[ntp:questions] shm + pps calibration

David Taylor david-taylor at blueyonder.co.uk.invalid
Sat Jul 7 06:02:10 UTC 2018

On 06/07/2018 21:18, Daniel Gearty wrote:
> In my case, I attribute the PPS delay to the serial to USB 1.1 conversion of the Prolific PL2303HXD in my NaviSys GR-8013W GPS.  The offset bounces around within +/- 0.5 msec.
> It appears the initial PPS offset should be zero if you receive your PPS signal directly from a proper serial port.
> I am using NTP "Generic NMEA GPS Receiver" driver 20 with the Windows loopback driver.
> ggavi is using "PPS Clock Discipline" driver 22 for PPS, but perhaps should be using "Shared Memory Driver" driver 28 for both NMEA and PPS.
> http://www.catb.org/gpsd/gpsd-time-service-howto.html#_feeding_ntpd_from_gpsd
> Another, newer driver for NTPD to receive time from gpsd is "GPSD NG client driver" driver 46.
> http://doc.ntp.org/current-stable/drivers/driver46.html
> Although I wonder whether chronyd receiving time from gpsd via a socket has the best NTP performance.
> https://chrony.tuxfamily.org/doc/3.3/chrony.conf.html

Using different software (Chrony or gpsd) is unlikely to improve 
performance which is inherently limited by the serial-USB conversion, 
unless someone were to write a special driver to average out the USB 
jitter.  NTP will do that to an extent, of course.

For best NTP performance, if you aren't using a portable PC (so no 
serial port), check whether your motherboard has a serial port header, 
or get an add-in PCIe serial port and swap the GPS for one which has a 
real PPS line.  Likely even the low-cost Chinese ones will be good enough.

On one PC I'm using an add-in PCIe card, a TTL-RS232 converter (tried 
without but the signal levels were too low) and a Chinese module sitting 
on the bench.  Needed to add one wire from the u-blox module's PPS pin:


but you could avoid that with the Adafruit module:


Here's a link to the PC's performance:


The variation on that PC is likely due to the CPU temperature as the 
load varies with receiving new Sentinel-3A satellite data every orbit 
(101 minutes).

Web: http://www.satsignal.eu

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