Hello Skip,
On 4/16/2010 4:24 AM, KH6TY wrote:
Hi Tony,
The aircraft reflections are usually recognizable.
They certainly are - especially at UHF.
You can hear the pulsations in the background noise change in rate as
the airplane flies around and sometimes even see the frequency shift
on the waterfall,
SBSpectrum (Peter Martinez) can reveal the inaudible Doppler shifts as
well. I suspect the weaker, underlying Doppler you can't hear can cause
interference with certain modes.
Since Contestia and Olivia keep printing after the transmission has
ceased, I suspect that the interleaving and redundancy is carrying
enough data over the peaks and valleys that we hear to produce perfect
print, but I am no theorist on this, for sure!
Seems like a reasonable analogy Skip.
It would be interesting to see these effects Skip. Feel free to send a
few screen-shots.
Thanks,
Tony -K2MO
Tony wrote:
FWIW, PSK125R does not survive the Doppler disturbances on UHF.
Olivia or Contestia does. Therefore, the mode we have found works
best under the severe conditions of multipath, Doppler shift,
Doppler spread, and very weak signals is Contestia 16-500 at 30 wpm.
the minimum S/N is -12 dB, which is essential for weak signal UHF
and VHF digital operation as every dB of S/N we can get is important
for weak signal work.
73 - Skip KH6TY
Skip,
Thanks for the info. The path simulator results seem to agree with
your observations on the high bands. I bet you have a fair share of
problems with aircraft Doppler? I've noticed multiple
reflections from multiple aircraft while monitoring VHF beacons.
Spectrum analysis reveals how great those Doppler shifts can be; the
mixing of 2 or 3 multipath signals can play havoc with throughput. .
While it's not quite the same, we didn't get a chance to test on HF
today. Andy tried his best to accomodate (always there for us Andy),
but conditions weren't good between us on 80 meters. Tomorrows
another day.
Thanks again.
Tony -K2MO
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