush's forthcoming book Shadows of the State seeks to visualise, locate and expose many of these stations. Long-documented by monitors and enthusiasts, with recordings collated by the likes of The Conet Project, Bush's new project takes the wealth of research available a number of steps further, using open source information, publicly available satellite imagery and inexpensive software to give these faceless stations visual identities.

I sat down with Bush in a south London pub to talk conspiracy, cryptography and obsession. And also to dwell on the merits of turning the tools of power to bear on their creators.

Warrenton Station C, United States of America. Station C is a high frequency transmitter facility employed by a number of US government agencies. In the mid-1980s, numbers monitors traced a numbers station known as Cynthia, thought to be operated by the Central Intelligence Agency, to the site.

VICE: You must have come up with a nice concise definition of what a numbers station is, after working on this book?

Lewis Bush: I guess the question of what constitutes a numbers station can be a bit divisive. But basically numbers stations are unidentified broadcasts that consist usually of a live announcer or mechanical voice reading out strings of seemingly random numbers. And at times these are accompanied by music, or weird sound effects. That's the gist.

What do you mean by "unidentified"?

Shortwave radio is very tightly regulated – stations are assigned frequencies and they are meant to stick to them, so you don't get the BBC broadcasting over an ambulance channel, say. Numbers stations operate a bit like pirate stations, in that they don't generally stick to assigned frequencies; they're all over the place, and generally are not included on formal telecommunications listings, though there are exceptions.

And it's the fact that these stations have been seemingly allowed – in some cases for a very long time – to operate like this that fuels the theory that these stations are government-run and part of intelligence operations and communications?

In part. While they behave like pirate stations in the way I just mentioned, what usually typifies pirate stations is that they are quite haphazard, quite basic. Certainly in south London, a pirate station is usually a bloke up a tower block with a transmitter playing bad music at 3AM. At least, that's my childhood memory.

But numbers stations are very professionally operated. They broadcast sometimes in the same formats for decades. They are clearly not being run, in those particular cases, by one person. They usually broadcast from high-powered transmitters too. So in all other respects they are not like pirate stations; they are more like the sort of thing that would be run by a well-funded organisation.

You could say that's still quite circumstantial, but on top of that the main thing that's been argued to link them to intelligence gathering is a handful of cases during the Cold War – and since – where spies have been caught, in some cases red-handed, transcribing coded messages and decoding them from these stations. The Czech government have actually admitted to formerly operating a numbers station, but most are, as you say, unclaimed by any government.

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There are also more, shall we say, exotic theories about what these stations could be used for, correct?

Yes. There's a station called The Buzzer, which for a while was thought to be a numbers station. For the record, I don't think the following is in any way true, or that The Buzzer is an actual numbers station, but The Buzzer has broadcast a continuous pulse for about 40 years, and there are some slightly conspiratorial theories suggesting that it's linked to a soviet doomsday programme called Perimeter, which was meant to launch an automated nuclear attack if the USSR was attacked. Some people theorise that when The Buzzer goes off the air it will be the end of the world.

Can you explain a little more about how people have interpreted these strings of numbers being broadcast?

Usually the strings of numbers broadcast from these stations appear random, but actually, on inspection, follow a format. If you listen to the same station again and again you start to see a pattern. That's what the numbers monitors and enthusiasts are very good at doing. Usually when that pattern starts to emerge it fits the profile of the kind of cypher called a one-time pad – a very secure, but very basic, way to code a message. If they are used properly they are virtually unbreakable.

I remember coming across the Conet Project – a set of recordings of numbers stations – and being really chilled by the recordings. Maybe I read too much le Carré, but to me these ethereal messages were very scary. Was that part of what hooked you and got the project started?

I got into them through a different documentary project I was finishing off. I was doing what we all do at that stage of a project, procrastinating on Wikipedia, and I went down a Wikipedia-hole and somehow ended up on a page about numbers stations. I listened to a few, and then found the Conet Project too. And, as you say, there's something chilling about the recordings. Listening to them, I felt I was listening to a voice from the past – literally, in some cases. On certain stations, the technology – the transmitters used – hasn't changed in 30 or 40 years. Some stations are running on hardware produced in countries that don't exist any more.

Beyond that, I also think it's very interesting that intelligence gathering – a world we read so much about, especially in the wake of the Snowden revelations, something we think is so distant from our lives – is actually all around us, pretty much every minute. Right now, in this pub, there are probably signals we could harvest if we knew how.

RAF Akrotiri, Cyprus. A signals annex within the base is thought to have been one of the transmitter sites for the Lincolnshire Poacher, one of the best-known numbers stations. Numbers stations monitors tracked transmissions to the site by process of elimination and with local radio observations.

How was it approaching this subject as a photographer specifically?

I think that documentary photography's weakness has been a tendency to fixate on things that are obviously visual. There are massive areas of human life and experience that are never touched on because they are not obviously visual. It was certainly quite a challenge – to make a photo project about something that was invisible.

That side of your project reminded me of Edmund Clark and Crofton Black's Negative Publicity, and to an extent Trevor Paglen's work on top secret installations. The idea of rendering visible things that the state tries to keep out of sight. And to an extent using the information or tools provided by the state to do so.

I think in terms of Ed and Crofton's work there are definite parallels – that interest in the stuff that goes on in these seemingly banal settings. The idea that the business down the street could be involved in extraordinary rendition, or equally, that cluster of aerials you pass every day could be transmitting coded broadcasts. There was a point early in the project where I wasn't sure about how active I should be in collecting the material – was I going to go out and file FOIAs for example? In the end I settled on the idea of only using information that was readily available and in the public domain.

How much of this project would you say is your interest in numbers stations, and how much is a broader interest in using this technology that's available to scrutinise state activity?

That idea – of taking a technology, a coercive technology – and turning it back on the people who built it is very interesting. Certainly imaging satellites, for example, are very much the direct descendants of spy satellites. In some cases, commercial satellites are still used by intelligence agencies.

Talk me through the actual process and structure of the book. For example, take the Lincolnshire Poacher numbers station.

I should clarify that I can never say with total certainty that any of these sites are 100 percent confirmed. That said, in a way that station is a good example in terms of how easy it can be. The Lincolnshire Poacher was allegedly an MI6 numbers station that transmitted first from Buckinghamshire, and later from an RAF base in Cyprus. Quite a lot of people had gone looking for it before, because it's one of the best known of these stations; the voice on it sounds like a Radio 4 continuity announcer. So there was already a lot of corroborative information pointing to Cyprus for that one, including some fairly reliable examples of people claiming to have stood outside the airbase and actually picking up high-powered transmissions. So selecting that site wasn't too hard.

There were other stations that were far tougher, where there were no smoking guns and only fractious information available. A nice example is a site in Cuba that's actually still probably being used for covert broadcasts. With that one there was some direction finding done – similar to standard triangulation – but also people noticed that the numbers station broadcast was getting interference from Radio Habana Cuba, which tends to suggest the two signals are coming from the same site. There was also some information to be found in declassified CIA documents. On a fun side note, people also realised the computer being used on that station was using Windows XP, because a few times they accidentally broadcast the XP shut-down noise, not that XP ties it to Cuba. That's an example of a less well-known station which had a variety of pointers to its location.

Tyson's Corner Communications Tower, United States of America. Built in 1952 as part of a program to harden US government systems against nuclear attack, some more fanciful numbers monitors suggested the tower was a transmitter for numbers broadcasts. While there is little evidence to support this, declassified documents suggest the tower acts as a relay between Warrenton Station C, CIA headquarters, and other sites.

Once you got as close as you felt you were likely to, to a probable transmission location, what was the next step?

Sometimes it's easy to find a site on satellite images. Sometimes it's a large, well-known shortwave transmitting site and you can find the coordinates in five minutes. Then again, sometimes the best information I can get is "this transmitter is roughly five miles from X village in a roughly north easterly direction". In those cases it can be anything from an hour to maybe days of looking at satellite photos for telltale signs of transmitter sites. A couple of days ago Twitter helped me find an Iraqi jammer that I had spent maybe half a day looking for without luck.

There are a stock number of transmitter configurations, which create certain patterns that after a while you start to recognise. I got very interested in satellite image interpretation and read a few books on it. It's a whole separate set of skills.

Of course, a transmitter, from above, in a satellite photo, could well be only one pixel big. So you have to start looking for the shadows of aerials, for example, or for other things. Some of the stations included in the book ceased transmitting years ago, ones run by the Stasi for example. In those cases you are looking for the traces of the transmitter sites. You are looking for marks in the ground, or variations in vegetation growth, grass colour, maybe. It brought me into learning about aerial archaeology too.

The third stage of your "exposure" of these stations and sites is the spectrograms. Can you talk me through those?

A spectrogram is a way of making visible a sound wave or a radio broadcast. It's a way of visualising frequencies. In some cases in the book, the older transmissions that are not broadcasting any more, I reverse-engineered the spectograms from recordings, but the way I prefer to do it is to tune into a station live. When you listen using software-defined radio you have various options for visualisations – it generates the spectrogram for you. I would screen grab those, then composite the grabs into these very long strings of signals. It's a digital visualisation of an analogue transmission.

How much did working on this bring you into that world, into the subculture of numbers enthusiasts?

I totally can't take credit in the sense that a lot of the work in the book, the sources I used, were compiled by those enthusiasts and monitors. This wouldn't have been possible without those communities that, in some cases, have been listening to these stations for decades. Total props to them, basically. I did talk to quite a few of these enthusiasts, because at one point I had the idea of them being part of the project – I think that was a bit of me clinging onto the old school photography idea: that I should take some of the photos in my book. But while I did talk to some monitors, my not being a real shortwave radio listener meant that I didn't have much to contribute to them, or their groups. So I didn't really get involved. But looking in from the outside, I didn't really want to – it seemed quite a fractious world, with different groups holding different views. I was concerned that if I got too involved I might have to almost take sides, which I didn't want to do.

Did you find working on this project made you more or less concerned by government activity? Were you reassured by the fact that you could do this work, and use the publicly available information to do so, or did the whole thing reinforce your views about government accountability and secrecy?

It was a mixture. The project certainly led me into reading about some pretty dark activities undertaken by intelligence agencies of all nationalities and ideologies. I think what concerns me even more than these specific activities, though, is the general sense of this world as one which is almost totally lacking in accountability. Whether you look at a Cold War dictatorship or a contemporary democracy, there is a real sense that some of these agencies operate like a state within the state, above democratic oversight, and really beyond the control of politicians.

The response of lots of people to that will be, "Well yeah, obviously these things need to be secret," and they'll defend that and the power these agencies hold by pointing out the role they play in protecting us, protecting democracy and so on. But it seems a pretty scary line of reasoning to me that the only way to defend democracy is by having something inherently undemocratic at its core. And that, I think, is what the book is ultimately about.