On 05/07/2018 21:30, Daniel Gearty wrote:
server 127.127.20.3 iburst minpoll 3 mode 17
# COM3, 8 second polling, NMEA GPRMC, 9600 bps, NaviSys GR-8013W u-blox M8 USB 
PPS GPS

fudge 127.127.20.3 time1 0.000142 time2 0.111111 flag1 1 flag2 1 flag3 0
# PPS offset, NMEA offset, enable PPS, pulse on falling edge, disable kernel PPS

I am using a u-blox M8.

I had trouble with it until I realized that the pulse is on the falling edge 
rather than the default of the rising edge.

I used a u-blox utility to see timestamps for the NMEA GPRMC sentence coming 
through.  It took about a tenth of a second.  If it is accurate to within four 
tenths of a second, it works.  It not, then it does not work.

Barring access to a more accurate clock, PPS offset is a blind guess.  All I do 
is offset by an average of the NTP loopstats offset over time.  I use 142 
microseconds.

I've compared a number of GSP/PPS sources using an oscilloscope and the PPS offset between them is usually within 100 nanoseconds, so I would be surprised if the PPS leading edge is as much as 142 microseconds out. With a Raspberry Pi, I typically I see offsets reported by ntpq -pn well under 20 microseconds:

C:\Windows\System32>ntpq -pn raspi-1
remote refid st t when poll reach delay offset jitter
==============================================================================
o127.127.22.0 .kPPS. 0 l 13 16 377 0.000 -0.001 0.004 127.127.28.0 .GPSD. 0 l 12 16 377 0.000 367.025 5.593 *192.168.0.3 .kPPS. 1 u 6 32 377 0.542 0.054 0.008
.
.

https://www.satsignal.eu/mrtg/raspi1_ntp.html


--
Cheers,
David
Web: http://www.satsignal.eu

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