This content of this page contains a mixture of Amateur Radio related topics such as Amateur Radio High Altitude Balloons, aircraft scatter propagation (ACS) and RADAR, together with other sections about aircraft in general.

I have a personal interest observing aircraft in flight and this page also reflects that and provides a number of useful links to other related sites. I am particularly interested in military helicopters as well as experimental, unusual or exotic aircraft. Living in the Lake District National Park with its many mountains and valleys, provides me with an abundance of jets, helicopters and other aircraft, flying at low level, to see.

My old amateur radio QSL card design, showing my interest in unusual aircraft.

 

Aircraft Scatter Propagation (ACS)

Aircraft scatter, occurs where an aircraft is flying in the air along a signal path.  The signal may not be available until the aircraft is at the midpoint, half way between your location and the transmitter, but it can occur at any point.  The metal of the aircraft may be able to reflect the signal, which would otherwise be attenuated along the ground, either because of the large distance involved or because the signal has been attenuated due to nearby terrain. The maximum distances involved are around 800km (500 miles) and the duration can be for several minutes.

 

 

The most common effect of aircraft scatter is to cause a rapid flutter of the propagated signal.  An aircraft flying at low altitude travels close to the signal path and begins to affect the received signal.  The fist signs are when the signal becomes choppy.  A rapid flutter becomes audible where the signal fades cyclically.  The cycle becomes shorter and after a minute or so the fades appear to stop.  A few seconds pass and the fluttery fading starts again, becoming more and more rapid until the effect passes.  This happens because the signal is arriving from two angles: from the ground wave and via the aircraft reflection.  The distance travelled by each signal is slightly different and as the aircraft moves around a phase cancellation effect occurs which produces the fading effect.

 

In order to cause a radio reflection an aircraft must have both the transmitter and receiver within its line of sight "radio horizon." The minimum altitude for a reflection is lowest at the transmitter-receiver baseline midpoint, with the minimum altitude about four times as high over either the transmitter or receiver.

 

A few examples of the minimum midpoint altitudes are:

 

 

I have had many QSOs using this mode usually on 144 MHz (2 metres) and in the region of up to 250km distance. I am on the main transatlantic flight path between Heathrow and the Hebrides. Most civilian aircraft fly at a cruising altitude of no more than 37,000feet, but there are other military aircraft, which fly much higher. However due to their Radar Absorbing Materials (RAM) and Stealth technology, they are intended to deliberately not reflect back any radio waves and thereby defeat enemy Radar, which might otherwise detect them.

 

           

 

 

The shorter the distance between the transmitter and receiver, the more levels of air traffic that become available to cause reflections, over a broader geographic area. In experimenting with this effect, it was found that aircraft reflections generally affect (to any significant degree) only those systems having a baseline of less than about 250 km (150 miles). This is not to say that aircraft will never cause reflections on systems of larger baselines; however, these reflections will constitute only a very small minority of the overall total events. Due to their slow speed, aircraft tend to have much longer reflection durations, when compared to meteors.

 

 

The aircraft will simulate an extremely slow "head" echo, with its associated oscillating diffraction pattern. The stronger the normal atmospheric scatter signal is at the receiver, the more pronounced these oscillations will be. This will create a symmetrical signal of a series of either accelerating or decelerating oscillations. The centre of these oscillations is usually occupied by a large peak which last from about one to fifteen minutes. The higher the operating frequency, the more likely it is for an aircraft to cause a reflection.

 

An experimental data mode SlowFeldXPAS designed for use specifically for aircraft scatter can be downloaded here. Depending on how close you are to major air routes will affect how many reflections you can detect.

 

 

Watch here how Rex VK7MO beat the Australian distance record for Microwave using ACS

 

 

 

 

Now in March 2013 I have just discovered some fantastic software called AirScout by DL2ALF for live real-time Aircraft Scatter propagation analysis with live mapping too.

 

Here is a screen shot showing the projected 144 MHz signal path between my station G0ISW at IO84OQ and the GB3VHF beacon at JO0EH. The area for mutual Aircraft scatter propagation, roughly the mid point distance wise between the two amateur radio stations, is shown in the 'Pathinfo' box at the lower part of the screen. Flights in this area will appear in the box. However that potential ACS area is above the set maximum flight height of 12200m set in the software indicated by the dark blue line in the 'Pathinfo' box, so is suggested as being non viable for this distance of 431 km.

 

For the live aircraft flight tracking, required by AirScout, I used the Flightradar24 website.

 

 

In this second alternative screenshot example below the projected 144 MHz signal path between my station G0ISW at IO84OQ and G4VLC at IO81PV, a distance of 311 km, is shown. The maximum aircraft altitude has been set at 16000m in the software (unlikely!), the Aircraft Scatter Propagation (ACS) zone is shown in purple. Flight UAL934 is on the path between the two stations at an altitude of 11887m, just not quite high enough to be in the purple mutual scatter zone! The distance remains too far apart at this frequency.

 

 

In this third and final example screenshot below, the projected 144 MHz signal path between my station G0ISW at IO84OQ and an imaginary location for G1MOG at IO83OQ, a distance of 112 km, is shown. The maximum aircraft altitude has been set at a realistic 12200m in the software, the large Aircraft Scatter Propagation (ACS) zone is shown in purple as almost anything flying between will potentially reflect signals. Several flights are on the path between the two stations at an altitude high enough to be in the purple mutual scatter zone! ACS propagation has a good chance at this frequency for this distance.

 

 

 

Of course apart from reflecting your radio signals, the aircraft themselves can also be detected using radio - Radio Detection and Ranging - RADAR

 

 

 

 

 

 

Introduction to RADAR principles

 

 

During WW2 the German Freya radar system, which was the first operational early warning radar system, used VHF frequencies very close to the current Amateur 144MHz (2m) range. View the Battle of the beams for more info about WW2 radar development. Or visit this page by LA8AK for an interesting tour of what remains in Norway now of these radar systems.

 

 

 

 

 

 

You can visually observe aircraft scatter for yourself using passive Doppler Radar. Have a look at the site of G3CWI for a full description. Or you may be interested in the new Kinetic Aviation SBS-1 real time virtual radar system, simply plug it into your PC, load the software and it is ready to track aircraft straight away.