Bouncing Radio Off of Airplanes

Amateur radio operators are always trying some new stunt or other. It’s like they’ve got something to prove. Take Aircraft scatter for instance: the idea is to extend your radio’s range by bouncing it directionally off of overhead airplanes.

Radio signals travel in straight lines, which is a bummer because the Earth (despite what you’ve heard) is round. Inevitably, if you want to talk to someone far enough away, they’re over a hill. We’ve covered various oddball propagation methods recently, so if you don’t know about moonbounce, you’ve got some background reading to do. But airplane scatter was new to us.

Actually pulling it off requires knowing where the airplanes are, of course. To do so, you could simply look up the aircraft in your target area on the web, using something like FlightRadar24, but where’s the fun in that? There’s also the possibility of tracking local aircraft yourself using RTL-SDR if you’re feeling hard core.

The rest is just details. Hams [Rex Moncur (VK7MO)] and [David Smith (VK3HZ)], for instance, got 10 GHz signals to skip off airplanes over 842 km (PDF). If you’re an old-school ham operator, you’re double-checking the “gigahertz”, but it’s not a mistake. It’s tremendously impressive that these guys got a link over such a long distance using only 10 watts — but note that they’re doing it with highly directive dishes, and telescopes to aim them.

Not to discourage you from trying this at home, but there are all sorts of difficulties that you’ll encounter when you do. Airplanes moving perpendicular to the path between sender and receiver will Doppler-shift the signal, and there’s still quite a chunk of atmosphere to get the signal through. Finally, although airplanes look pretty big when they’re on the ground, they’re actually tiny when they’re up in the sky at 35,000 ft and 500 miles away; you’re bouncing your signal off of a small target.

The good news? People like [W3SZ] are sharing their well-documented results, and at least it’s 20dB easier than bouncing signals off the moon!

Thanks [Martin] for the tip!

44 thoughts on “Bouncing Radio Off of Airplanes

  1. “Airplanes moving perpendicular to the path between sender and receiver will Doppler-shift the signal”

    Mmmm…”along” the path? A velocity at right angles to the path shouldn’t affect the wavelength, should it?

    1. Nope, scratch that. At right angles, the path length increases or decreases. ALONG the path, the path length remains constant, though the path from the reflector to one station will increase, but the path from the reflector to the opposite station will decrease by the same amount.

    1. Not as long as you stay in the ham band and below recommended health safety levels. 10W @ 500miles, inverse square law…Yeah, way below anything dangerous.

      Now, if you go out of band, point at the wrong aircraft, and set off a threat receiver, all bets are off. ;)

    2. The general rule is that you can’t cause interference with other devices, so long as you don’t cause interference you should be OK. If you really want to get into the nitty-gritty details you can read the FCC Part 97 which lays out the rules for Amateur Radio use.

    3. I’m not an expert of the issue, nor do I have a ham radio license, etc, etc.
      But from a logical point of view
      If several signals are being bounced off of various airplanes, it might be possible to end up jamming/interfering with the airplane’s operation (or causing spotty communication between the pilots, tower, and other planes). (think of this like DDOS for radio or the pitfalls of a wifi saturated neighborhood).

      Now, as others have stated the power is, too, low to cause any serious damage.
      But what happens if the pilots/air-traffic controllers cannot communicate with one another (or have intermittent issues).

      i’d recommend not trying to use airplanes to increase range…..
      You don’t want to be that guy thinking you might be responsible for some catasropphe after watching the new, like David Lightman (Matthew Broderick in WarGames), thinking that he might have just started World War III — or at least thinks he just stepped in a deep pile of sh|t.

      1. Now I too know nothing about planes or technology, but directing a weak radio signal on a plane might cause damage to a passenger’s brain causing him to become stupid and subsequently make him run for president, and then it would be a very costly and confusing thing for CNN.
        Not sure about all of this but I thought I’d give the warning so you can take care it doesn’t happen.

    4. nonsense! we’ve been pointing magnetrons at airplanes for decades! (usually as a means of disrupting enemy radar, employing ‘jerry-rigged’ microwave ovens, modified to operate with the door open.)

      1. Well it’ll bounce right off of the aircraft, for starters – that’s how airport radar works. You don’t see aircraft falling out of the skies when they are scanned by radars, and those put out huge amounts of tightly focused electromagnetic energy.

    5. Given the idiocy of not even being able to use personal electronics while on a plane, I have no problem imagining someone being prosecuted for this.
      I’m sure they’ll throw a “terrorism” charge in for good measure.

      In other words I’d never do this in today’s world of paranoia and stupidity.

      1. That is like saying you will throw people in jail for pointing a 10 watt light bulb at an airplane that is 500 miles away. The amount of energy hitting a plane is going to be miniscule. The same inverse square law applies for the entire electromagnetic spectrum. A signal at 10 GHz travelling 842 km will be attenuated by ~171 dB if you plug the numbers into the Free Space Path Loss Equation. For every 3dB of attenuation, the strength of the signal will be half. If I ignore the transmitting antenna gain, which I don’t know, 10 watts (+40dBm) transmitted would be attenuated to around 0.1 femtowatt of energy by the time it reached the plane (-131 dBm).

    1. Not a fair comparison. Stars move in a constant and easily predictable path. You can track stars using nothing more than a hinge in the correct orientation and a synchronous motor– no computation necessary (look up barn door tracking mounts). Looking at it the other way, the stars are stationary and all you need to do is compensate for the earth’s rotation.

      Tracking planes on the other hand is much more difficult. Unless you know the plane’s exact flight path with corrections for wind drift, you probably need to track optically.

      1. Actually, even though it is reasonably fair to say star movement is constant, it highly depends on FOV. A camera pointed at the sky can get reasonable results just using a barndoor tracker. The problem gets compounded as the FOV gets more narrow. Tracking errors magnify quickly. Most objects in the sky that have resolvable detail are at a much more confined space of the sky. So how do you improve the tracking? Well equitorial mounts are wonderful and highly accurate. The downside is the amount of time you’ll spend aligning to polar north. Photographically speaking, you don’t want rotation in the image so it must be tightly turned and calibrated. This is something that doesn’t depend on perfect rotation, it just needs to know where north is, the rest is math.

  2. The ultimate extension of this is how it is possible to use a radio telescope to sweep across a planet to measure it’s diameter. I think they used this to measure Mercury’s size and also orbital velocity and some aspects of it’s atmospheric composition based on phase change and dispersion/absorption.

    This is still rad to the max though.

    1. Composites won’t matter, usually it’s carbon fiber. Carbon fiber is still conductive, and a conductive material will have an effect on radio waves. Just not as strong as an impact.

  3. I used to live close to a major commercial airport. One of the fun things to do was to tune a receiver to 75MHz, the frequency of the marker beacons. These are navigation beacons located at prescribed distances from a runway, and charted on the approach plates. A marker transmitter beams a narrow, highly directional signal straight upwards. A 75 MHz receiver in the cockpit sounds a tone when the plane crosses the beacon. My receiver at home would also sound the same tone at the same time, because I would pick up the 75 MHz signal reflected off of the aircraft, only for the couple of seconds that the plane was over the marker.

    In combination with listening to approach control and the tower, and looking at the approach plates, I could get a very good feel for where the aircraft were, without radar.

  4. Airscout.eu http://www.airscout.eu/index.html is a site with special software to help calculate where to point your antenna. It uses the aircraft position, and two stations position, and tells each station which direction to beam. The user manual explains more about how the calculations work and has lots of graphics:
    http://www.airscout.eu/downloads.html

    Similar to moonbounce, these techniques have been used for years. I know stations that bounce signals off mountains, I even pointed West once for maximum signal from a station to the North, bouncing off a mountain. Planes routinely taking off on a nearby airport can create regular conditions for extra distances.

    It is even used in commercial situations where a microwave link is deliberately pointed at a passive reflector such as a high building, or in one case a special metal reflector was installed on a hillside, which is cheaper than an active repeater.

    https://en.wikipedia.org/wiki/Passive_repeater

    The main issues with these set ups is high path losses and possibly relatively strong interference from unwanted other reflections.

    1. You can also use edge diffraction off the corner of a building or mountain.

      One of the interesting hacks used in New England for the VHF/UHF contest is a reflection off of a water tower.

  5. This is old, old news.

    Hams have been bouncing signals of Aircraft for many decades. Actually what is hapening is that the signals are reflected by the ionised trail left behind by the aircraft’s engines…

  6. I dont know if you’d call it ‘airplane scatter’ proper, but I’m A: east of Denver and B: on the most common western approach route for Denver International Airport. At least four or five times I’ve been working 2 meter (144 mhz) single sideband, and have had the other station (almost always west of me, near town) suddenly start fluttering upwards in signal strength, with the flutters slowing until their signal is just 20-30 dB above normal. Then the fading reverses, and 1 minute later, I hear the plane overhead making its final approach.

    It’s almost regular enough to be useful!

  7. i don’t have any experience with radio so pardon my ignorance if i’m way off base with theory and missing some obvious atmospheric interference like, you know, water vapor or something, but why don’t you guys just blast a wider area with the signal, perhaps something like a beam splitter for a laser? it’d be far less complicated and require far less precision to rotate/manipulate/vibrate the transmitting dish or a reflector to cover a wider area.

    (i only ask for a negotiable 5% if someone develops this idea into a massively profitable new industry :)

  8. Back in the days when I used a FLEX pager as the ringer for my AMPS cell phone, I got a page when I was out hiking in the hinterlands. The pager had much better coverage than the cell phone, especially in the not-really wilderness around Sunol, CA, so I climbed up onto a vantage point and tried to call out ..

    .. for a long time: nothing, nothing, nothing, and then suddenly I get signal, and have part of a conversation. And then nothing, nothing, nothing, and then I get signal again, and finish the conversation. As I was walking back down to the trail, I noted that during both connections I had had to compete continuously with the sound of overhead jet aircraft, and it seemed clear that the airframes in the sky had something to do with my radio connection at 850 MHz.

  9. Hi all, a friend, with whom I performed Aircraft Scatter tests wiith lowest power in digital modes told me about this discussion.

    In Europe AS is commonly in use on frequencies of 1 GHz and up. We arrange skedules in the ON4KST chat and check for planes with the AirScout software mentioned above. It works fine on distances up to 800 km. Forward scatter, when a plane is in the middle of the path performs best. On distances of 400 to 600 km a window will open for up to 30 seconds. For sidescatter more power is required. Even backscatter is possible, but causes much doppler shift. On short distances the antennas have to be elevated and planes to be tracked.

    As far as I know, in the US planes in virtual radars will be shown with a certain delay.

    Bouncing signals off the ISS is possible too, but requires tracking of the bird and constant correction of the doppler shift, which can be as high as 600 Hz/second on 1.3 GHz. Recently we performed even Satellite bounce on a small object called OKEAN-O (Norad #25860).

    Find more details in my blog: http://www.dj5ar.de

  10. One need note use Gigahertz or VHF to try this phenomenon. 40 years ago, I, myself, and friend were chatting on 15 meters on cw. I had a window open and heard a plane coming in from the northwest – a familar route used to come into the airport. It may have been 500 feet high. As our conversation went on, his signal was going up and up and up till it pinned the meter. He was running a homebrew thing one tube and I a lower powered HW 100, as I was bothering the neighbours 70’s vintage cheap wood box stereo. I informed my bud of the plane and his sigs, and he reported my sigs rising rapidly. I told him to open his window and listen for an aircraft. He heard it and said my sigs were blasting his speaker on his receiver. Our distanse between us was some 20 miles Now, always my question is – how do the radio waves reflect off the aluminum shell of the plane. What is the scientific principle behind it. It’s a lot more than “reflect”.Something like the radio wave is making the electrons or atoms of the skin of the aircraft are being made to move symmetrically and in unison to produce a “phantom” generated equal wave. Remember, electrons in motion creates what ? And electric current. Right ? I don’t dig that bouncing of radio to a light in a mirror – the principles are black and white.

    1. All of them. It’s just a matter of beamwidth and power.

      How many are practical at ham frequencies and power levels is the real question. I’d suspect that list is pretty short, include ISS, and not much more. It needs to be a) large enough, b) close enough, c) reflective enough in the right direction. All 3 are clearly related. And remember that the satellite may be spinning, which just adds to the fun.

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