[time-nuts] Four hour cycle in GPS NMEA jitter

[Chris Albertson]

> Does this sound like something that one would expect with the NMEA output of a non-timing GPS?  Is it related to satellite orbits?  Or perhaps is has something to do with the design of the SiRFStar IV?
>

Remember the phone based time service? "At the tome time time will be
.... BEEP"  With a GPS the NEMA sentence take the place of the spoken
words on the phone.   The NEMA specification allows the sentences to
up to one second "off".   That said most GPS receivers do MUCH better
than the NEMA spec but you should never count on non-specified
performance in a professional design.   The PPS is of course doing the
job of the BEEP on the old phone system.  Set you watch based on the
beep, not NEMA.

If you have an FPGA available then you could significantly improve
system time keeping.   Currently the PPS interrupts the CPU to
snapshot internal counter.   Unpredictable interrupt latency lifts NTP
timekeeping to about 1 or 2 microseconds but is the counter snap
shooting could be moved out to FPGA hardware there would be no unknown
latency and you could get NTP to break a "magic" 1uS barrier.   I've
only hear of 1 uS being broken with hardware.    You would actually
not ned to write much software to make this happen, just move the
counter outside the CPU to the FPGA and you about have it.


Chris Albertson
Redondo Beach, California