Forwarded on behalf of Andy Talbot G4JNT
Re JT65 and LF
Forgive the second hand posting, I cannot send
directly to the group as my
EMail provider has decided to throw a wobbler and
stop allowing any outgoing
email to radiate. Incoming ones still arrive
though ! (perhaps the
tranmitting licence has been
rescinded:-)
Anyway..
I have been looking into JT65, and its earlier
incarnation JT44, in some
depth recently for use on beacons. It is very
amenable to being generated
in a simple driver based around a DDS
chip.
However, I don't think that JT65 as it stands, even in its
narrowest
version, has a place on LF. It is too wide and the mode has
been targetted
specifically at areas where frequency, and to a lesser extent
timing,
uncertainty is appreciable such as EME, and does not make the most of
the
stabilities inherent in the LF path. Coding issues notwithstanding -it
does
allow 13 character random messages so the callsign prefix limitation may
not
bee too bad if used sensibly.
Some years ago, Alberto introduced
us to Jason, but even that is an
incoherent mode and doesn't make the most of
the LF path. But if the error
correction capabilities of JT65, were
used with a reduced synchronisation
vector allowed for by better frequency
accuracy, and with the tone / symbol
spacing of Jason we may be geting
something formidable.
BUT it is still incoherent.
In
spite of Mike, G3XDV's reservations about the 'complexity' of GPS
derived
techniques this is still clearly an obvious and straightforward way
to
coherent signalling. Any individual can have sub microsecond timing
for a
few tens of pounds cost, plus the effort needed to build a small amount
of
hardware.
Dictating that any mode used on LF must be
capable of interfacing with a
soundcard only, to all intents dictates
incoherent modes and loses us the
6dB plus that could be recovered with a bit
of effort.
BUt (again) there may still be a
way to make soundcard / GPS simple
....
Some years ago, Andrew Senior
G0TJZ wrote a chirpsounder monitoring prog
that took the output of a receiver
to one side of a soundcard, and the 1 PPS
signal to the other channel. He
also fed the NMEA data string from the GPS
into the serial port. Now,
the computer could get timing to the nearest
second from the NMEA data, and
then look though the digitised audio samples
of the right channel to get the
time to the nearest sampling interval.
Using 8kHz sampling, the received
audio in the left channel could then be
correlated to UTC to an accuracy of
125us. (In fact he managed a bit better
than this by various
methods). The software measured time of flight of HF
signals from
chirpsounders around the world, at any frequency at HF just by
tuning an SSB
Rx roughly to the frequency wanted, then informing the
software of the exact
tuning point. It workd brilliantly, and gives a
great, simple, HF
propagation tool showing the reflections from the
different layers as time
progresses. Do a Google search on G0TJZ
So, how about a dual
channel signalling scheme, audio and PPS. It may even
be possible
to dispense with the NMEA data and use only the 1 PPS, provided
the PC clock
can be set accurately enough. GPS modules are cheap, simple
and
easy to get going.
Andy G4JNT
