Date: Sun, 4 Jun 2017 00:22:40 +0200 (CEST)
From: Jacek Lipkowski <sq5bpf@lipkowski.org>
To: rsgb_lf_group@blacksheep.org
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Subject: Re: VLF: tomorrow morning...
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Hello,

I'm posting the description of the decode, people which use the linux 
software will find it obvious, but maybe someone will find it 
interesting/helpful for his attemps. BTW I got Stefan's permission to 
disclose the message to the mailing list.

The message didn't decode correctly at first with the noiseblanker 
settings i've used (-a2 -d0 -t 10). Tried the -f9 option:

$ vtraw -oa < resampled_file | ./ebnaut -d -N7 -p 16K21A  -S 24 -k 5 -r240 -c2  -PS -v -L 20000 2>&1 |tee decwy

However with only 5 bits of crc there are many valid decodes:

$ grep found decwy |cut -d \[ -f 2 |cut -d \] -f 1 |sort -u | wc -l
67495

67495 of unique decodes is not good, we can't sift through this manually 
and it will have a lot of valid looking messages.

Lets cheat a bit: Paul wrote that it is a famous name, so we can 
hypothesize that it will be only letters:

$ grep found decwy |cut -d \[ -f 2 |cut -d \] -f 1 |sort -u |egrep '^[A-Z]+$' |wc -l
352

352 is much better, i can look through all of the messages manually:

$ grep found decwy |cut -d \[ -f 2 |cut -d \] -f 1 |sort -u |egrep '^[A-Z]+$' | less

And the only message which makes sense is MARCONI :)


I dug out the best message from the ebnaut decoder logs:

found rank 13961 ber 4.0858e-01 Eb/N0 -1.4 M -8.404992819e-01 [MARCONI] ps[ 83  -90  -60  -90  -60]
carrier phase: 5.1 deg
carrier Eb/N0: -4.1 dB
carrier Es/N0: -18.44 dB
carrier S/N: 11.87 dB in 38.9 uHz, -32.24 dB in 1Hz, -66.22 dB in 2.5kHz
elapsed 1334


Now let's try to find the best noiseblanker settings:

PLIK2=plik_dk7fc_20170603_1_raw1
FREQ=6470.1
TT=2017-06-03_03:00:00,+25728
vtread -T$TT /mnt/rawvlf | vtfilter -h bp,f=${FREQ},w=3000 > $PLIK2

for a in 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6
do
for t in 10 100
do
         echo a=$a t=$t
         vtblank -v -a$a -d0 -t$t < $PLIK2 | \
         vtmult -f ${FREQ} | \
         vtresample -r 240 > ${PLIK2}_${a}_${t}
done
done

This will create files called plik_dk7fc_20170603_1_raw1_XX_YY (where XX 
is the -a vtblank setting and YY is the -t setting).

Lets go through the files and see how they match our hypothesis:

$ for i in  plik_dk7fc_20170603_1_raw1_*; do  echo $i; vtraw -oa < $i |  ./ebnaut -d -N7 -p 16K21A  -S 24 -k 5 -r240 -c2  -PS -v -L 20000 -f15 -f16 -M MARCONI; echo; done

... after a lot of output this one looks best:

plik_dk7fc_20170603_1_raw1_1.4_100
padded 0.267 seconds at end
initial reference phase -0.2 amplitude 2.277e-02
carrier phase: 2.4
carrier Eb/N0: -2.7 dB
carrier S/N: 13.28 dB in 38.9 uHz


Let's try to decode the plik_dk7fc_20170603_1_raw1_1.4_100 file without 
the -f9 option and see how that works:

$ vtraw -oa < plik_dk7fc_20170603_1_raw1_1.4_100 |  ./ebnaut -d -N7 -p 16K21A  -S 24 -k 5 -r240 -c2  -PS -v -L 200000

padded 0.267 seconds at end
initial reference phase -0.2 amplitude 2.277e-02
prep [  0    0    0    0    0]
found rank 141 ber 4.1884e-01 Eb/N0 -2.4 M -6.664184332e-01 [MARCONI] ps [  0    0    0    0    0]
carrier phase: 2.6 deg
carrier Eb/N0: -2.7 dB
carrier Es/N0: -17.02 dB
carrier S/N: 13.28 dB in 38.9 uHz, -30.82 dB in 1Hz, -64.80 dB in 2.5kHz
elapsed 42


Yes, it works. Turns out that the best vtblank settings for this 
transmission were -a1.4 -d0 -t100.


VY 73

Jacek / SQ5BPF