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
