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:
https://www.satsignal.eu/ntp/NEO-6M-under-initial-test.jpg
https://www.satsignal.eu/ntp/RasPi-3-with-u-blox-RX.jpg
but you could avoid that with the Adafruit module:
https://www.adafruit.com/product/746
Here's a link to the PC's performance:
https://www.satsignal.eu/mrtg/lund-ntp-2.html
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).
--
Cheers,
David
Web: http://www.satsignal.eu
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