Re: LF: More EbNaut 137.777 - Dec. 1/2

[Markus Vester <markusvester@aol.com>]

Hi Paul,

 

that's an interesting plot!

 

> That means the phase isn't being preserved from one message to the next.

 

Presumably the carrier is not really interrupted, only the frequency seems to be slightly off. The first few spikes show a regular alternating pattern with ~180° per half hour, consistent with the observation that Joe's QRG was approximately 0.3 mHz low.

 

All the best,

Markus

 

 

-----Ursprüngliche Mitteilung----- 
Von: Paul Nicholson <vlf0403@abelian.org>
An: rsgb_lf_group <rsgb_lf_group@blacksheep.org>
Verschickt: Mi, 2 Dez 2015 11:49 pm
Betreff: Re: LF: More EbNaut 137.777 - Dec. 1/2

 

I ran last night's signal through vtmatch.  First I retrieve 

14 hours of signal and mix down to baseband I/Q at 100 samples/sec. 

 

  vtread -T2015-12-01_19:00,2015-12-02_09:00 /raw | 

     vtmult -f 2777 |  # 137.777 kHz is at 2777 Hz in /raw 

     vtresample -r 100 > v9.vt 

 

The file v9.vt is two channels, ch1 = I, ch2 = Q. 

 

Then I create a template for matching, using the known message, 

 

    echo VO1NA | 

      ebnaut -e -r100 -S2 -p8K19A -N5 | 

      awk '{print $1}' > ref9.txt 

 

ref9.txt is just a single column of sample values, 1024 seconds 

long. 

 

Then 

 

  vtmatch -t ref9.txt v9.vt | vtplot -gs 2000,480 

 

produces 

 

  http://abelian.org/vlf/tmp/151202a.gif 

 

in which I is the top channel and Q is below and the time axis 

is seconds from 2015-12-01 19:00. 

 

Each spike is a VO1NA message at half-hour intervals. Some spikes 

are +I, others -Q, and some are a mix.  That means the phase isn't 

being preserved from one message to the next. 

 

The little spike in both channels at offset 3600 seconds is 

the 20:00 message which gave Eb/N0 = 3.4 dB.  Messages that 

decode below about 1 or 2dB are not really visible on the 

cross-correlation.   They might be if I knew the phase and 

rotated I/Q to put the signal all into one channel. 

 

I'll upload a new vtmatch.c just as soon as I've persuaded it 

to pass its regression tests. 

 

-- 

Paul Nicholson 

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