Geez, disabusing sounds awfully harsh I 'm just very cautious of using a serial port for any kind of precise timing.

The PPS signal from the GPS receiver is going to be good to ~1microsecond when you have a navigation solution with good geometry or when you are stationary at a known position with one or more satellites tracked and the receiver in the time-keeping mode. The delays through the serial port will be on the order of 1/baud seconds, plus some unknown and probably variable processing delays. All of this may well be good enough for your rally application as it could be well under 100 milliseconds.

I suspect that you can do well enough for your needs by using the NMEA data along with a calibration factor for all of the delays to set the Empeg clock periodically. I doubt you would need to set it more often than once a day at the first good fix after turn on. I suspect that we can come up with a proceedure for determining the calibration factor though it would be tricky to take into account various receivers and their different processing contributions along with their variations in NMEA sentence outputs.

Lynn