Aircraft Scatter: Bouncing VHF and Microwave Off Planes

Look up at a clear sky and you will often see a white trail where a passenger jet is crossing at 10,000 metres. To a radio amateur that aircraft is also a large, fast, metallic reflector hanging in the middle of the sky, and for a few minutes it can bounce a signal between two stations that cannot hear each other by any other route. This is aircraft scatter, a propagation mode that has produced contacts of 500 to 900 kilometres on the VHF, UHF and microwave bands without any need for the weather or the ionosphere to cooperate. It is predictable, available every day, and particularly well suited to the higher bands where other modes are scarce. This guide explains the physics, the geometry, the Doppler effect and the practical tools that make it work.

How an Aircraft Reflects a Radio Signal

A radio wave that strikes a metallic object is scattered in many directions, and a small part of it returns towards the source or continues towards other places. The strength of the reflection depends on the radar cross section of the object, which is the apparent size of the target as seen by the wave. For a large airliner at VHF and microwave wavelengths this area is of the order of tens to hundreds of square metres, much larger than the physical outline would suggest because the wings, the engines and the fuselage form resonant structures. A smaller business jet has a smaller cross section, and a light aeroplane a lot less.

In aircraft scatter, two stations point their antennas at a common point in the sky where an aircraft is flying. The first station illuminates the aircraft, which reflects a tiny fraction of the power in the direction of the second station. The loss is large, but at a distance of a few hundred kilometres it is within reach of a modest amateur station on the bands from 144 MHz upwards, since the path is made of two line-of-sight legs. This is the same idea as in radar, with the transmitter and the receiver at different places, and bistatic radar uses the same geometry.

Which Bands Work Best

Aircraft scatter works on 144 MHz and above, with the best results on 1296 MHz, 2.3 GHz, 3.4 GHz, 5.7 GHz and 10 GHz, where the antennas are small and highly directional. On the lower bands the aircraft is small compared with the wavelength and scatters less, and the antennas are wide, so the reflection appears over a long time but with a smaller enhancement. On microwaves a modest dish of 60 to 90 centimetres with a gain of 30 dBi or more concentrates the energy on the aircraft, and the loss is reduced by the gain of both antennas. The 10 GHz contest community, described in the article on the ARRL 10 GHz and up contest, is where many amateurs first try the mode.

The table below summarises the typical features of the bands. The distances are what can be achieved with a good station and a favourable aircraft. Typical numbers are only a guide, because the cross section and the geometry change from one flight to the next.

The Geometry: Where the Aircraft Should Be

The signal is strongest when the aircraft is near the middle of the path between the two stations, because that minimises the sum of the two distances for a given total range, and when the angle at the aircraft between the two stations is close to a right angle or wider, since the aircraft is then seen from the side with the largest cross section. In practice a path of 400 kilometres means an aircraft about 200 kilometres from each station, at a height of 10 kilometres, which is just above the radio horizon of both ends. The article on the radio horizon shows why an aircraft at cruising height is visible to both ends at that distance.

At the two stations the elevation angle to the aircraft is very small, a degree or two above the horizon. This is why a clear horizon in the direction of the path matters more than anything else. A hill or a block of flats in front of the antenna destroys the contact, and a mast on a high point helps a lot. The aircraft also needs to be in the beams of both antennas at the same time, so the operators at both ends need to know where it is. This is the major practical issue, and it leads to the use of flight tracking tools.

Flight Tracking and AirScout

Most airliners broadcast their position, speed and height through the ADS-B system, and the data are collected by thousands of volunteer receivers and shown on public maps. The data show where the aircraft are right now, and a program can also predict where they will be in the next minutes. The best known tool for amateurs is AirScout, free software that takes the locators of two stations, finds the aircraft that are close to the line between them, and calculates which of them will be at a suitable position and at what time. It shows the expected strength of the reflection in a colour scale, so that the operators can choose the most promising aircraft instead of watching all of them. With the software, you can see for example that a large aircraft will be at the right point on the path in five minutes, and you can both turn your antennas to the heading.

You do not need AirScout for a first attempt. Open a public flight tracking site on a second screen, draw a line between your locator and your partner’s, and watch for aircraft that cross it. Agree the frequency and the mode in advance on a chat or a calling frequency. The grid locators you need are explained in the article on the Maidenhead locator system.

Doppler Shift and Spread

An aircraft that moves at 250 metres per second changes the frequency of the reflected signal. The shift in hertz is about twice the speed divided by the wavelength times the cosine of an angle that depends on the geometry. On 144 MHz the largest value for a fast aircraft is a couple of hundred hertz. On 10 GHz it can reach 10 to 15 kilohertz, enough to move an SSB signal out of the receiver passband. Because the geometry changes as the aircraft flies, the shift changes during the contact, from a high positive value when it approaches, through zero when it moves perpendicular to the line to the stations, to a negative value when it flies away. The same physics is described in the guide to Doppler shift in amateur radio.

The practical result is that the signal is not on a fixed frequency. The operator at the receiving end must tune up and down or use a wide filter, and the CW signal will sound like a swept note. Spread also occurs, since different parts of the aircraft move at different speeds relative to the stations. Wide receive filters, a digital mode with a built-in search of a frequency range or a software tool that corrects for the predicted shift all help. Sending a longer carrier first, a call on CW or a recognisable tone, lets the other side find the signal in the sweep.

Signal Character and Operating Technique

An aircraft scatter signal appears for a short time, often a minute or two, grows in strength as the aircraft approaches the best geometry and then fades away. It is accompanied by a fast flutter, because propellers, turbines and the changing aspect of the aircraft modulate the reflection. On SSB the speech is shaky and distorted, and on CW the dots and dashes are clearly readable if the signal is above the noise. The usual practice is to keep the exchange short and standard: callsigns, a report and a locator, with each piece sent several times as long as the aircraft is in the right place. The overview of weak-signal exchanges in the article on meteor scatter with MSK144 shows a similar style of rapid short messages.

Plan the schedule. Aircraft scatter is not a mode where you call CQ and wait for a reply, since there is no way for the other station to know where to point. Arrange a time, a frequency, a mode and which operator transmits first. With the sked arranged, many contacts are completed within minutes, and a band such as 10 GHz can bring unexpectedly long distances. Keep a record of the aircraft types that gave good results, because large wide-body jets on the airways are consistently better than regional planes.

Equipment Considerations

The key points are a clear horizon, good pointing accuracy and enough power. On 144 and 432 MHz a long yagi or a stack and 100 to 500 watts are typical, and the narrow beam must follow the aircraft, so a rotator with elevation is a help. On the microwave bands the antenna beamwidth is just a few degrees, so a reliable mount and an accurate pointing method, such as a compass bearing and a bubble level or a computerised tracker, are critical. A sensitive receiver with a low-noise amplifier at the antenna, as discussed in the article on the mast-mounted preamplifier, is more valuable than extra transmit power because the reflected signal is so weak. A stable frequency reference matters on 10 GHz, where drift of a few kilohertz can put the partner out of range.

Safety and Courtesy

The power radiated at amateur stations, even on microwaves, is aimed at the sky where aircraft fly, and it is low compared with air traffic radar. Nevertheless, respect the exposure rules of your licence and of your country around the antennas, especially on the microwave bands where a dish concentrates the energy. Never transmit towards a nearby airfield at low elevation without considering the safety of the area. Be considerate about the frequencies: use the weak-signal sections and the calling frequencies of the band plan, so that you do not disturb the beacons and the other users.

The Bottom Line

Aircraft scatter uses a passing aircraft as a moving reflector between two stations on 144 MHz and above, with the best results on 1296 MHz and the microwave bands. The path works best when the aircraft is near the middle between the stations and is seen from the side, and both antennas must point at it at the same time, which is why flight tracking and tools such as AirScout are so useful. Expect a Doppler shift that grows with frequency, from a couple of hundred hertz on 144 MHz to ten kilohertz or more on 10 GHz, and a signal that lasts one or two minutes. Arrange a schedule, use short messages and keep the horizon clear.

Frequently Asked Questions

What is aircraft scatter?

It is a propagation mode in which a signal from one station is reflected by an aircraft towards another station, so that two stations beyond each other’s horizon can make a contact.

Which bands are best for aircraft scatter?

From 144 MHz upwards, with the best results on 1296 MHz, 2.3 GHz and 10 GHz, where highly directional antennas are small and the reflection is strong.

How far can aircraft scatter reach?

Typical distances are 300 to 600 kilometres on VHF and UHF, and microwave contacts of 700 to 900 kilometres have been made, depending on the aircraft and the geometry.

Do I need a special mode or software?

No. SSB and CW are common. Flight tracking sites or the AirScout program help find the right aircraft and the right time, but a first contact can be arranged with a simple map.

Why does the signal sound shaky?

The Doppler shift and the movement of the aircraft, including its engines and changing angle, modulate and spread the reflected signal.

Can I call CQ for aircraft scatter?

It is rarely successful. Arrange a schedule with a specific partner, because both antennas must point at the same aircraft at the same time.