Can I point out that a carier that appears at a
known time and on a known frequency IS modulated. It
is on off keyed. (Well...off/on anyway :-) The
only way that a carrier can be unmodulated is if it
has been there since the beginning of (RF)time and is
never switched off, ever, never, before the end of
(RF)time. Its appearance or disappearance is
information; what that info means depends on how it
has been coded.
Any switching, at all, introduces modulation and
widens the bandwidth, and therefore counts as a means
of identification if the meaning of the on/off is
pre-arranged.
In comms-theory-speak If G9BOF makes the
announcement "I'll be transmitting on Sunday at 1300z
on 8910.0004Hz" he has now generated a codebook. The
appearance of the carrier at that point in time
indicates a single bit of information, which by
reference to the code boook means G9BOF is sending a
'1'. No carrier detected,he is sending a '0', or
there has been a bit error. In other words, no error
correction.or detection has been performed. If G9BOF
now makes another announcement that "I'll switch it
off at 2358z" he has added another entry to the code
book. If the carrier really does go off then, the
two bits of information together make a
dual-redundant pair, the minimum needed for error
detection.
If G9BOF also makes the statement that "my signal
is stable and accurate to 1mHz" and another signal
appears in the passband that is 2mHz away, or wobbles
by 1.5mHz, then is is not G9BOF, and error detection
will flag it as not valid. If a receiving station
cannot detect this error, and incorrectly assesses the
carrier then the Rx has used the wrong codebook, and
therefore cannot be deemed to be listening to G9BOF at
all.
All a rather extreme and somewhat petty example of
coding theory and error detection, but its exactly
the same as simple Hamming codes all the way thorough
to reed Solomon. And... I'm not sure how G9BOF
getting his frequency or time wrong would fit into the
coding theory.
So, conclusion, a carrier as stable and on the
exact frequency as stated, at the right time is going
to be valid. If you have no way of proving it really
is as stable as quoted, or in the right place, you've
no right to question its validity.
And anyway, how many spurious signals can even give
any pretence to being stable in these terms - if you
think they are, get a better receiver. We are
talking about 0.1ppm over the duration of a signalling
element as a minimum frequency stability requirement
for most serious VLF through LF signalling -
which means a quite good TCXO, an average to
middling OCXO or ideally a locked source. If you
can't manage this, do not cry 'foul' when others do.
Andy
On 11 March 2011 21:30, Markus
Vester
<[email protected]>
wrote:
Dear Mal,
with all due respect, I very much disagree here.
Whenever I see a clear trace appear at the right
time and within a milliHz
of a confirmed and correctly calibrated frequency, I
have hardly a doubt
about the validity of the reception. Even more so if
the transmission can be
verified by comparison to a calibrated grabber in
another location.
It seems that the G3KEV antenna does have excellent
sensitivity at VLF. It
would be fantastic if you could augment that with a
stable, properly
calibrated, and narrowband reception system. SpecLab
has made that so easy
now - throw out the wildly drifting "receiver",
connect your antenna
directly to the soundcard instead, lock to one of
your MSK neighbours (GBZ
or GQD), and your in the game.
Best regards,
Markus (DF6NM)
From: mal hamilton
Sent: Friday, March 11, 2011 5:57 PM
To: rsgb
Subject: LF: VLF traces
I can see a couple of vy weak traces around 8970 but
this is no guarantee
who it might be because there is no ID. Frequency
alone is not a VALID
report- it could be anything.
G3KEV
