I have been working with a HP58534A GPS Timing Antenna which outputs extended
NMEA sentences including a custom sentence which specifies the Timing Error of
the NEXT 1PPS Pulse as a number between +00 and +85 Nanoseconds. I am wondering
whether using a PIC to correct each 1PPS pulse using this sentence would be
worthwhile to clean up the saw tooth in the 1PPS and if so what would be the
best way to correct each 1PPS.
One solution might be to send the receiver the custom NMEA sentence which lets
me set the antenna delay +/-nnnnnn Nanoseconds. I would use the timing error
plus the actual cable delay to calculate the nanosecond delay to use for the
next pulse and send the cable delay sentence to the receiver before the next
1PPS pulse. I have at least 100ms to send this message at 9600 baud before the
specified pulse arrives. This should be sufficient time assuming the receiver
acts on the message immediately.
A second approach would be to use a programmable delay line (about $14 from
jameco) to correct the the 1PPS by 85ns - Timing Error from message. I would
set the cable delay to 85ns more then the actual cable delay to bring the 85ns
in the delay line calculation.
I'm sure all the timenuts will have better or simpler solutions to this concept
and many different views on whether it is worthwhile to do.
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