On 5 Jan 2019 at 22:44, bil...@classdesign.com [wsjtgroup] wrote: > Hi Martin, > > you must be careful not to confuse first and second order Doppler > effects. A signal reflected from a moving object (or emitted from a > moving transmitter, or received at a moving receiver) will be > frequency shifted by a fixed offset proportional to the frequency and > to the relative velocity of the wave source with respect to the > receiving aerial. That shift in frequency is the Doppler frequency. > > If the relative velocity of the wave source with respect to the > receiving aerial is changing, i.e. there is acceleration, then the > Doppler frequency will also change over time.
Yes Bill, I know and understand all of that... :-) > The former should lead to a decodable FSK signal the latter will not > if the acceleration or the frequency are sufficiently high to shift > the tones more than half the inter-tone spacing during a single > transmission period. > > Here's an example of a JT65A signal on 6m showing several aircraft > reflections exhibiting accelerating reflectors (sloping waterfall > traces) which are shifting by more than would be acceptable for a > single FT8 receiving period: > > Note that I am located close to several large airports and air traffic > around here is considerable. I think the key point here is that there must be enough change of velocity, ie. acceleration, to create a change in the Doppler frequency that's more than half the inter-tone spacing. My interpretation of the points made by Joe K1JT in his posting back in the middle of last year is that in many circumstances the acceleration of the reflector(s) won't be sufficient to cause such a shift, but clearly in your particular scenario, with a lot of local air traffic that's accelerating and deccelerating, there is presumably enough change in velocity to cause significant time-varying shifts and hence the sloping traces on the waterfall. As I mentioned previously I have on a few occasions observed a noticeable slope on the waterfall trace of a 6m FT8 signal but the instances of this are fairly few and far between, probably because I'm 50-60kms away from a major airport so there aren't many aircraft around whose velocities are changing by enough to cause the frequency of the Doppler-shifted signal to vary significantly over time. My guess is that the few sloping waterfall traces I have observed might have been caused by an aircraft taking off from my local airport, which is about 16kms away and ain't very busy! -- 73, Martin G0HDB --- This email has been checked for viruses by Avast antivirus software. https://www.avast.com/antivirus _______________________________________________ wsjt-devel mailing list wsjt-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/wsjt-devel
