Paul,
Thank you! I'm always looking forward to higher SNRs that could enable
applications of the types that you mentioned below. The rarity (that you so
aptly described) of useable (calibrated or validated or well-documented)
data on atmospheric phase variation at the faster time scales that could
impact those applications keeps me interested in any new, well-considered
phase data. In that regard I always consider your work groundbreaking. Much
of the valuable work of past decades is limited by comparatively low or
unknown experimental precision and/or unknown experimental conditions, your
data and analyses are always most illuminating.
Thanks especially for the artificial carrier and NAA reference points and
your summary (amplitude SNR for this test probably contributes phase error
of magnitude similar to that of actual path-delay variation); nicely done.
73,
Jim AA5BW
-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of Paul Nicholson
Sent: Tuesday, January 6, 2015 12:53 AM
To: [email protected]
Subject: Re: VLF: Transatlantic messages at 8822Hz
Jim AA5BW wrote:
> I'm guessing that the multiple inflections could be > manifestations of
amplitude noise alone (given ~14dB SNR > and < 7 degrees between
inflections)
Yes. To demonstrate...
I mixed a locally (software) generated carrier into the audio recording of
1st Jan 00:00 to 07:00, placing it at 8823 Hz and setting the level to match
approximately the average amplitude of W4DEX at 8822 Hz.
Now we can measure the artificial carrier (which has exact constant phase of
-110) against the real background VLF noise.
----W4DEX------ --Simulated----
00:00 to 01:00 11.6 dB -122.1 11.9 dB -110.7
01:00 to 02:00 13.1 dB -119.4 14.1 dB -100.8
02:00 to 03:00 12.6 dB -131.9 14.9 dB -118.7
03:00 to 04:00 14.3 dB -135.0 13.0 dB -104.1
04:00 to 05:00 12.7 dB -111.0 13.8 dB -121.6
05:00 to 06:00 15.2 dB -118.9 14.4 dB -100.2
06:00 to 07:00 13.4 dB -111.7 13.0 dB -88.1
The phase variations due to propagation are probably similar size...
For example, here's the phase of NAA 24kHz for the same time period
http://abelian.org/vlf/tmp/naa150101a.gif
The overall increase in path length during the night is apparent from the
reducing phase lead but there is variation at a wide range of time scales.
I'd guess that as we go lower in frequency, the phase variations reduce in
proportion. So the ~50 degrees range at 24kHz probably drops to ~20 degrees
at 9kHz.
Propagation measurements at low VLF are as rare as hen's teeth.
Radio amateur transmissions could be useful to some researchers.
There is great reliance on propagation models for lower ionospheric
research, lightning measurements and long range lightning location systems
(which have to adjust arrival time measurements to allow for propagation
effects). Data from amateur signals would enable spot checks on model
accuracy.
A challenging experiment would be to measure the actual path length. This
requires transmitting two fairly close frequencies
either simultaneously or alternating with FSK. Accurate phase
measurements allow the path length to be calculated.
--
Paul Nicholson
http://abelian.org/
--
|
