Radio Photography
For a long time I’ve considered setting up a corner on my site as a repository for random bits and pieces I’ve done that didn’t quite fall into the narrower ‘mountain and hillwalking’ mould. So I’ll start with an essay from 2010 - a home project on radio photography - spur-of-the-moment and DIY in nature. Probably as good a place to begin as any!
The two photographs above show exactly the same scene.
The lower (‘normal’) photo was taken by a normal camera (in light visible to the human eye), in February 2010.
The upper photo was taken in radio waves, a wavelength not visible to the human eye whatsoever.
In the radio photo, the arc of bright light in the sky is the ring of geostationary satellites that are arranged in a band around the Earth’s equator. This very ‘light’ delivers some of our telecommunications, including television.
The radio photo came as a result of two factors:
1) That radio waves are just light, albeit they are not a wavelength that is visible to us.
2) That thanks to satellite dishes, we have a (single-point) way of measuring the intensity of that ‘light’.
Origin
In 2010 I was 18 years old, and figured out this unusual project together with my dad, David Woods, who brought experience and rigour to an otherwise quirky idea.
The text below is an amalgamation of both our writing and input.
We wondered what the world would look like in frequencies of electromagnetic radiation beyond visible light. IR and UV pictures were easy to find on the net but we wondered about the TV transmitter 20 miles away. It sprays 400-800MHz (far below visible red light and microwaves) across the landscape. We wondered whether, if you could ‘see’ in that ‘light’, could you produce an image that would show even the nearby hills being illuminated by this 'light'?
Discovering techniques
Consider that a normal digital camera, when taking a photograph, simply samples many points of light all at once, and arranges the different values of light into a grid of pixels.
In the case of picking up radio waves, we realised we had a ‘camera’ of sorts at home - a satellite TV dish. It was little camping satellite TV kit, with a 40-cm dish and it included a LNB (low noise block - at the dish's focus) and a receiver. Think of this dish as being a one-pixel camera. It will measure high and low signals - which could also be interpreted as being light and dark. But it will only ever measure one value in one location at a time - the place in which you point it. To measure the high/low signal (light/dark tone), we bought a simple signal-finder meter and wafted the dish around the garden to begin. This was showing profound differences in signal (or ‘brightness’!) between, say, the lawn, and the path running across the lawn. And since the TV satellites transmit around 11GHz, we might be able to build up a 'photograph' in that 'light'.
Building our 'camera'
We now had a single-point ‘camera’. But to build up a meaningful photograph, we would have to mount the dish on a structure, which would allow us to move it by calibrated amounts. By going back and forth in a grid pattern, and individually measuring the signal (light/dark!) at each point, we would be building up our pixels one by one.
To build the calibrated mount for the dish, we used drum hardware, which I (Kevin) could use to pan and tilt the dish. To move the dish by the correct increments, we fashioned protractors from CDs with labels stuck on them, sporting scales made on a drawing package on the computer.
Once the dish was pointed in a known and calibrated direction, we would need to take a signal reading from this. We bought a signal meter, conventionally used to align satellite dishes.
This would produce a numerical reading from 0 to 10 when the dish was pointed around. Slightly counter-intuitively, we actually wanted to keep this figure at ‘5’ (the middle of its scale) by using a knob on the meter, and instead measure the angle of the knob’s adjustment as a measure of the signal level - i.e. ‘brightness of the pixel’.
To measure the angle of the knob accurately, we made a small protractor to mount behind the signal meter. To create a dial out of the little knob in the centre that you could turn, we bored a tiny hole through the knob and passed a sewing needle through to give our degrees of rotation.
Collecting Data
We set up and started at 20:30 one evening to take measurements for a 66 x 30 pixel image - 1,980 individual samples in all - with our single-pixel ‘camera’!
It took 6 hours (!), me out in freezing temperatures pointing the dish, and my dad in the relative warmth adjusting the meter and entering angles into a spreadsheet. We were shattered by the end. As we worked, there were variations in the signal levels that seemed to be interesting.
Interpreting Data
The next day, we used Paint in Windows to directly transfer the angle measurements into grey values in a 66 x 30 pixel image. Another 2 hours! Once finished, the levels were inverted and stretched in Photoshop and the image enlarged to make the final 'photograph'. Amazingly, we could see the arc of the geostationary satellites! The brightest splodges across the sky are us looking straight up the boresight of the TV satellites that serve this area. I'm guessing the dimmer splodges are leakage glow from other TV satellites or other downlinks at a similar frequency, but aimed at other parts of the world. It was very satisfying to see the outline of the trees and a hint of the path across the lawn. There is also an obvious reflection from the powerful satellites off the wet wooden deck.
Now if someone could provide a computer-controlled pan/tilt platform, wideband signal measuring gear that will directly attach to a computer and software to directly translate level measurements to pixel values, we'd be laughing!
We were told that the Cassini probe, which visited Saturn, was made to do a very similar thing. Just as we endlessly panned the dish back and forth across the Glasgow sky, the Cassini probe tediously rocked backwards and forwards to use its large antenna to image in the Saturnian system in 13GHz.
Postscript, 13 Oct 2024
When we first did this project, we had never heard of anything like it, outwith the interplanetary probes (!) as described above. For years afterward, I had an article on this website about it. Subsequently, I did receive a couple of emails from folk who were interested in doing similar projects, or had done similar. But I haven’t kept my ear to the ground since - it would be interesting to know if anyone has indeed developed the idea further.