With the help of Colin G6VAKs Grabber I was able to see my WSPR signal on 6m arriving at his QTH, strong enough, but not decoding. I could see my signal was accompanied by multiple lines descending in frequency and modulated by WSPR. I consider the WSPR decoder is confused by these.
I assumed them to be Aircraft reflections and set out today to duplicate the effect over a shorter range on 70cm with the help of G8EUX.
At my QTH I am well provided with high altitude Aircraft on the North South UK Air Corridor and also the Daventry DTY Navigation Beacon Chain.
G8EUX QTH is 14 Km to the South and Transmitted a slightly drifting WSPR signal which barely showed on the S meter. Under these conditions any reflections were rare but soon we were treated to a steep strong trace, modulated by WSPR which apparently crossed our signal path and went through flutter and zero beat. The steepness meant less time within the Bandwidth of the WSPR signal and it decoded OK.
At first the straight lines always descending in frequency and crossing the transmission frequency may cause confusion. The explanation lies in the fact that we have two stations and one moving Aircraft. As the Aircraft approaches the signal path the distance decreases. After crossing the path the distance increases. The same holds true from which ever way the the Aircraft approaches. The offset depends on the rate of increase or decrease.
The capture shows the 70cm signal with multiple reflections and one long strong one probably flying on the DTY beam.
The map shows the Track of the DTY Navigation Beacon chain to/from the Luton Control Area, the North South Air Corridor and the 70cm path between G8EUX and G3ZJO.
G6AVK QTH lies 127 Km distant to my South East and the path is largely parallel to the North South Air Corridor. Consequently the trace of many Aircraft Reflections is less steep and spends time almost parallel to the wanted WSPR signal. Unlike the 70cm trace which carries only a few phase changes from the modulation, the 6m reflections were modulated with an obvious image of the Direct Path signal.
G6AVK had a sample of this stored.
The capture shows 2 transmission periods from me with a blank 2 minutes when G6AVK transmits. The second period is the one that has an Aircraft which is on an almost parallel track to the signal path. The reflection from it quickly arrives at the transmission frequency. At this time we can assume that the Aircraft is at the midway point between G3ZJo – G6AVK, flying either North – South or South – North. At this point Doppler Shift is Zero. The plane moves toward one station at the same speed as it moves away from the other, result Zero.
G4FFC at my QTH, some multi reflections here.
The Radio Path crosses the UK Jet Motorway.
In this one, a parallel tacking aircraft produces a parallel reflection. The decoder in WSPR is confused by this and produces no decode.
Interestingly this is Early Radar at work. The first Radar tests used a Fixed Radio Transmitter, [Daventry](G8EUX) and Aircraft [Heyford Bomber] (Thanks RAF or who ever) and a Receiver Station [Watson Watt/Wilkins] (G3ZJO).
Where did this first historic test by Watson Watt take place? Why right below our 70cm Test Aircraft track and only a short distance West of G8EUX.