Studying Airplane Radio Reflections With SDR

A property of radio waves is that they tend to reflect off things. Metal surfaces in particular act as good reflectors, and by studying how these reflections work, it’s possible to achieve all manner of interesting feats. [destevez] decided to have some fun with reflections from local air traffic, and was kind enough to share the results.

The project centers around receiving 2.3 GHz signals from a local ham beacon that have been reflected by planes taking off from the Madrid-Barajas airport. The beacon was installed by a local ham, and transmits a CW idenfication and tone at 2 W of power.

In order to try and receive reflections from nearby aircraft, [destevez] put together a simple but ingenious setup.

ADS-B data was plotted on a map and correlated with the received reflections.

A LimeSDR radio was used, connected to a 9 dB planar 2.4 GHz WiFi antenna. This was an intentional choice, as it has a wide radiation pattern which is useful for receiving reflections from odd angles. A car was positioned between the antenna and the beacon to avoid the direct signal overpowering reflected signals from aircraft.

Data was recorded, and then compared with ADS-B data on aircraft position and velocity, allowing recorded reflections to be matched to the flight paths of individual flights after the fact. It’s a great example of smart radio sleuthing using SDR and how to process such data. If you’re thirsty for more, check out this project to receive Russian weather sat images with an SDR.

[Thanks to Adrian for the tip!]

11 thoughts on “Studying Airplane Radio Reflections With SDR

  1. This process, for all intents and purposes, is basically a form of radar. Transmit a signal, bounce it off a plane, receive it, get (by cross checking ADS-B) position and velocity. Rise of the SDR radar?

      1. Depends on what you mean by ‘sdr’. Almost all hobby radar uses a computer for processing the received IF signal, but since this usually has only a few khz of bandwidth they get away with using a computer sound card or similar to digitize the signal. Ex

        There has been some interest in using more traditional SDR’s, in particular for passive radar where they make a bit more sense. Ex

  2. As I understand this method was also used by Serbia with Russian help to detect and in one case down an F-117 stealth fighter(bomber).
    Radiating antenna array was just existing cell sites wired into the system by fiber optic links and the passive RX site was then slaved to the AA weaponry with a command guidance missile link.
    Eventually the stealth radar tech will get into public hands, perhaps we can already do it using ambient radiation(like this project does) as well as using spread spectrum below noise floor legal for non-amateurs even TX like the latest stealth fighters do with it’s long range stealth radar.
    I have always though it would be fun to strap a home made QRP radar set to one of the wing supports of a C-172 and go out hunting friendly ‘Migs and Zeros” in air to air mode, it would probably be fun to be able to radar gun LEO satellites as well; with work maybe even radar navigation and mapping like bombers have had forever.
    FWIW you can already radar RX UHF TV station signals(not good enough to watch TV) off the moon with a massive but DIY-able Yaggi or dish antennas and careful preamplification.

    1. As for the F-117 intercept, it’s not surprising considering these planes (and all other stealth planes) were built to absorb (but not 100%) and deflect radar from a single tx/rx station. If you split the receive station from the transmission, these planes should become somewhat more “visible” to radar.

      1. Exactly, like driving with headlights off in a lit parking lot.
        Until recently the radar systems were primitive things ie can you or cant you(blip) see the reflecting stop sign or other obstacle by scanning around through a pipe with a flashlight attached, hide benind a mirror at 45 degrees and you are invisible.
        These anti-stealth radar systems are probably close to what we might consider normal binocular 3D daytime vision and spacial awareness of a baseball player except working in radio frequencies.
        They require knowing exact location and timing of the signal emitters and some computing to turn these into a usable 3D image from a single or multiple receiver stations.

      2. Probably more like several observers and several flash lights talking to each other to find the person in a dim stadium hiding behind some clever mirrored enclosure even if sprayed with lampblack. Eventually we will probably be able to even see with some distributed phased array system that radio black blob in the sky when compared to the cosmic microwave background or something similar.

    1. The transmitter is presumably licensed. Reason #1 to get an amateur radio license — more power! (But with great power…)

      If you follow the rules, you can legally run at 1500 W (!) in the US with an Extra license, and I believe it’s similar in the EU. 5 W is “low power” for hams, and 1 W and below is generally “extra low power”.

      Does this mean that garden-variety hams are using much more power than they need to?

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