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Linux timing server with gps source

How to build your own ntp server which is using a usb gsp module as timing source.

get a gps module

GPS-Modul VK-162 with TCXO EEPROM

install tools

sudo apt update
sudo apt install gpsd gpsd-clients chrony pps-tools

find device

ls /dev/ttyA* /dev/ttyU*
ls: Zugriff auf '/dev/ttyU*' nicht möglich: Datei oder Verzeichnis nicht gefunden
 /dev/ttyACM0

config server

sudo nano /etc/default/gpsd

START_DAEMON="true"
USBAUTO="false"
DEVICES="/dev/ttyACM0"
GPSD_OPTIONS="-n"

if it failes - maybe you have deactivated ipv6 due to your isp provider

sudo systemctl edit gpsd.socket

[Socket]
ListenStream=
ListenStream=/run/gpsd.sock
ListenStream=127.0.0.1:2947
BindIPv6Only=no

sudo systemctl daemon-reload
sudo systemctl restart gpsd.socket
``

## check gps status
```bash
gpsmon

#or

gpspipe -w | grep TPV

#"mode":3 is important
#0	Unknown -> no data
#1	No Fix -> time, satellite data, but no gps data
#2	2D Fix -> At least 3 satellites found
#3  3D Fix -> At least 4 satellites found

configure chrony to use gps to get the time

sudo nano /etc/chrony/chrony.conf

# GPS timeing source with Shared Memory (SHM)
refclock SHM 0 refid GPS precision 1e-3 offset 0.0668 delay 0.1 poll 3 filter 16 trust prefer
# Local network access
allow 192.168.178.0/24

sudo systemctl restart chrony

parameters explained

Parameter Value Description
refclock SHM 0 Interface to the Shared Memory segment (NTP0) provided by gpsd.
refid GPS A unique 4-character ID displayed in chronyc sources.
precision 1e-3 Declared precision ($10^{-3}$s).
offset 0.0668 Calibration: Static compensation (66.8ms) for USB bus latency and processing time.
delay 0.1 Estimated fixed delay. Low values prioritize local hardware over network peers.
poll 3 Polling interval ($2^3 = 8$ seconds) for synchronization of a large value sample.
filter 16 Median Filter: Uses the last 16 samples to eliminate USB-scheduling jitter.
trust N/A Trusted local timing source
prefer N/A Ensures the GPS remains the primary time source over network-based servers.

check chrony sources

chronyc sources -v


  .-- Source mode  '^' = server, '=' = peer, '#' = local clock.
 / .- Source state '*' = current best, '+' = combined, '-' = not combined,
| /             'x' = may be in error, '~' = too variable, '?' = unusable.
||                                                 .- xxxx [ yyyy ] +/- zzzz
||      Reachability register (octal) -.           |  xxxx = adjusted offset,
||      Log2(Polling interval) --.      |          |  yyyy = measured offset,
||                                \     |          |  zzzz = estimated error.
||                                 |    |           \
MS Name/IP address         Stratum Poll Reach LastRx Last sample               
===============================================================================
#? GPS                           0   4     0     -     +0ns[   +0ns] +/-    0ns

-> Chrony knows about the source but does not get any information

set groups right

sudo usermod -aG dialout gpsd
sudo systemctl restart gpsd
sudo systemctl restart chrony

check if data is coming through shared memory

sudo ntpshmmon

check chrony sources

chronyc sources -v

  .-- Source mode  '^' = server, '=' = peer, '#' = local clock.
 / .- Source state '*' = current best, '+' = combined, '-' = not combined,
| /             'x' = may be in error, '~' = too variable, '?' = unusable.
||                                                 .- xxxx [ yyyy ] +/- zzzz
||      Reachability register (octal) -.           |  xxxx = adjusted offset,
||      Log2(Polling interval) --.      |          |  yyyy = measured offset,
||                                \     |          |  zzzz = estimated error.
||                                 |    |           \
MS Name/IP address         Stratum Poll Reach LastRx Last sample               
===============================================================================
#* GPS                           0   3   377     5   +549us[+1029us] +/-   51ms
^- xxxxxxxxxxxxxxxxxxxxxxxx>     2   6   377    37  +1455us[ +257us] +/- 9430us
^- xxxxxxxxxxxxxxxxxxxxxxxx>     2   6   377    36  +2361us[+1129us] +/- 9321us
^- xxxxxxxxxxxxxxxxxxxxxxxx>     3   6   377    38  +1414us[ +237us] +/-   19ms

check statistics for your own to optimize config values

cat /var/log/chrony/statistics.log | grep GPS

check statistics with simple script

./chrony/stats.sh