Message-ID: <01C35502.5E19CAB0.g4jnt@thersgb.net>
From: "Andy talbot" <g4jnt@thersgb.net>
To: rsgb_lf_group@blacksheep.org
Date: Mon, 28 Jul 2003 12:18:02 +0100
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Subject: LF:GPS Locked signalling at LF
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At the end of this month a long term writing task will come to an end and I can 
begin to look at a number of various shelved projects again.  One of these is 
GPS locked BPSK for LF.

With a low cost GPS module (and they are becoming very cheap now !) anyone can 
get a 1 pulse per second signal accurate to within a microsecond of UTC.  Which 
makes all the usual requirements for decoding coherent transmissions such as 
clock and carrier recovery redundant.   I've had one or two ideas about simple 
equipment to demodulate, and not using soundcards, and will be looking for 
someone rto do some experimenting with.

Can anyone can generate synchronised PSK with symbols that are an integral 
number of seconds in length and timed to within a few milliseconds of UTC ? 
 Frequency needs to be accurate to within about a tenth of the symbol period, 
ie for 1 second symbols an accuracy of about 0.1Hz will suffice, for 30 second 
signalling 3mHz is needed.

I have already made a start, using a PIC based two channel A/D converter to 
read the output of an I/Q detector working on the 1kHz tone from a 
converntional receiver. This reads the level from the I and Q mixers every 
second synchronised to the 1 PPS from a GPS module.  The integrated I and Q 
voltages are sent every second to a PC which in turn integrates over a longer 
period and calculates the phase, plotting to a graphical display.

At the moment all I have for a test signal is the GPS locked DDS source (see 
RadCom last year) sending a locked BPSK waveform with a sufficiently low signal 
leakage from a 60mm piece of wire as the antenna,  to make the signal virtually 
undectable on ARGO with 10s dots and just about visible with 30 second dot mode 
and probably too weak to resolve any SlowCW signalling information.    Using 30 
second BPSK symbols a very rough and ready quick test last night indicated a 
very definite square wave phase plot, mirroring exactly the transmitted 
waveform that would have given reliable decoding.

That's as far as I am going for now until the writing task is complete and sent 
off, but I also have an outline idea for a even simpler PIC based interface 
that does away with the I/Q detector, taking in the 1kHz directly and 
generating a GPS synchronised quadrature LO internally, but that will have to 
wait a few weeks.   Meanwhile, if there are any Windows programmers who would 
be interested in taking this on board with a view to producing a soundcard 
version can you get in touch.   In the Chirpview software by G0TJZ, Andrew 
locks the Soundcard to GPS by feeding the pulse into one of the audio channels 
which gives a resolution equal to the sampling rate.

For slow signalling at LF a few tens of milliseconds timing accuracy is 
adequate for symbol timing, and given an accurate external 1 PPS signal, GPS 
itself may not even be essential if this can be set with reference to another 
timing reference - such as MSF, DCF etc.

Andy  G4JNT