In earlier experiment we compared ‘doppler footprints’ or radar signatures of large airliners and demonstrated that similar planes on same routes also plot similar doppler amplitude tracks, allowing aircraft to be recognized by type. Then, what causes those characteristic sequences of strong and weak signal on aircraft tracks? Main reasons for these distinctive fly-by footprints are aircraft size and shape. In this experiment, we can compare differences between doppler footprints of small and large aircraft.
The small aircraft of first doppler pics is a Gulfstream G550. Owned by Wells Fargo, it reminds early stagecoach wagons familiar to us from old western movies. Like those Wild West stagecoach wagons, also this small business jet carries a small party of people through uninhabited lands. Again quite like wagons, the small jet leaves recognizable tracks, although it does not even touch ground on its long voyage.
If the Gulfstream reminds of Wild West stagecoach, then a gigantic Boeing 747 is like the first trains, which brought the crowds of people to the West. Again, the track it plots on these strips, is as as much stronger as the rails are compared to wagon tracks. However, a stronger track is not the only difference between their AS trails…
Tracks of Sky Train and Flying Stagecoach
Comparing 50 MHz TV band dopplers of these two aircraft of different sizes, we can see that the small ‘stagecoach’ Gulfstream trail recorded by my Spectrum Lab software has less variations of strong and weak signal, than the doppler track of large Boeing 747 ‘sky train’ has.
Why does a big aircraft make more intensity variation in its doppler track? An explanation is that a radio wave reflecting aircraft behaves similar to an aerial. A large plane reflects radio waves with numerous directional lobes, while the number of directional lobes a small aircraft reflects is smaller. Each pair of directional lobes and dips show as a sequence of strong and weak doppler variation on the strips recorded during aircraft bypass.
The number of directional lobes of reflection is roughly related to aircraft size, measured by half-wave lengths of the radio wave reflecting from aircraft. As a result, we have a means to estimate bypassing aircraft size, by counting how many dips its bypass doppler has. Examples of both are illustrated by the screenshots below.
Links
‘Stagecoach in the Sky’ registered for Wells Fargo Bank - Gulfstream G550, N155AN, Nissan Corporation: abpic.co.uk/photo/1423846/
Comparison of Boeing 777 and 747 ‘Fingerprint’ Dopplers: Aircraft Scatter - Lentokoneheijastus ja sen doppleri.
AS Intensity Profile Experiment: Aircraft Scatter - Lentokoneheijastus ja sen doppleri.
- Juha -
Ominaisuuksia on kuvattu maanpuolustusnetin ketjussa ‘Monipaikkatutkajärjestelmä’: 
