Switched the GPS configuration to one optimized for timing / 2020-07-03

2020-07-03 Switched the GPS configuration to one optimized for timing 1 month ago
Based on A Raspberry Pi Stratum 1 NTP Server - Phil's Occasional Blog I switched the gps to a configuration optimized for timing. The default settings are optimized for location services, but I want an NTP server.

I used gpsctl to configure the ublox chip in the GPS/RTC Hat:
$ gpsctl -a -B 115200 --configure_for_timing -vv
Serial port ("/dev/ttyAMA0") open...
Serial port open and configured...
Automatically determining baud rate...
Trying 230400 baud...
Trying 115200 baud...
Trying 57600 baud...
Trying 38400 baud...
Trying 19200 baud...
Trying 9600 baud...
Synchronized on 9600 baud...
Changing baud rate to 115200...
Successfully changed baud rate to 115200...
After that I got location data at a high speed. I changed the /etc/ntp.conf parameters to use the GPS_NMEA and PPS drivers, with:
# PPS reference
server 127.127.22.0 minpoll 4 maxpoll 4
fudge 127.127.22.0 refid PPS

# GPS NMEA driver
server 127.127.20.0 mode 89 minpoll 4 maxpoll 4 iburst prefer
fudge 127.127.20.0 flag1 0 flag2 0 flag3 0 time2 0.043 refid GPS
And now I get much better numbers:
$ ntpq -p
     remote           refid      st t when poll reach   delay   offset  jitter
==============================================================================
oPPS(0)          .PPS.            0 l   14   16  377    0.000   -1.656   0.134
*GPS_NMEA(0)     .GPS.            2 l   13   16  377    0.000  -11.730   0.517
+ntpritchie.idef 131.211.8.244    3 u   44   64  377    4.263    1.436  62.373
+metronoom.dmz.c 192.87.106.3     2 u   44   64  377   12.141   -2.250  49.247
koos@henkp:~ $ ntpdc -c kern
pll offset:           -0.00142676 s
pll frequency:        7.468 ppm
maximum error:        4.934e-06 s
estimated error:      3.372e-06 s
status:               2001  pll nano
pll time constant:    4
precision:            1e-09 s
frequency tolerance:  500 ppm
The time offset factors still need work, but I'm getting close!

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