24 Jun 2014

6m WSPR - G0OQK spotted (with Doppler) and CN8LI

For the first time G0OQK (98km) has been on TX so this was my chance to see if reception here was pure tropo or aircraft assisted.

Well, looking at his last traces here I can see 2 or 3 traces in each 2 minute burst suggesting reflection from planes. This is far cleaner than I've seen some stations. I guess it says something about the flight paths.  WSPR seems to  decode the transmission with little Doppler, but there is little doubt that Nick's 2W to a V2000 is helped by aircraft reflection. Of course, being in the Chilterns I wonder if it is reflections from gliders?

This suggests almost all (or all?) 6m GDX signals copied here are aided by aircraft reflection and few/none are traditional tropo. For more local stations most signals are tropo but even over 25km paths people frequently see Doppler from aircraft on my traces. I guess it depends whether the direct path, or aircraft reflected path, dominates? For locals, the direct (tropo) path usually dominates.

UPDATE 1510z:  No 6m Es here so far today and just a couple of spots received  of G0OQK (98km).  I noticed more traces of Nick than proper decodes, either Doppler wrong or MS pings too short.

UPDATE 1516z:  My WSPR software decided to close on its own this afternoon (Win 8.1 issue with updates) so I have resync'd the internet clock and restarted the WSPR software. It is now working OK as G4IKZ is spotting me at 1506z. Seeing 50Hz sidebands on the very strong reports from this Nick. Not sure if it is his issue or mine. Not really worried as it only appears, sometimes, on strong signals.

UPDATE 1532z:  I spoke too soon as CN8LI (2113km) spotted me at 1516z just after I spotted G4FFC (45km). There is Es about today.


David (G0LRD) said...

I think it's unlikely that decodes are ever 'aircraft assisted'. As for reflection-only decodes, the WSPR decoder can cope with about 2 Hz/min drift. I rarely see Doppler with anywhere near that little drift. Receiving a reflected signal is possible; it has happened, when deliberately arranged: http://g3zjo-radio.blog.co.uk/2014/06/09/aircraft-reflections-on-2-meters-wspr-18625666/

Roger G3XBM said...

David, I disagree.

Several instances in my WSPR logs on 2m and 6m where the direct path is likely to be MUCH weaker than the signal by aircraft reflection. To work, Doppler HAS to be low enough for a WSPR decode. Sometimes there are several different Doppler shifted signals at the same time. I have waited nearly a day on 2m for just 1 successful WSPR decode at 134km on 2m. Several times the traces show lots of Doppler and just occasionally low enough Doppler for a successful decode. Aircraft scatter is one of the main DX modes on microwave bands.

David (G0LRD) said...

My point is that it is incredibly rare. The tone spacing and symbol rate of WSPR just isn't well suited for doppler. Most of the decodes you are getting are likely by direct path, one of the Tropo enhancement modes, or Es.

Aircraft scatter for microwave DX works far better with a fast, wide tone-spaced digital mode such as JT6M.

Roger G3XBM said...

David, I agree that where there is Doppler there are MUCH better modes than WSPR, but I believe my evidence proves beyond reasonable doubt that 6m GDX paths out to around 200km are possible on WSPR solely because of aircraft, but ONLY if the Doppler is low enough. WSPR GDX because of aircraft is much less rare than you think, in my view. I doubt some of my recent paths would have been spanned without planes. I'd like to think pure tropo was enough! HI. Also my antenna is a vertical and the random nature of plane reflection helps when stations are cross polarised. G0OQK is one station who uses a vertical like me. Interesting discussion - thank you .

David (G0LRD) said...

Perhaps a look at the numbers might add to the debate? For a WSPR decode, the frequency shift cannot be greater than about 2Hz/min, or 33.3mHz/sec. At 50MHz this means the path length cannot change faster than about 200mm/sec, or 12m/min.

Roger G3XBM said...

David, generally agreed. But sometimes, with multiple or single plane reflections, decodes occur on the lower Doppler shifted signals. Also, I have seen decodes with 3 or even 4Hz frequency shifts. My point is aircraft reflection decodes DO occur on 6m and 2m (ask G3WKW and others) and far less rarely than might have been expected. I'd prefer it if some/all of these GDX signals were pure tropo, but they are not. I'm pretty sure G8JNJ/A (184km) is only copied here because of planes. Some decodes from him, for example,are at -3Hz shift! Some at shorter range MAY be are stronger /as strong on tropo, but my feeling/evidence points to a lot of assistance on longer paths from planes. Wish it were NOT the case! HI.

David (G0LRD) said...

I don't think we're in disagreement. I'm not saying it's impossible at all. Perhaps what is at issue is the term "aircraft assisted". The decode will either be entirely via regular propagation or entirely via reflection. There is no possible 'augmentation' from reflections (unless there is zero Doppler). It would be interesting to see the traces from some of your reflection-only decodes.
Cheers, David

Roger G3XBM said...

David, I agree. I am not expecting any "gain" as a result of aircraft reflection. Just on some long (for me) paths the path seems to be ONLY there as a result of aircraft reflection. Personally I'd be very pleased if the long (for me) paths were truly tropo and not artificially helped. Guess a real test would be to do controlled tests with distant stations when we can be certain there are no planes along the path to see if there are decodes by pure tropo. BTW, WSPR is far from ideal for paths with Doppler. I already know that much. JT6M or similar I think would be better at coping as you rightly said.