Whilst looking at the destruction that Aircraft Scatter (AS) can do to WSPR signals I received a puzzling series of signals decoded by WSPR from G8EUX.
The grab above shows a straight line signal 59Hz below the original signal which itself appears to be effected by AS.
This lead to G8EUX testing his TX for spurious emissions and me saying it cannot be an RX odd response due to the fact that the second signal has AS. To add to my identification as a Scattered signal I never see odd decodes from G8EUX even when signals are much stronger.
After some thought I came up with an explanation.
The straight line is AS and the other related curve is from another aircraft reflecting the fundamental signal. Being a straight line reflection means the Aircraft is moving away from both stations on a constant path relative to both stations. The Doppler shift of 59Hz thereby gives us the Aircraft speed. Which works out at 633KmHr.
One example of this phenomenon does not prove anything so I began to consider a method of proof.
Beacon GB3BAA on 50MHz lies to the South East of me, not parallel to the Controlled UK Airspace South North routes but knowing the flight paths I decided to give it a try.
Pointing my beam North away from GB3BAA I set Spectran so that the beacon was just at the top of the display, only the Morse Ident Keying, lower sidebands can be seen.
I was most interested in the minus 50 to 60Hz Doppler shift region. In a short time an example of 60Hz straight line shift occurred. There are short periods of no AS then just like buses several come at once and many with a straight line portion with a Doppler shift of 50 to 70Hz.
So what are we seeing? One explanation I can put forward is.
As the AS signal descends in frequency we are witnessing Aircraft travelling North away from both GB3BAA and G3ZJO and at an angle having already crossed the direct line signal path. As the Aircraft turn to cross the Atlantic there is a period when the trigonometry produces the straight line. I do not show the signal path from GB3BAA to G3ZJO. Its not critical, just think of GB3BAA as being down London way.
Equally we could suspect that the Aircraft involved could be turning earlier to take the route across via Ireland, there is an example in the animated plot doing just this, I consider the angle to be too acute for this to be the case.