How-to use a Garmin 18x with Chrony and PPS

[elvisimprsntr]

@PoolMUC

Try running ntpshmmon with the -o option, that shows the offset instead of the “seen at” timestamp, and also look at NTP0 to get an impression of the timing of the serial data.

Looks like PPS time stamp is correct when the serial NMEA message offset is > 100 msec. I am currently running at 38400 baud (fastest it supports). I assumed faster is always better. How is that for an edge case?

Ubuntu 24.04

root@ntp:~# ntpshmmon -o 
ntpshmmon: version 3.25
#      Name     Offset            Clock                 Real                 L Prc
sample NTP0          0.085228630  1715539516.085228630  1715539516.000000000 0  -1
sample NTP1         -0.873629879  1715539516.126370121  1715539517.000000000 0 -20
sample NTP0          0.081718522  1715539517.081718522  1715539517.000000000 0  -1
sample NTP1         -0.884069774  1715539517.115930226  1715539518.000000000 0 -20
sample NTP0          0.075880041  1715539518.075880041  1715539518.000000000 0  -1
sample NTP1         -0.895254918  1715539518.104745082  1715539519.000000000 0 -20
sample NTP0          0.078816731  1715539519.078816731  1715539519.000000000 0  -1
sample NTP1         -0.899284870  1715539519.100715130  1715539520.000000000 0 -20
sample NTP0          0.084647225  1715539520.084647225  1715539520.000000000 0  -1
sample NTP1         -0.899280238  1715539520.100719762  1715539521.000000000 0 -20
sample NTP0          0.089901582  1715539521.089901582  1715539521.000000000 0  -1
sample NTP1         -0.899164593  1715539521.100835407  1715539522.000000000 0 -20
sample NTP0          0.095393339  1715539522.095393339  1715539522.000000000 0  -1
sample NTP1         -0.899273486  1715539522.100726514  1715539523.000000000 0 -20
sample NTP0          0.101733133  1715539523.101733133  1715539523.000000000 0  -1 # NMEA data offset
sample NTP1          0.000697857  1715539524.000697857  1715539524.000000000 0 -20 # PPS assert offset
sample NTP0          0.067291492  1715539524.067291492  1715539524.000000000 0  -1
sample NTP1         -0.899164022  1715539524.100835978  1715539525.000000000 0 -20
sample NTP0          0.061891719  1715539525.061891719  1715539525.000000000 0  -1
sample NTP1         -0.900270046  1715539525.099729954  1715539526.000000000 0 -20

@PoolMUC

Would be interesting to see what gpsd would see from the device with kernel PPS (i.e., under BSD), if it is having similar issues in that case (there are some differences, but also a lot of stuff in common which might also give clues as to what goes wrong).

Installed FreeBSD 14.0, GPSD and Chrony (for some reason I could not get the NTPD SHM devices to work with GPSD, so I switched to Chrony). Added the PPS_MODE flag to the loader. PPS offset looks great. You are right Linux <> FreeBSD. Config files are not in the same locations and setup is completely different.

FreeBSD - 14.0

root@ntp-2:/boot # cat loader.conf.local 
dev.uart.0.pps_mode="1"

root@ntp-2:/boot # sysctl -a | grep -i pps_mode
dev.uart.0.pps_mode: 1


/dev/ttyu0                    NMEA0183>
┌──────────────────────────────────────────────────────────────────────────────┐_minor":14}
│Time: 2024-05-12T21:39:06.000Z   Lat: XX XX.599400' N   Lon:  XX XX.263300' W │r":"NMEA0183","readonly":"true","activated":"2024-05-12T21:38:5
└───────────────────────────────── Cooked TPV ─────────────────────────────────┘]}
┌──────────────────────────────────────────────────────────────────────────────┐false,"timing":false,"split24":false,"pps":true}
│ GPGGA GPRMC GPGSA GPGSV PGRME                                                │
└───────────────────────────────── Sentences ──────────────────────────────────┘
┌───────────────────────┌─────────────────────────┌────────────────────────────┐
│ SVID  PRN  Az El SN HU│Time:     213906         │Time:      213906           │
│GP  5    5   4 57 46  Y│Latitude:    XXXX.5994 N │Latitude:  XXXX.5994        │
│GP 15   15 191 54 17  Y│Longitude:  XXXXX.2633 W │Longitude: XXXXX.2633       │
│GP 20   20  39 36 41  Y│Speed:    000.1          │Altitude:  2.3              │
│GP 29   29 310 54 50  Y│Course:   000.0          │Quality:   1   Sats: 04     │
│GP 23   23 250  6 25  N│Status:   A        FAA:A │HDOP:      1.5              │
│SB138   51 228 46  0  N│MagVar:   007.2W         │Geoid:     -28.7            │
│                       └───────── RMC ───────────└─────────── GGA ────────────┘
│                       ┌─────────────────────────┌────────────────────────────┐
│                       │Mode: A3 Sats: 5 15 20   │UTC:           RMS:         │
│                       │DOP H=1.5  V=2.3  P=2.8  │MAJ:           MIN:         │
│                       │TOFF:  0.149858632       │ORI:           LAT:         │
│                       │PPS:  0.000011444        │LON:           ALT:         │
└──────── GSV ──────────└────── GSA + PPS ────────└─────────── GST ────────────┘
root@ntp-2:/usr/local/etc # gpsmon -a | grep -i pps
TOFF= 1715550025.068900268 real= 1715550025.000000000
PPS= 1715550026.00002302 clock= 1715550026.00000000 offset= 0.000023023
TOFF= 1715550026.075223465 real= 1715550026.000000000
PPS= 1715550027.00002048 clock= 1715550027.00000000 offset= 0.000020485
TOFF= 1715550027.081342965 real= 1715550027.000000000
PPS= 1715550028.00002379 clock= 1715550028.00000000 offset= 0.000023797
TOFF= 1715550028.087593513 real= 1715550028.000000000
PPS= 1715550029.00002874 clock= 1715550029.00000000 offset= 0.000028747
TOFF= 1715550029.094062283 real= 1715550029.000000000
PPS= 1715550030.00002342 clock= 1715550030.00000000 offset= 0.000023421
TOFF= 1715550030.099321907 real= 1715550030.000000000
PPS= 1715550031.00002090 clock= 1715550031.00000000 offset= 0.000020908
TOFF= 1715550031.105047413 real= 1715550031.000000000
PPS= 1715550032.00002083 clock= 1715550032.00000000 offset= 0.000020833
TOFF= 1715550032.070664659 real= 1715550032.000000000
PPS= 1715550033.00002333 clock= 1715550033.00000000 offset= 0.000023330
TOFF= 1715550033.067559462 real= 1715550033.000000000
(122) {"class":"WATCH","enable":true,"json":false,"nmea":false,"raw":2,"scaled":false,"timing":false,"split24":false,"pps":true}
------------------- PPS offset:  0.000023023 ------
------------------- PPS offset:  0.000020485 ------
------------------- PPS offset:  0.000023797 ------
------------------- PPS offset:  0.000028747 ------
------------------- PPS offset:  0.000023421 ------
------------------- PPS offset:  0.000020908 ------
------------------- PPS offset:  0.000020833 ------
------------------- PPS offset:  0.000023330 ------

@PoolMUC

Run gpsd with logging (-D option). I think one needs to crank the log level up to 4 or even 5 to see also informational messages from the PPS handling.

Did that, but not sure where the logs are.

@PoolMUC

Your use of the -g option to gpsmon at least seems to suggest that some data from the 18x is in Garmin binary.

I used the -a option you suggested when GPSMON crashed on Ubuntu 24.04. I had to go back and make sure I did not have a typo in my post. An “a” and “g” look the same without my glasses.
It’s a NMEA 0183 GPS 19X (GPS+GLONASS). Basically the same as the 16X and 18X GPS only serial units.

OTHER COMMENTS

• Couldn’t figure out how to get NTPSHMMON installed on FreeBSD
• I tried several times to recompile the Linux kernel with the required KPPS support enabled, but all the wikis and google searches that provide instructions how to recompile the linux kernel are horribly out of date.
  GPSD Time Service HOWTO

QUESTIONS

• So is slowing down the baud rate the next step? FreeBSD did not seem to have a problem at the same baud rate.
• For giggles, I set the PPS_MODE=2 (CD) and GPSD would not detect the PPS signal.

UPDATE

Created an issue report. NOTE: You need a GitLab account to view.