Re: FT8 performance (or, lack of) on 432

James Duffey

Well, there is a major E-W flight path between you and Phoenix.I guess you probably have planes on that path one way or another every 10 minutes or so. If you look at the slope of the frequency shift and know the frequency, you should be able to calculate the velocity of the aircraft pretty accurately. 

James Duffey KK6MC
Cedar Crest NM

On Aug 1, 2020, at 18:09, Keith Morehouse <w9rm@...> wrote:

So far, the 222+ contest has convinced me (or, I have rediscovered) that FT8 on 432 is unusable on paths with lots of Doppler shifted multi-path, such as my paths to DEN and PHX.  These paths are heavily influenced by aircraft scatter which causes various Doppler shifted signals.  The rate of frequency shift I'm seeing is in the range of 25-100 hz over a 15 second FT8 sequence, which is, apparently, too much for the decoder algorithm to handle.  If you calculate how much Doppler shift you are likely to get on 432 MHz from an aircraft traveling at various tangential paths and speeds, it roughly matches what I'm seeing (and what the other end of the path also sees).

I'm thinking a mode such as ISCAT (specifically ISCAT-A, which was designed for microwave rain scatter paths) might be a vastly superior mode for this situation.  FT8 certainly sucks wind under these conditions and proved it repeatedly today.

I have verified to the best of my ability that my system is stable.  My transverters are reference locked to GPS and I have another, independent, reference locked source which I can monitor on the air at whatever frequency I set it to.  I have also completed at least one 432 Q on FT8 that showed no 'drift'.  Evidently there are fewer aircraft on this path (DM58 to SLC area) or they are moving on a course which does not cause Doppler shift on any reflection.

Comments or further discussion welcome !


Keith Morehouse
via MotoG

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