Over on YouTube Meine Videokasetten has posted a video showing how he's been using an RTL-SDR to detect aircraft landing and taking off via the scatter on a VOR beacon. VOR (aka VHF Omnidirectional Range) is a navigational beacon that is transmitted between 108 MHz and 117.95 MHz from a site usually at an airport. Although as it is an older technology it is slowly being phased out in some places.
An interesting observation can be made that is unrelated to the actual operation and use of VOR navigation. When an aircraft passes near the VOR beacon it results in the signal reflecting and scattering off the metal aircraft body. As the aircraft is moving quickly, it also results in a frequency doppler shift that can be seen on an RF waterfall display.
In his video Meine Videokasetten uses an RTL-SDR and OpenWebRX to receive the VOR signal. He then pipes the audio output of that signal into Speclab which allows him to get significantly increased FFT resolution for the waterfall. This increased resolution allows him to clearly see the doppler scattering effects of aircraft on the VOR transmission. He notes that it's possible from the scattering to determine if an aircraft is taking off or landing.
Passive doppler radar on VOR beacon transmitter .:°:. A let's test it out
We note that back in 2015 we posted about the ability to "fingerprint" aircraft using this technique. Different types of aircraft will result in unique patterns on the waterfall. In that post they used analogue TV carriers which are not very common in most countries anymore, so it's good to see that this can be used with VOR signals too.
Over on the SWLing post, Thomas has shared an interesting video showing aircraft scatter reception in action. Alps DX [FR] shows on his YouTube channel a broadcast FM signal coming in clearly as a plane passes by, then fading away to nothing as it leaves. This effect is due to the scattering of radio waves that occurs when radio waves reflect off aircraft.
In order to predict when the scattering occurs he uses the free AirScout software which allows you to do the following:
Calculate a propagation path as a cross circle path between two QSO – partners
Calculate a path profile between both QSO – partners using a Digital Elevation Model
Calculate the mutual visibility of an aircraft from both QSO – partners for each point on the path using their elevation and any possible obstruction between them
Calculate a “hot area” in which an aircraft is mutually visible from both QSO –
partners where a reflection is theoretically possible
Show calculated path and aircrafts in real time on a map
Predict Aircraft Scatter potential for each single aircraft according to position, track and altitude
Over on his blog author Daniel Estevez has described how he's been listening to aircraft reflections from a 2.3 GHz 2W beacon. The beacon is 10km away from Daniels location and transmits a tone and CW identification at 2320.865 MHz. As aircraft fly nearby to his location Daniel was able to observe aircraft reflections of the beacon, and was able to match them with ADS-B position and velocity reports.
The hardware that he used was a LimeSDR and a 9dBi 2.4GHz planar WiFi antenna patch. By aiming the antenna away from the transmitter, and using his car as a shield to block the transmitter he was able to receive some reflections. Daniel recorded several reflections including one produced by a nearby car.
By combining his results with ADS-B data he was able to superimpose the results, and color aircraft tracks by either a negative or positive doppler shift which was observed from the reflection. By combining the ADS-B data with the time stamps, he was also able to mark the reflections from each aircraft.
In the thread the original poster used a standard hardware radio, but an RTL-SDR dongle or other software defined radio could also be used. He tuned to a strong analogue TV carrier and plotted the audio spectrograph in Spectrum Lab. If analogue TV is no longer available in your country other strong signals such as amateur radio beacons or radar signal carriers could also be used for aircraft scatter.
Below we show a small selection of some of the interesting images from page 9 of the thread, please see the actual thread for the rest. There is also more information and images contained in the other pages of the discussion thread too.