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Saturday, April 27, 2019
A Raspberry Pi Stratum 1 NTP Server
Overview
The following instructions are how to make a cheap Pulse Per Second (PPS) disciplined Stratum 1 NTP Time server using one of the Raspberry Pi U-blox M8Q based GPS boards sold by Uputronics.
Our basic requirement is for an NTP server which will work standalone without connectivity to other NTP servers, so if a datacentre loses connectivity its servers will still have a stable time source.
As we’re only interested in time and not height, motion, or location, all we’re interested in is the NMEA xxZDA and xxRMC sentences and the highly accurate PPS signal from the GPS module, both of which we feed straight into the ntp daemon. The xxZDA sentences (which give us full 4-digit years) are not output by default by the M8Q so we have to enable them at boot time.
Many guides on the net use gpsd to feed ntp via shared memory. That’s an additional overhead and complexity, especially if we want to enable Galileo reception (gpsd initialises the U-blox the way it sees fit). I prefer to let the NTP daemon read the NMEA stream from the GPS receiver itself, avoiding the middle man.
In this guide we’ve chosen to use NMEA output from the GPS module via the ntp Generic GPS Receiver driver 20 ( https://www.eecis.udel.edu/~mills/ntp/html/drivers/driver20.html ) and the PPS signal via driver 22 ( https://www.eecis.udel.edu/~mills/ntp/html/drivers/driver22.html ).
You will need a Raspberry Pi 4B, the Uputronics Raspberry Pi+ GPS Expansion Board and a suitable GPS antenna.
This guide assumes that you’re using Raspbian Buster Lite or later. It should work on Bullseye as there's now no dependency on wiringPI. Download and write this to an SD card.
See https://www.raspberrypi.org/documentation/installation/installing-images/windows.md
Create a file named ssh in the boot folder after burning the MicroSD card.
Attach the Uputronics Raspberry Pi+ GPS Expansion Board to the Pi, insert the SD card, connect the antenna and network cable and boot the Pi up. Either connect locally or via SSH to the Pi. If you can’t SSH in and don’t have a monitor see this
https://www.raspberrypi.org/documentation/configuration/wireless/headless.md
Follow the instructions carefully if you miss steps things won’t work.
The Uputronics board has u-blox firmware 3.01 on it, dated 2016.
The week number rollover is set to 1867 (October 2015). All transmitted week numbers are mapped to the ~19.5 year period between week 1867 and week 2990 (April 2035).
A Note About Accuracy
In theory, GPS-based time receivers can give a very high accuracy, with the PPS (Pulse per Second) signal being accurate to within 10ns.
However, fix data from gpsctl shows:
pi@ntp2:~ $ /usr/local/bin/gpsctl -Q fix
Time (UTC): 2019-04-18 15:31:27 (yyyy-mm-dd hh:mm:ss)
Latitude: 52.05994980 N
Longitude: 2.72698960 W
Altitude: 198.789 feet
Motion: 0.338 mph at 53.114 degrees heading
Satellites: 5 used for computing this fix
Accuracy: time (39 ns), height (+/-18.199 feet), position (+/-103.911 feet), heading(+/-8.230 degrees), speed(+/-0.132 mph)
Note the 39ns time accuracy from that fix. The more satellites, the better.
Errors are also caused by signal delay (4ns per foot) in the cable from the aerial to the GPS receiver, etc.
So we’d be lucky to get 100ns accuracy from the PPS pulse.
Add to that the processing overheads of the PPS interrupt, and processor clock jitter in the Raspberry Pi, and overheads transferring and decoding the NMEA sentences. The jitter on the PPS signal is less than 5 microseconds.
The output from the ntp NMEA driver without PPS correction can have a jitter of a few milliseconds, and an offset from real time which has to be tweaked manually to get within 5ms of the correct time.
NTP sources on the internet can show offsets / jitter of over 5ms, local LAN 50us or more.
On the Pi 4B we can expect an average jitter of less than 1 microsecond.
Required Components
I sourced my components from the Pi Hut unless noted elsewhere:
1 Raspberry Pi Model 4B 1GB RAM
1 8GB or larger micro SD card
(a Transcend High Endurance 32GB micro SD card would be better choice than a generic one for longevity)
1 Uputronics GPS Hat
1 Pi Hut Pi 4 GPS case
1 Raspberry Pi 4 power supply
1 GPS SMA Antenna
(optional) 1 SMA-male to TNC-female (or BNC, as needed) adaptor to connect to existing GPS aerial (from Amazon or eBay)
Prerequisite Settings
Login as pi / raspberry, and immediately change the password from the default
passwd
sudo raspi-config
Advanced Options
Expand filesystem
Interfacing Options
SSH -> Would you like the SSH server to be enabled – Yes (Recommended)
I2C -> Would you like the ARM I2C interface to be enabled? - Yes
Serial -> Login Shell (no) Hardware (yes) (Optional)
Quit but no need to reboot at this point.
for i in systemd-timesyncd avahi-daemon alsa-state bluetooth triggerhappy hciuart rng-tools autologin getty;
do
sudo systemctl disable $i.service; sudo systemctl stop $i.service;
done
sudo systemctl disable serial-getty@ttyAMA0.service
sudo systemctl mask serial-getty@ttyAMA0.service
sudo apt install pps-tools ntp cpufrequtils i2c-tools
sudo apt purge bluez bluez-firmware wpasupplicant (assuming no wifi required)
sudo apt update
sudo apt full-upgrade
sudo nano /boot/config.txt and add at the bottom :
dtoverlay=disable-bt
dtoverlay=disable-wifi
dtoverlay=pps-gpio,gpiopin=18
Make sure that enable_uart=1 is set in /boot/config.txt
sudo nano /boot/cmdline.txt and add at the end
nohz=off
sudo nano /etc/modules and add at the bottom :
pps-gpio
Save and Quit Nano.
sudo nano /etc/udev/rules.d/80-gps-to-ntp.rules (needed for refclock 20 in ntp)
# Change MODE of ttyAMA0 so it is readable by NTP and provide
# a symlink to /dev/gps0
KERNEL=="ttyAMA0",
SYMLINK+="gps0", MODE="0666"
#
Symlink /dev/pps0 to /dev/gpspps0
KERNEL=="pps0",
SYMLINK+="gpspps0", MODE="0666"
and then sudo reboot
Enabling Galileo Satellites and Setting Stationary Mode
Stationary mode gives a faster GPS fix.
See Tom Dilatush’s gpsctl:
http://www.jamulblog.com/2017/11/paradise-ponders-gpsctl-functionally.html
I’ve customised it further to enable the use of a config file and put it in my GitHub repo:
https://github.com/philrandal/gpsctl
Download and build gpsctl.
cd ~
wget https://github.com/philrandal/gpsctl/archive/master.zip
unzip master.zip
mv gpsctl-master gpsctl
cd gpsctl
./build.sh
sudo cp /home/pi/gpsctl/gpsctl /usr/local/bin
sudo cp /home/pi/gpsctl/etc/gpsctl.conf /etc/gpsctl.conf
Individual settings can be tweaked in /etc/gpsctl.conf.
To set port speed to 115200 baud, enable Galileo satellites, set stationary mode, tweak antenna delays, PPS timing, etc:
/usr/local/bin/gpsctl -a -B 115200 --configure_for_timing -vv
To reset the device to its defaults
/usr/local/bin/gpsctl -a --reset -vv
To view info:
/usr/local/bin/gpsctl -a -Q satellites
/usr/local/bin/gpsctl -a -Q config
/usr/local/bin/gpsctl -a -Q fix
Note that these commands can only be run when gpsd / ntp are not using /dev/ttyAMA0
Example /etc/gpsctl.conf which configures the U-blox M8Q for this environment:
# # example gpsctl.conf which enables Galileo as in --galileo parameter
#
[gpsctl]
port = /dev/serial0
# sync method: ASCII = 1, NMEA = 2, UBX = 3
sync method = 3
verbosity = 0
[NMEA]
enabled = true
version = 41
GGA = off
GLL = off
GSA = off
GSV = off
RMC = on
VTG = off
GRS = off
GST = off
ZDA = on
[GPS]
enabled = yes
minimum channels=8
maximum channels=16
[SBAS]
enabled = no
minimum channels=1
maximum channels=3
[Galileo]
enabled = yes
minimum channels=4
maximum channels=8
[Beidou]enabled = no
minimum channels=8
maximum channels=16
[IMES]
enabled = no
minimum channels=0
maximum channels=8
[QZSS]
enabled = no
minimum channels=0
maximum channels=3
[GLONASS]
enabled = yes
minimum channels=8
maximum channels=14
[Navigation Engine]
# Dynamic model: Portable = 0, Stationary = 2, Pedestrian = 3, Automotive = 4,
# Sea = 5, Air1G = 6, Air2G = 7, Air4G = 8, Watch = 9
Dynamic model = 2
# Fix mode: 2D only = 1, 3D only = 2, auto 2D/3D = 3
Fix mode = 3
Fixed altitude (2D) = 0.00 meters
Fixed altitude variance (2D) = 1.0000 meters^2
Minimum elevation = 5 degrees
Position DoP mask = 10.0
Time DoP mask = 10.0
Position accuracy mask = 100 meters
Time accuracy mask = 300 meters
Static hold threshold = 0 cm/s
Dynamic GNSS timeout = 60 seconds
Threshold above C/No = 0 satellites
C/No threshold = 0 dBHz
Static hold max distance = 0 meters
# UTC Standard: AutoUTC = 0, USNO_UTC = 3, GLONASS_UTC = 6, BeiDou_UTC = 7
UTC standard = 3
[Time Pulse]
# the nanoseconds / microseconds after the numbers are just reminders,
# they don't mean anything to the config parserAntenna cable delay = 56 nanoseconds
RF group delay = 20 ns
Unlocked pulse period = 1000000 microseconds
Unlocked pulse length = 0
Locked pulse period = 1000000 microseconds
Locked pulse length = 500000 microseconds
User configurable delay = 0
To run gpsctl at system startup,
sudo cp /home/pi/gpsctl/etc/systemd/system/ublox-init.service /etc/systemd/system/ublox-init.service
or
nano /etc/systemd/system/ublox-init.service
and add the following contents
[Unit] Description=u-blox initialisation
Before=gpsd.service
Before=ntp.service
[Service]
Type=oneshot
Environment="PARAMS=-q -a -B 115200 --configure_for_timing"
EnvironmentFile=-/etc/default/gpsctl
ExecStart=/bin/bash '/usr/local/bin/gpsctl ${PARAMS}'
[Install]
WantedBy=multi-user.target
This will enable Galileo satellites, set stationary mode, configure comms at 115200 baud, and restrict NMEA output to RMC and ZDA records.
To use the default baud rate of 9600, create /etc/default/gpsctl with the contents
PARAMS=-q --configure-for-timing
Then
sudo systemctl enable ublox-init.service
sudo systemctl enable ntp.service
sudo systemctl daemon-reload
Verifying that PPS Is Working
Ensure the GPS has a lock and the Green PPS LED on the Uputronics Pi+ GPS Expansion Board is blinking once a second.
lsmod | grep pps
Output should be similar to :
pps_gpio 3089 1
pps_core 8606 4 pps_gpio
dmesg | grep pps
Output should be similar to :
[ 2.735586] pps_core: LinuxPPS API ver. 1 registered [ 2.738121] pps_core: Software ver. 5.3.6 - Copyright 2005-2007 Rodolfo Giometti <giometti@linux.it>
[ 2.763842] pps pps0: new PPS source pps@12.-1
[ 2.766361] pps pps0: Registered IRQ 169 as PPS source
This indicates that the PPS Module is loaded.
sudo ppstest /dev/pps0
Output should be similar to:
trying PPS source "/dev/pps0" found PPS source "/dev/pps0" ok, found 1 source(s), now start fetching data...
source 0 - assert 1418933982.998042450, sequence: 970 - clear 0.000000000, sequence: 0
source 0 - assert 1418933983.998045441, sequence: 971 - clear 0.000000000, sequence: 0
(Press CTRL+C to quit). This indicates that the PPS Module is working.
Enabling PPS/ATOM Support in NTPD
The supplied version of NTPD on the Raspberry Pi in Raspbian Stretch 2018-11-13 and later supports PPS so there is no need to roll your own NTP.
You need to pick a few local NTP servers to use. The easiest way to do this is pick your region:
https://support.ntp.org/bin/view/Servers/NTPPoolServers
Select your region then you get a list of the country servers. E.g for the UK its uk.pool.ntp.org.
Type:
dig uk.pool.ntp.org
You will get four IP’s back:
;; <<>> DiG 9.10.3-P4-Raspbian <<>> +answer uk.pool.ntp.org ;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 51647
;; flags: qr rd ra; QUERY: 1, ANSWER: 4, AUTHORITY: 0, ADDITIONAL: 1
;; OPT PSEUDOSECTION:
;; EDNS: version: 0, flags:; udp: 4000
;; QUESTION SECTION:
;uk.pool.ntp.org. IN A
;; ANSWER SECTION:
uk.pool.ntp.org. 24 IN A 193.150.34.2
uk.pool.ntp.org. 24 IN A 176.58.109.199
uk.pool.ntp.org. 24 IN A 195.195.221.100
uk.pool.ntp.org. 24 IN A 185.53.93.157
sudo nano /etc/ntp.conf
Add
# PPS Driver server 127.127.22.0 minpoll 4 maxpoll 4
fudge 127.127.22.0 time1 +0.000000 flag3 0 refid PPS
#flag3 Controls the kernel PPS discipline: 0 for disable (default), 1 for enable.
#time1 PPS time offset
# use local clock at stratum 10 if no others available
tos orphan 10
tos mindist 0.002
# NMEA driver (/dev/gps0 and /dev/gpspps0)
# 115200 baud, RMC and ZDA messages
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 0.050 refid GPS stratum 2
#flag1 Disable PPS signal processing if 0 (default); enable PPS signal processing if 1.
#flag2 If PPS signal processing is enabled, capture the pulse on the rising edge if 0 (default); capture on the falling edge if 1.
#flag3 If PPS signal processing is enabled, use the ntpd clock discipline if 0 (default); use the kernel discipline if 1.
#time1 PPS time offset
#time2 NMEA time offset
#mode
# bit 0 - process $GPRMC (value = 1)
# bit 1 - process $GPGGA (value = 2)
# bit 2 - process $GPGLL (value = 4)
# bit 3 - process $GPZDA or $GPZDG (value = 8)
# bits 4/5/6 - select serial bitrate (0 for 4800 - the default, 16 for 9600, 32 for 19200, 48 for 38400, 64 for 57600, 80 for 115200)
# mode 25 = process only xxRMC and xxZDA NMEA records at 9600 baud
(Note that I used “fudge 127.127.20.0 time2 0.050 …” to adjust the GPS time according to the other NTP servers. You can try some higher/lower values to have the offset of your NMEA driver compared to the offsets of those other NTP servers very small. We’ll cover that later.)
We want to make sure that the second reported by the NMEA driver is the second that the last PPS pulse referred to, so it needs to be within a few hundred milliseconds of the correct time so that NTP does the right thing. This is why it’s desirable to mark another NTP server as “prefer” to help the NTP daemon get it right.
Note that we use NTP PPS discipline, not kernel PPS (which in my testing results in warnings from ntp that kernel PPS discipline isn’t supported).
Comment out all the pool lines.
Add the servers from the dig command, or use servers of your choice) with the top one saying prefer on it (example only, don't all use these IP addresses):
server 176.58.109.199 iburst prefer server 195.195.221.100 iburst
server 185.53.193.157 iburst
# By default, exchange time with everybody, but don't allow configuration.
restrict default kod nomodify notrap nopeer noquery
restrict -6 default kod nomodify notrap nopeer noquery
# Local users may interrogate the ntp server more closely.
restrict 127.0.0.1
restrict -6 ::1
restrict 10.0.0.0 mask 255.0.0.0
restrict 172.16.0.0 mask 255.240.0.0
restrict 192.168.0.0 mask 255.255.0.0
# Drift file etc.
driftfile /var/lib/ntp/ntp.drift
Note You MUST add a preferred server or PPS doesn’t work.
Save and close nano.
sudo systemctl restart ntp.service
After a few minutes run
ntpq –p
If you get oPPS(0) this indicates source selected, Pulse Per Second (PPS) used and everything is working.
pi@ntp2:~ $ ntpq -pn remote refid st t when poll reach delay offset jitter
==============================================================================
o127.127.22.0 .PPS. 0 l 6 16 377 0.000 -0.002 0.001
*127.127.20.0 .GPS. 1 l 5 16 377 0.000 -0.323 1.052
+195.195.221.100 .GPS. 1 u 49 64 377 18.307 0.190 0.127
+193.150.34.2 85.199.214.101 2 u 56 64 377 7.953 0.067 0.114
And then
pi@ntp2:~ $ ntpq -csysinfo associd=0 status=0115 leap_none, sync_pps, 1 event, clock_sync,
system peer: PPS(0)
system peer mode: client
leap indicator: 00
stratum: 1
log2 precision: -21
root delay: 0.000
root dispersion: 2.015
reference ID: PPS
reference time: e0601d81.248b4b37 Tue, Apr 16 2019 10:23:13.142
system jitter: 0.000477
clock jitter: 0.003
clock wander: 0.001
broadcast delay: -50.000
symm. auth. delay: 0.000
If you aren’t seeing the settings its possible the NTP server is picking up the NTP information via DHCP which is overriding your settings above. Do this :
rm /etc/dhcp/dhclient-exit-hooks.d/ntp
rm /var/lib/ntp/ntp.conf.dhcp
At this point you have a NTP server which will use an external time source and use your local PPS to discipline it.
Note: Once you've configured ntp to use your GPS source, you'll need to stop the ntp service before running gpsctl, otherwise they'll both be trying to access ttyAMA0 at the same time.
GPS Offset Tuning
Your PPS time is going to be more accurate than NTP pool servers. Unless you have specialised equipment and local LAN GPS / DCF77 / MSF PPS sources to calibrate against, it’s not really possible to determine the appropriate PPS offset. It’s likely to be in the order of a few microseconds at most ( htt://lists.ntp.org/pipermail/questions/2011-September/030338.html ).
The PPS is a precise signal with around 10 ns of jitter. On the other hand, the offset of the GPS serial data output (GPSD) generally will have much more variation than the PPS because of the variables involved in sending data over an asynchronous serial port. However, the GPSD offset can be reduced somewhat by adjusting the GPSD reference clock fudge parameter time2 in the ntp.conf file.
The initial value used in our ntp.conf file is 43ms (0.043s). This means that the GPS ZDA packet arrives in the NTP daemon approximately 43ms after the PPS pulse corresponding to it. On our test device, we see a jitter of under 3ms on the GPS_NMEA clock.
We might need to adjust the GPS ntp.conf time2 (offset) value to get the NMEA time offset low enough that the offset isn’t great enough to confuse NTP as to which second the PPS pulse referred to.
This option can be used to compensate for a constant error. The specified offset (in seconds) is applied to all samples produced by the reference clock. The default is 0.000s.
Start with these ntp.conf settings, they should get you close enough to get everything working properly:
sudo nano /etc/ntp.conf
#kernel-mode PPS server 127.127.22.0 minpoll 4 maxpoll 4
fudge 127.127.22.0 time1 +0.000 flag3 0 refid PPS
tos mindist 0.002
#GPS (NMEA)
server 127.127.20.0 mode 89 minpoll 4 maxpoll 4 iburst prefer
fudge 127.127.20.0 flag1 0 flag3 0 time2 0.043 refid GPS stratum 2
Add these lines at the top of ntp.conf:
statsdir /var/log/ntpstats/ statistics peerstats
filegen peerstats file peerstats type day enable
This enables logging of the peer server statistics.
To calculate the GPS offset we must disable GPS by placing a noselect in the ntp.conf GPS line. We'll run the time server for a few hours and then compare the ntpq -p GPS offset to the average public time server offset. For accurate tuning use a bunch of known-good Stratum 1 servers in ntp.conf.
sudo nano /etc/ntp.conf
#PPS server 127.127.22.0 minpoll 4 maxpoll 4
fudge 127.127.22.0 time1 +0.000 flag3 0 refid PPS
tos mindist 0.002
#GPS (NMEA)
server 127.127.20.0 mode 89 minpoll 4 maxpoll 4 iburst prefer noselect
fudge 127.127.20.0 flag1 0 flag3 0 time2 0.043 refid GPS stratum 2
sudo systemctl stop ntp.service
sudo rm –f /var/log/ntpstats/*
sudo systemctl start ntp.service
Start ntpd and let it run for at least four hours. Periodically check progress with "ntpq -p" and wait until change has settled out.
Look for the row with GPS_NMEA(0) and refid GPSD. The offset values probably will be different for each
query. Note that the ntpq offsets are in milliseconds, but the peerstats file offsets and NTP’s time2 parameter are in seconds.
Calculate the average GPS offset (in seconds) using this script:
sudo nano ~/nmeaoffset
#!/bin/sh #
# Generate an estimate of your GPS's offset from a peerstats file
#
awk '
/127\.127\.20\.0/ { sum -= $5 ; cnt++; }
END { printf("%.6f\n", sum / cnt); }
' </var/log/ntpstats/peerstats
Then sudo chmod +x ./nmeaoffset
Run by typing
pi@ntp2:~ $ ./nmeaoffset
-0.001212
That’s within a few milliseconds, close enough for anyone as PPS is going to do its magic to give us accuracy within a few microseconds.
Adjust the "time2" value for the GPS source of your ntp.conf by adding the average offset from above.
sudo systemctl stop ntp.service
sudo rm –f /var/log/ntpstats/*
sudo systemctl start ntp.service
Repeat the procedure above until -5ms < offset < +5ms (or under 10ms, if that’s OK with you). When you’re done, remove the noselect from the server 127.127.20.0 line in ntp.conf.
If you decide to recalculate the average offset using the above procedures, wait at least another day or two.
Avoid unnecessarily changing the time2 value. A typical value for the Adafruit GPSD driver is +0.534 s when using its default 4800 baud interface and in the HAB Supplies/Uputronics GPSD it’s +0.043 s when using 115200 baud and the gpsctl --config_for_timing tweaks.
To save wear and tear on the SD card, comment out the statistics line in /etc/ntp.conf when done.
Automatically updating GPS leap seconds
These semi-annual changes will be made no later than 1 June and 1 December of each year to indicate what action (if any) is to be taken on 30th June and 31st December, respectively.
In Buster, the default ntp.conf points LeapFile at the tzdata-provided leap-seconds.list. That's fine, as long as you auto apply system updates via cron or manually on a regular basis.
Automatically updating Raspbian
To automatically update Raspbian on a schedule, follow these steps:
sudo apt install unattended-upgrades mailutils
sudo nano /etc/apt/apt.conf.d/50unattended-upgrades
Edit to include these sources (remove all others)
"origin=Raspbian,codename=${distro_codename},label=Raspbian";
"origin=Raspberry Pi Foundation,codename=${distro_codename},label=Raspberry Pi Foundation";
Unattended-Upgrade::Automatic-Reboot "true";
APT::Periodic::Update-Package-Lists "1"; APT::Periodic::Download-Upgradeable-Packages "1"; APT::Periodic::Unattended-Upgrade "1"; APT::Periodic::Verbose "1"; APT::Periodic::AutocleanInterval "7";
sudo dpkg-reconfigure --priority=low unattended-upgrades
Set the systemctl apt timer to fire at 2AM daily
[Timer] OnCalendar= # OnCalendar=DayOfWeek Year-Month-Day Hour:Minute:Second # see https://wiki.archlinux.org/index.php/Systemd/Timers OnCalendar=02:00 RandomizedDelaySec=0
Static IP and Hostname
If you want to fix your LAN IP you do it by amending /etc/dhcpcd.conf adding the following lines (adjust to suit your environment):
# It is possible to fall back to a static IP if DHCP fails: # define static profile
profile static eth0
static ip_address=192.168.1.7/24
static routers=192.168.1.254
static domain_name_servers=8.8.8.8 8.8.4.4
# fallback to static profile on eth0
interface eth0
fallback static_eth0
This way DHCP will work if plugged into a client switch port, but static IP will default when no DHCP is available.
Great for testing the box on one’s desktop before going live.
Amend your hostname by editing /etc/hostname and then adding the below to /etc/hosts.
i.e if you call your machine ‘ntp’ fix the /etc/hosts 127.0.1.1 line:
127.0.1.1 ntp
Further Reading
The original text this document was based on can be found at https://ava.upuaut.net/?p=951
Updated to include information from comments at https://blog.webernetz.net/ntp-server-via-gps-on-a-raspberry-pi/
David Taylor’s website https://satsignal.eu/ntp/Raspberry-Pi-NTP.html goes into much further detail about the process above and covers graphing, remote access monitoring.
Rob Robinette has a good write up on Pi-based NTP servers
https://robrobinette.com/pi_GPS_PPS_Time_Server.htm
Whitham D. Reeve’s GpsNtp-Pi Installation and Operation Guide
http://www.reeve.com/RadioScience/Raspberry%20Pi/GpsNtp-Pi.htm
Rich Laager’s Raspberry Pi 3 Stratum 1 NTP Server has the best stuff on offsets
https://coderich.net/2016/11/21/raspberry-pi-3-stratum-1-ntp-server/
Tech Solvency’s similar setup
https://www.techsolvency.com/ntp/systems/tackleberry-uputronics/
Gary Miller’s GPSD Time Service HOWTO
http://www.catb.org/gpsd/gpsd-time-service-howto.html
Jack Zimmerman’s Raspberry PI NTP Server LCD Display
https://github.com/jacken/Raspberry-Pi-ntp-server-LCD-display
Jack Zimmerman’s Pi U-Blox Stationary Mode
https://github.com/jacken/Raspberry-Pi-U-Blox-Stationary-Mode
Tom Dilatush’s gpsctl
http://www.jamulblog.com/2017/11/paradise-ponders-gpsctl-functionally.html
My improved gpsctl with added .conf file support GitHub repo
https://github.com/philrandal/gpsctl
u-blox M8 Receiver Description (the bible)
GNSS Firmware 3.01 for u-blox M8
https://www.u-blox.com/sites/default/files/GNSS-FW3.01_ReleaseNotes_%28UBX-16000319%29_Public.pdf
Postscript, July 15th, 2019
You can enable either BeiDou satellites, or Glonass, but not both with gpsctl. You'll get an error if you try. Section 4.2 of the u-blox m8 receiver description manual has a hint as to why.
From version 1.5 of gpsctl, enabling either BeiDou or Galileo automatically enables NMEA 4.1.
Postscript, January 30th, 2020
Updated for the Raspberry Pi 4, used cpufrequtils to set processor speed, added nohz=off, fixed numerous typos, added automatic updating howto, and added a note about making sure that ntpd is not running when you run gpsctl.
Postscript, June 21st, 2020
Released gpsctl v1.9. Tried to fix an issue with gpsctl not initialising the u-blox module via systemd at startup. Still not convinced that it is right.
Added configure-rv3028.sh script to properly configure the rv3028 RTC chip on the new (June 2020) Uputronics boards.
Without this step I got lots of
kernel: [ 156.227408] rtc-rv3028 1-0052: Voltage low, data is invalid.
messages in /var/log/messages on boot.
To configure Raspberry Pi OS to use the rv3028,
sudo nano /boot/config.txt
Add the line
dtoverlay=i2c-rtc,rv3028
at the end.
Remove the fake hardware clock:
sudo apt-get -y remove fake-hwclock
sudo update-rc.d -f fake-hwclock remove
sudo systemctl disable fake-hwclock
Run sudo nano /lib/udev/hwclock-set and comment out these three lines:
#if [ -e /run/systemd/system ] ; then # exit 0 #fi
Also comment out the two lines
/sbin/hwclock --rtc=$dev --systz --badyear
and
/sbin/hwclock --rtc=$dev --systz
Configure the rv3028 by running
sudo configure-rv3028.sh
from the gpsctl archive.
Then,
sudo hwclock -w -v
to set the clock for the first time.
To debug, sudo systemctl disable ntp, then sudo reboot
After the reboot, sudo hwclock -r -v should show the retained time.
Then sudo systemctl enable ntp --now to enable and start ntp.
Postscript, October 17th, 2020
Due to the repeated unreliabilty of gpsctl's automatic baud rate synchronisation, I've reverted to using 9600 baud and not messing with the baud rate at all. The latest version in the github master branch allows you to specify the gpsctl parameters for ublox-init.service in /etc/default/gpsctl. e.g.
PARAMS=-q --configure-for-timing
If /etc/default/gpsctl doesn't exist, the behaviour will be as before.
Any edits of ublox-init.service or /etc/default/gpsctl require you to run sudo systemctl daemon-reload