Markus wrote:
> I would't rule out propagational Doppler.
I see what you mean, the carrier looks quite ragged on
there too. I guess I'm used to the nice sharp lines
we get a VLF. These super high frequencies are a bit
different!
But I certainly have a problem with the timing system
here. I'm capturing through the audio input of a
BT878 video chip sampling at 448k/second and discipling
the rate by centroid measurements of a pulse-per-second
with the LF mixed in, sharing the mono input. It's
pushing things a bit to say the least.
I've altered some magic numbers in vttime to get some
long term stability at the cost of extra short term jitter.
In a few minutes we'll see how the 20:00 message looks.
Nice decode from Andy. Eb/N0 from the symbol errors
is 3dB below the carrier measurement. I'm not sure
what that implies. This is all very new territory to
explore, and great fun!
The 20:00 just finished. Decoded!
found rank 0: GE PAUL TNX RPRT ps [ 15 -120 -120 -150 -150]
re-encode 290/1040 ber 2.79e-01 Es/N0=-7.6 Eb/N0=2.7 effrate=1/10.83
You'll notice that with weak signals, say less than about
Eb/N0 = +3dB, it becomes difficult to estimate the reference
phase by squaring the signal (or equivalently, costas looping).
If the initial guess of the reference phase is good, the message
will decode almost immediately with reference phase pattern
0 0 0 0 or 180 180 180 180. But you can see from mine an
Andy's decodes that the initial guess based on squaring was
poor, and the program really has to do some trial and error
to get the best decode, which is coming out some 60 deg
adrift from the initial guess. When you get down to around
zero Eb/N0 the initial guess is pretty worthless. This
is one of the key features that sets EbNaut apart from
other coherent modes: it doesn't rely at all on trying to
extract a reference phase. This is what allows it to
go well below 3dB.
--
Paul Nicholson
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