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LF:GPS Locked signalling at LF

To: [email protected]
Subject: LF:GPS Locked signalling at LF
From: "Andy talbot" <[email protected]>
Date: Mon, 28 Jul 2003 12:18:02 +0100
Importance: high
Reply-to: [email protected]
Sender: <[email protected]>
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




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