Last Updated on November 5, 2025 by John Berry
I’ve made the following video as an introduction to aircraft scatter as an amateur radio propagation mechanism. Aircraft scatter is interesting to radio amateurs because it enables communications when the normal path is too lossy.
This talk begins with a short clip which describes receiving the GB3NGI amateur radio beacon located on Slieve Annora in Northern Ireland from a location in Scotland. The clip illustrates receiving the beacon via the normal path – a heavily obstructed non-line-of-sight path. The beacon is received at S0 – just on the receiver noise floor. The clip also illustrates receiving the beacon when the signal is reflected from a passing aircraft. Then the signal strength rises to S4.
The talk goes on to describe the geometry of the propagation with direct and reflected paths. Importantly, the antennas are pointed skywards and towards the beacon. As an airplane passes over, the amplitude rises and then falls, and the pitch of the Morse code signal from the beacon rises then falls. The airplane has enabled communication via what’s termed aircraft scatter.
The signal when the airplane is there is around 4 S-points or about 25dB higher than when it’s not.
The presentation goes on to discuss the normal path: the state when there’s no aircraft to reflect the signal. The normal path is important in discussing any radiowave propagation. Propagation mechanisms or modes are talked of with reference to a normal path.
Analysis of the normal path shows that it should be available for 50% of time at a level just above receiver threshold. So the clip describes normal propagation modified by aircraft scatter.
By way of initial explanation, aircraft scatter enables communication when the cross sectional area of the aircraft (also called the radar cross section) is very much greater than the wavelength of the communications signal. Typical cross sectional areas are from 1m2 for a small private plane to 40m2 for a big passenger jet like an A380. As a result of the efficiencies of reflection, the most optimum frequencies for aircraft scatter are the 144MHz, 432MHz and 1296MHz amateur bands. While reflection at higher frequencies will be good too, there are other engineering issues that make their use difficult.
Both the path between the stations and the aircraft track must coincide. Since the time for which a useful reflection is available can be short, the transmission mode must accommodate short ‘overs’. In the example in this presentation Q65-15C is used with a 15-second transmission time.
The signal transmitted towards the plane diffuses, spreads, and paints the whole aircraft.
To understand aircraft scatter, one needs to understand the theory of reflection. A radio wave incident on a metallic surface like an airplane excites the molecules of the metal causing them to re-radiate. This happens across the airframe of the plane. Over a very small area the re-radiation is insufficient to be useful. But when summed across the whole airframe, there is significant gain. The gain is compared with the small reflection and measured in dBi – decibels above an isotropic radiator. The gain is the power flux density when reflections are over the whole aircraft divided by the that when reflections would be over a small area.
Now, aircraft scatter is a misnomer. Waves are considered scattered when the surface doing the scattering obeys the Rayleigh roughness criterion. And a plane doesn’t. It’s not rough. More accurately it could be described as supporting a plurality of reflections which, when summed, give gain. And if the energy from the reflections is measured around the aircraft, they would enable a polar plot to be built like that used to represent antennas.
Despite the misnomer, radio amateurs refer to reflections from airplanes as scatter.
The presentation goes on to explain the gain equation. There are two possibilities for this – the bi-static radar equation, and the billboard equation. The two use the same science. The billboard equation (Gain = 22.2 + 40logf + 20logA + 20log (cos alpha)) is easier to understand because it separates the effects. First there’s the free-space loss up to the plane, then the billboard gain, then free-space loss on to the distant station. This equation gives the gain of a Boeing 747 as about 42dB at 432MHz.The result is around 212dBi total path loss – and this is workable using data mode Q65.
A path budget can be built considering the characteristics of the stations, the two free-space path losses and the billboard gain. This presentation shows an MS Excel budget and the fade margin is calculated on-screen for a big plane, a small plane and a stealth jet showing the types of aircraft that will work and those that won’t.
The talk then covers the incidence of Doppler frequency shift as the aircraft moves at speed across the path. Using the Doppler equation the shift is about 300Hz. This was about the shift seen in the clip at the start of the presentation when listening to the beacon GB3NGI. The Doppler shift is low when the aircraft flies along the path and highest when the aircraft cuts orthogonally across the path.
Geometry is important. A diagram showing the Earth bulge and the aircraft over a 400km path shows that any aircraft at above 15,000ft in the mid-path region are useful. But this is seriously constrained to above 25,000ft over a small area when local terrain losses are considered.
Now the presentation moves to look at practical examples of Q65 communications using aircraft. In the first example, an 18dB uplift is apparent. The path initially works at -16 (dB below the 2.5kHz receiver threshold). This rises to +2 (dB) when a plane is mid-path. The time for which the plane is useful is only a few seconds. The plane was an EasyJet B737.
In the second practical example, powers were reduced at the stations below receiver thresholds. Communications were tested as not possible. A full Q65 communications exchange was possible, however, using four airplanes over a period of 30 minutes. Signal levels were between -2 and -7 suggesting an uplift of around 20dB-25dB above normal. For reference the threshold of Q65 is around -28dB.
So aircraft scatter may need more than one plane to be useful.
The question then is how to exploit aircraft scatter. Two images were share – one of all aircraft over the skies in the UK at a particular time, and the other the airways criss-crossing the country. The airway on which the planes are flying in the examples is Bravo 4 around the Scotland-England border. There is then a discussion about what airways would be useful for stations in the south of England.
There are five tools useful in exploiting aircraft scatter – a terrain profiling tool, a report of the planes in the sky from ground radar, a chat forum where radio amateurs can announce their interest in a QSO, and a tool to predict which planes are likely to be useful (thereby setting the times for a QSO). Note that the online predictor of useful planes is AirScout, but this may use flat-Earth rather than considering radio wave refraction and hence may be optimistic. A screen plot from AirScout is discussed. Also referenced is a presentation by John Quarmby G3XDY on use of AirScout. This is particularly interesting because if discusses exploiting planes over a DX path from Norwich to Frankfurt.
The presentation ends with speculation about what sort of scenario might be interesting for using aircraft scatter in the south of England. I give an idea for using a small, crossed Yagi pointing skyward to illuminate all planes in the area effectively giving a low-altitude reflecting surface.