Hi Wolf and all,
triggered by John VE1ZJ's recent remarks on Loran as a skywave propagation
monitor, a couple of weeks ago I took a deeper look at what could be
received. John's "www.G4CNN.f2s.com/Loran_lines.htm" pointed me to the list
at "www.megapulse.com/table.html". With this at hand I tried to identify the
lines I could see on Argo around 100.0 kHz.
The key to their frequencies is the "GRI" (group repetition interval), which
is the number of 10us carrier periods between two repetitions of the
modulating pulse groups. Each group consists of 9 or 8 pulses, 1 ms apart.
Some of these pulses have an alternating phase, so that the periodicity of
the pattern is actually two times the GRI:
Master Secondary
++--+-+- + +++++--+
+--+++++ - +-+-++--
Thus the frequency spacing of the lines is Df = 100kHz/(2*GRI), eg.
100kHz/(2*7499) = 6.6676 Hz for the Sylt chain. The Loran-C carrier frequency
is generated by atomic clocks and claimed to be accurate on the order of
10^-13.
The chains I could clearly observe here were
GRI Df Chain (Wolf's AM line)
/10us /Hz
5930 8.4317 Canadian East Coast
6731 7.4283 Lessay
7001 7.1418 Bo (140*Df = 999.8572 Hz)
7030 7.1124 Saudi Arabia S
7270 6.8776 Newfoundland East Coast
7499 6.6676 Sylt (150*Df = 1000.1334 Hz)
8000 6.2500 Western Russia (160*Df = 1000.000 Hz)
8830 5.6625 Saudi Arabia N
9007 5.5512 Eide
Then there were additional weak lines which were too close to 100 kHz, at
offsets of 1.52, 3.04 and 4.56 Hz. Their explanation is a little more subtle:
Many loran stations are "dual-rated", they transmit in two chains with
different GRI's. In case of a collision between two pulses that would have to
be sent simultaneously, one of the pulses is simply left out. These dropped
pulses occur at the beat frequency between the two GRI's. For the Sylt
station, these "intermodulation" lines are multiples of
(100kHz/6731-100kHz/7499) = 1.5215Hz.
There were even more lines I could not identify, eg. on 5.14, 5.90, 7.24,
8.18, 8.95 Hz. This made me wonder if the table is really complete, as stated
by megapulse. Also, I can't explain Wolf's observed 999.96 Hz.
The fun got even more interesting when I went to time domain. Using a
programmable divider clocked by 100 kHz, I generated 2*GRI trigger signals
for a digital oscilloscope in 128-averaging mode, and viewed the SSB output
tuned to 100.0 kHz zero beat (thus allowing phase-sensitive averaging). With
this setup I could see the individual pulse groups of distant chains grow out
of the noise, identify their stations and measure the time-differences. These
matched calculated great-circle distances with a surprising accuracy of less
than 100us, and it even worked fine for all five of the Saudi-Arabian
stations, up to 4642 km from here.
Thus I think that Loran-C can be used not only as a precise frequency and
time standard, but also as a powerful instrument for worldwide LF propagation
monitoring, aided by its ability to resolve propagation delays.
73s and happy experimenting
de Markus, DF6NM
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