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
