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Saturday, April 27, 2019

A Raspberry Pi Stratum 1 NTP Server


Updated for the Pi 5 and Raspberry Pi OS Bookworm

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 ntpsec Generic GPS Receiver driver 20 ( https://docs.ntpsec.org/latest/driver_nmea.html ) and the PPS driver ( https://docs.ntpsec.org/latest/driver_pps.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 Raspberry Pi OS Bookworm, 64-bit Lite edition.  Download and write this to an SD card.

See https://www.raspberrypi.org/documentation/installation/installing-images/windows.md

You'll need a keyboard and display attached for first boot configuration.

Create a user (mine was ntpadmin, use whatever you like) and set its password.

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


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


sudo raspi-config

Advanced Options

Expand filesystem (this is automatically done on first boot)

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;


sudo systemctl disable $i.service; sudo systemctl stop $i.service;


sudo systemctl disable serial-getty@ttyAMA0.service

sudo systemctl mask serial-getty@ttyAMA0.service

sudo apt install gpsd-clients pps-tools ntpsec ntpsec-ntpviz i2c-tools linux-cpupower mlocate

sudo systemctl enable ntpsec

sudo apt purge bluez bluez-firmware modemmanager wpasupplicant (assuming no wifi required)

sudo apt update

sudo apt full-upgrade

If you want your Pi to run at full clock speed (active cooling is recommended),

sudo nano /etc/systemd/system/cpu-governor.service

Description=CPU Governor Service
ExecStart=/usr/bin/cpupower frequency-set -g performance


sudo systemctl enable cpu-governor.service --now

sudo nano /boot/firmware/config.txt and add at the bottom :


On a Pi 4 or below, make sure that enable_uart=1 is set in /boot/config.txt

On a Pi 5, make sure that dtparam=uart0=on is set in boot/config.txt

sudo nano /boot/firmware/cmdline.txt and add at the end


sudo nano /etc/modules and add at the bottom :


Save and quit nano, then sudo reboot

Enabling Galileo Satellites and Setting Stationary Mode

Stationary mode gives a faster GPS fix.

See Tom Dilatush’s gpsctl:


I’ve customised it further to enable the use of a config file and put it in my GitHub repo:


Download and build gpsctl.

cd ~
wget https://github.com/philrandal/gpsctl/archive/master.zip
unzip master.zip
mv gpsctl-master gpsctl
cd gpsctl
sudo cp ~/gpsctl/gpsctl /usr/local/bin
sudo cp ~/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
port = /dev/ttyAMA0
# sync method: ASCII = 1, NMEA = 2, UBX = 3
sync method = 3
verbosity = 0
enabled = true
version = 41
GGA = off
GLL = off
GSA = off
GSV = off
RMC = on
VTG = off
GRS = off
GST = off
ZDA = on
enabled = yes
minimum channels=8
maximum channels=16
enabled = no
minimum channels=1
maximum channels=3
enabled = yes
minimum channels=4
maximum channels=8
[Beidou]enabled = no
minimum channels=8
maximum channels=16
enabled = no
minimum channels=0
maximum channels=8
enabled = no
minimum channels=0
maximum channels=3
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 ~/gpsctl/etc/systemd/system/ublox-init.service /etc/systemd/system/ublox-init.service


nano /etc/systemd/system/ublox-init.service

and add the following contents

Description=u-blox initialisation
Environment="PARAMS=-q -a -B 115200 --configure_for_timing"
ExecStart=/bin/bash '/usr/local/bin/gpsctl ${PARAMS}'


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


sudo systemctl enable ublox-init.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.

You need to pick a few local NTP servers to use. The easiest way to do this is pick your region:


Select your region then you get a list of the country servers. E.g for the UK its uk.pool.ntp.org.


dig uk.pool.ntp.org

You will get four IP’s back:

;; &lt;&lt;&gt;&gt; DiG 9.10.3-P4-Raspbian &lt;&lt;&gt;&gt; +answer uk.pool.ntp.org
;; global options: +cmd
;; Got answer:
;; -&gt;&gt;HEADER&lt;&lt;- opcode: QUERY, status: NOERROR, id: 51647
;; flags: qr rd ra; QUERY: 1, ANSWER: 4, AUTHORITY: 0, ADDITIONAL: 1
;; EDNS: version: 0, flags:; udp: 4000
;uk.pool.ntp.org. IN A
uk.pool.ntp.org. 24 IN A
uk.pool.ntp.org. 24 IN A
uk.pool.ntp.org. 24 IN A
uk.pool.ntp.org. 24 IN A

sudo nano /etc/ntpsec/ntp.conf

Amend the start of ntp.conf to look like this

## nts enable

# You must create /var/log/ntpsec (owned by ntpsec:ntpsec) to enable logging.
#statsdir /var/log/ntpsec/
#statistics loopstats peerstats clockstats
#filegen loopstats file loopstats type day enable
#filegen peerstats file peerstats type day enable
#filegen clockstats file clockstats type day enable

# This should be maxclock 7, but the pool entries count towards maxclock.
#tos maxclock 11

# Comment this out if you have a refclock and want it to be able to discipline
# the clock by itself (e.g. if the system is not connected to the network).
#tos minclock 4 minsane 3

# Specify one or more NTP servers.

# Public NTP servers supporting Network Time Security:
# server time.cloudflare.com nts

# PPS Driver
refclock pps unit 0 minpoll 4 maxpoll 4 ppspath /dev/pps0
#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)
# process $GPZDA only (mode 8)
refclock nmea prefer minpoll 4 maxpoll 4 mode 8 stratum 2 baud 115200 time2 0.060 path /dev/ttyAMA0 #time2 NMEA time offset #mode bit 0 - process $GPMRC (value = 1) # bit 1 - process $GPGGA (value = 2) # bit 2 - process $GPGLL (value = 4) # bit 4 - process $GPZDA or $GPZDG (value = 8) # mode 9 = process only xxRMC and xxZDA NMEA records #--------------------------- # pool.ntp.org maps to about 1000 low-stratum NTP servers. Your server will # pick a different set every time it starts up. Please consider joining the # pool: <https://www.pool.ntp.org/join.html> server 0.uk.pool.ntp.org iburst server 1.uk.pool.ntp.org iburst server 2.uk.pool.ntp.org iburst #pool 0.debian.pool.ntp.org iburst #pool 1.debian.pool.ntp.org iburst #pool 2.debian.pool.ntp.org iburst #pool 3.debian.pool.ntp.org iburst

(Note that I used “time2 0.060 …” 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 iburst prefer
server iburst
server 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 -6 ::1
restrict mask
restrict mask
restrict mask
# Drift file etc.
driftfile /var/lib/ntpsec/ntp.drift


Note You MUST add a preferred server or PPS doesn’t work.

Save and close nano.

sudo systemctl restart ntpsec.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
* .GPS. 1 l 5 16 377 0.000 -0.323 1.052
+ .GPS. 1 u 49 64 377 18.307 0.190 0.127
+ 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 60ms (0.060s). This means that the GPS ZDA packet arrives in the NTP daemon approximately 60ms 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/ntpsec/ntp.conf

# PPS Driver
refclock pps unit 0 minpoll 4 maxpoll 4 ppspath /dev/pps0
#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)
refclock nmea prefer minpoll 4 maxpoll 4 mode 8 stratum 2 baud 115200 time2 0.060 path /dev/ttyAMA0
#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 $GPMRC (value = 1)
# bit 1 - process $GPGGA (value = 2)
# bit 2 - process $GPGLL (value = 4)
# bit 4 - process $GPZDA or $GPZDG (value = 8)
# mode 9 = process only xxRMC and xxZDA NMEA records at 115200 baud
# pool.ntp.org maps to about 1000 low-stratum NTP servers.  Your server will
# pick a different set every time it starts up.  Please consider joining the
# pool: <https://www.pool.ntp.org/join.html>
server 0.uk.pool.ntp.org iburst
server 1.uk.pool.ntp.org iburst
server 2.uk.pool.ntp.org iburst

To calculate the GPS offset we must disable GPS by placing a noselect in the ntp.conf "refclock nmea" 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. Remember to remove the "noselect" after tweaking.

sudo systemctl stop ntpsec.service

sudo rm –f /var/log/ntpsec/*

sudo systemctl start ntpsec.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

# Generate an estimate of your GPS's offset from a peerstats file
awk '
/NMEA/ { sum -= $5 ; cnt++; }
END { printf("%.6f\n", sum / cnt); }
' </var/log/ntpsec/peerstats


Then sudo chmod +x ./nmeaoffset

Run by typing

pi@ntp2:~ $ ./nmeaoffset


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 ntpsec.service

sudo rm –f /var/log/ntpsec/*

sudo systemctl start ntpsec.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 "refclock nmea" 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 +0.060 s when using 115200 baud and the gpsctl --config_for_timing tweaks.

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=Raspberry Pi Foundation,codename=${distro_codename},label=Raspberry Pi Foundation";
and add
Unattended-Upgrade::Automatic-Reboot "true";
sudo nano /etc/apt/apt.conf.d/20auto-upgrades
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

sudo systemctl edit apt-daily-upgrade.timer
# OnCalendar=DayOfWeek Year-Month-Day Hour:Minute:Second
# see https://wiki.archlinux.org/index.php/Systemd/Timers

To test,

sudo unattended-upgrade -d -v --dry-run

Static IP and Hostname

If you want to fix your LAN IP you do it by running nmtui to configure your network.

Amend your hostname via raspi-config.


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


Whitham D. Reeve’s GpsNtp-Pi Installation and Operation Guide


Rich Laager’s Raspberry Pi 3 Stratum 1 NTP Server has the best stuff on offsets


Tech Solvency’s similar setup


Jack Zimmerman’s Raspberry PI NTP Server LCD Display


Jack Zimmerman’s Pi U-Blox Stationary Mode


GPSD Time Service HOWTO


Tom Dilatush’s gpsctl


My improved gpsctl with added .conf file support GitHub repo


u-blox M8 Receiver Description (the bible)


GNSS Firmware 3.01 for u-blox M8


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/firmware/config.txt

Add the line


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

Also comment out the two lines

/sbin/hwclock --rtc=$dev --systz --badyear


/sbin/hwclock --rtc=$dev --systz

Configure the rv3028 by running

sudo configure-rv3028.sh

from the gpsctl archive.


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.

Occasionally, run

sudo hwclock --update-drift -w -v

to calibrate the RTC accuracy.

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

This comment about unreliability of baud rate sync no longer applies - it all seems to work fine for me with current builds of Raspi OS and current kernels. Also tested on bookworm.

Postscript, October 19th, 2023

This blog post has been revised for Raspberry Pi OS Bookworm and ntpsec. The old udev rules are no longer needed as the PPS and GPS devices are now specified explicitly in ntp.conf.

Postscript, January 23rd, 2024

I've just found Conor Robinson's excellent blog series on how to build a Raspberry Pi NTP Server using a Uputronics HAT and gpsd. Worth a read and a revisit when Conor has filled in the blanks in Part 6 (optimisation).

Postscript, February 3rd, 2024

Another excellent build recipe, but for the Adafruit Ultimate GPS HAT etc: Raspberry Pi 3B+ NTP Server - Stratum 1 (with Adafruit Ultimate GPS HAT) 

Postscript, February 18th, 2024

Sad news. The original author of gpsctl, Tom Dilatush, passed away on February 14th.

Posted by Phil at 7:12 PM
Edited on: Friday, February 23, 2024 1:34 PM
Categories: IT, Raspberry Pi, Software