Introduction

This material was supplied by Arising Technology Systems Pty Ltd and has contributed to the installation of two Stratum 1 time servers used internally in their network. This is an advanced project.

What you will need hardware wise:

raspberry pi (preferably a 3 or better)
- a good micro USB 2.5 to 3 Amp power supply (otherwise things won't go good for you)
- heatsinks for the processors, and maybe a fan in a box
a GPS module with a serial port and 1 PPS, and one that supplies an external antenna socket (my cost U$8.14 each - ranging up to $36)
a plastic case with some way of mounting the GPS module so its integral antenna can see the sky (for portable use)
to improve reception, you can use an external active patch antenna of at least 28dB gain and at least 1.5 dB NF (for a max gain of approximately 50 dB) - these are available from ebay ranging from $6 to $36. They are just a plastic dongle, with a ceramic patch antenna inside of questionable radiation pattern, with an active amplifier that works with a supply voltage between 2.5 and 3.7 volts, and a cheap magnet and plastic covering the base.
a 20*2 header socket (if you can't find one of those use an IDC socket, and you can bend the pins down on the IDC socket and solder to them if you do not want to use a full ribbon cable).
soldering iron and solder
wire-wrap, hookup-wire or ribbon cable - your preference.

I originally used a hat board in my first unit - but these are better for prototyping, they make your pi heavy and hot. It's better to have air circulating over the processors, preferably with a fan, rather than filling the space with a PCB, and screw stand-offs.

I initially tried #using NTP as the time daemon, and it is woefully inadequate, being unable to handle jitter and differences that occur as your system is starting from fresh, particularly if you have disassociated your pi from the NTP servers during the dhcp assignment and from system-time synchronisation. (See #stabilise).

It's a bit of a mechanical hack (what can I say?) I made my piaware pi have dual purpose with the installation of a GPS module.

Errors are of the order of 10's of metres when running with the intrinsic antenna, but reduced to the order of 1 meter hozizontal when running with the 28dB antenna (so get a better antenna).

GPS module (those nicks are how the Chinese tune the patch antenna)

This is a hobbyist module, supplied for Arduino use. I originally tried the one that had an external ceramic dipole-like patch and it never acquired any satellites after 24 hours. Admittedly it was a 10 year old unit that as far as I know had never been turned on. Thus the need for the above unit with an external antenna connector, and its intrinsic patch antenna (for use on an aeroplane glare-shield when portable). This unit's signals improved to reported 30 dB S/N ratio (from a max of 10 dB S/N) with the external antenna sitting on top of my UPS working through a concrete tile roof without sisalation on the top of my office cabinet (poor-man's computer rack). Thus the antenna is closer to having 20 dB of gain (what do you expect for $6?).

system updates

update your SDCard/OS using the piaware repository, because it's at buster and faster than the official raspberry pi repo. You might even want to run piaware on your time daemon box like I do, it won't hurt it, it's only power and money.

Via the following

use the piaware repo

sudo wget http://flightaware.com/adsb/piaware/files/packages/p/piaware-support/piaware-repository_3.7.1_all.deb
sudo dpkg -i piaware-repository_3.7.1_all.deb

update the package list using the new repo

sudo apt update

optionally install piaware stuff

sudo apt install piaware
sudo apt install dump1090-fa

install the gpsd daemon for monitoring your GPS module (it automatically chooses parser/handlers to service messages, which includes being able to handle the very common NMEA-0183 format)

sudo apt install gpsd

install the gpsd clients - which you can use to monitor gpsd

sudo apt install gpsd-clients
sudo apt install python-gps

install the ppstest command - which you will use to watch /dev/pps1

sudo apt install pps-tools

install the following so you can find open file descriptors

sudo apt install lsof

using pps

Looks like a sysop had to use an expensive GPS box sporting a IEEE-802 cable and resort to a serial patch cable to obtain the more accurate 1 PPS - lol.

1PPS

It turns out that the PPS signal is compatible with the logic levels of the DCD line

It is a little easier on the raspberry pi ^{[note 1]}

source a GPS module that operates at raspberry pi favourable voltages; an Arduino GPS module would be suitable, provided it has 1 PPS and has been manufactured since the last GPS date roll-over.
suitable wire and a header socket or similar and a means to mount your GPS module
decide on which GPIO pin you will connect to 1 PPS (I used GPIO 23), connect the 1 PPS to the GPIO pin.
connect the board TX line to the pi RX line; I left the module RX line disconnected because the pi was sending garbage to the module
connect the module Vcc to 3.3v (and not the 5 volt)
connect the ground
disable the console access to the pi serial port (advanced option in config serial disable)
- select 5 interfacing options > 6 serial to disable shell and kernel messages on serial connection.

sudo rasp-config

assign the UART (e.g. the mini UART is connected to /dev/stty0) - this requires using the mini UART overlay.
enable gpio overlay

Edit /boot/config.txt and include:

# ################################################
#
# RBH clocking

arm_freq=1000
arm_freq_max=1000
# fix the clock
force_turbo=1
nohz=off

# RBH enable uart

enable_uart=1
dtoverlay=pps-gpio,gpiopin=23

edit /boot/commandline.txt and remove the console=tty1 to stop it polling console and waiting for login on it as tty1 will be used for the gpsd

dwc_otg.lpm_enable=0 root=PARTUUID=d3e1179d-02 rootfstype=ext4 elevator=deadline fsck.repair=yes rootwait net.ifnames=0 quiet splash plymouth.ignore-serial-consoles

and after wiring is complete turn your pi on and login
set the serial port baudrate

sudo stty -F /dev/ttyS0 speed 9600 cs8 -cstopb -parenb && echo -n > /dev/stty0

watch the serial line for messages from the GPS module

sudo cat /dev/ttyS0

If you see messages, particularly GSV with satellites in view, then all good and you can do some further testing:

watch the 1 PPS transition:

sudo ppstest /dev/pps0

You should notice the system time stamp reported at the time the 1 PPS rising edge was detected.

now is a good time to source the leap-seconds.list from a reputable supply

cd /usr/share/zoneinfo
sudo wget https://www.ietf.org/timezones/data/leap-seconds.list

configure gpsd

# Default settings for the gpsd init script and the hotplug wrapper.

# Start the gpsd daemon automatically at boot time
START_DAEMON="true"

# Use USB hotplugging to add new USB devices automatically to the daemon
USBAUTO="true"

# Devices gpsd should collect to at boot time.
# They need to be read/writeable, either by user gpsd or the group dialout.
#DEVICES="/dev/ttyAMA0"

# RBH it works on the simple UART too
DEVICES="/dev/serial0 /dev/pps0"

# Other options you want to pass to gpsd
GPSD_OPTIONS="-n"

A technical paper describing gpsd https://www.aosabook.org/en/gpsd.html ^[1]

How to hack https://gpsd.gitlab.io/gpsd/hacking.html ^[2]

The gpsd project https://gpsd.gitlab.io/gpsd/index.html ^[3]

Clients https://gpsd.gitlab.io/gpsd/gps.html ^[4]

Example client code https://gpsd.gitlab.io/gpsd/gpsd-client-example-code.html ^[5]

stabilise

you may wish to stop the CPU frequency modulation to reduce jitter (I did not bother), add to your /boot/cmdline.txt

nohz=off

turn off systemd-timesyncd (I did this)

sudo systemctl stop systemd-timesyncd.service
sudo systemctl disable systemd-timesyncd.service

You may configure the system to run at full performance which uses more power to lower 1PPS latency (I did the opposite and set my governor to powersave),

sudo apt -y install cpufrequtils
sudo echo 'GOVERNOR="performance"' > /etc/default/cpufrequtils
sudo systemctl restart cpufrequtils

or lock your CPU to one frequency via the /boot/config.txt

# Meaningful word.. This means 'enable cpu freq scaling'
force_turbo=0
# Set lowest cpu freq. By default, it's 700.
arm_freq_min=1600

ntp-servers are specified in /etc/dhcp/dhclient.conf

...
#supersede domain-name "fugue.com home.vix.com";
#prepend domain-name-servers 127.0.0.1;
request subnet-mask, broadcast-address, time-offset, routers,
     domain-name, domain-name-servers, domain-search, host-name,
     dhcp6.name-servers, dhcp6.domain-search,
     netbios-name-servers, netbios-scope, interface-mtu,
     rfc3442-classless-static-routes , ntp-servers;
#require subnet-mask, domain-name-servers;
#timeout 60;
...

Remove or comment out "ntp-servers" from /etc/dhcp/dhclient.conf

...
# A list of options to request from the DHCP server.
option domain_name_servers, domain_name, domain_search, host_name
option classless_static_routes
# Most distributions have NTP support.
# option ntp_servers
# Respect the network MTU.
...

Delete these related files:

/etc/dhcp/dhclient-exit-hooks.d/ntp
/lib/dhcpcd/dhcpcd-hooks/50-ntp.conf
/var/lib/ntp/ntp.conf.dhcp (might not exist)

A reboot is warranted for this to take effect. At this point, you will be able to configure your time daemon entirely without interference from the rest of the system.

chrony & PPS

Chrony is much better at synchronising to hardware PPS without marking the PPS as a bad-ticker just when timing is about to start.

obtain ntpdate for snooping etc

sudo apt install ntpdate

install chrony

sudo apt install chrony

synchronize your system clock with the www pool by running an initial chrony config without your GPS and PPS refclocks active

keyfile /etc/chrony/chrony.keys

driftfile /var/lib/chrony/chrony.drift

log tracking measurements statistics
logdir /var/log/chrony

maxupdateskew 100.0

# no RTC on pi
#rtcsync

allow

# Step clock by 1 second for each up to 5 seconds of drift
makestep 1 5

# the pool I use when in au
server 0.au.pool.ntp.org
server 1.au.pool.ntp.org
server 2.au.pool.ntp.org
server 3.au.pool.ntp.org

# gpsd feeding NMEA time to SHM 0 and 1 PPS connected to /dev/pps0 (delay just stops hunting between PPS and NMEA references)
#refclock SHM 0 offset 0.1225 delay 0.2 refid NMEA precision 1e-9 trust
#refclock PPS /dev/pps0 lock NMEA

start gpsd and chrony

 sudo systemctl start gpsd chrony

run gpsmon to see the adjustment required to set your GPS time to net time

 gpsmon

take the offset you see in gpsmon and insert it into your /etc/chrony/chrony.conf refclock offset and uncomment the two refclock lines

# gpsd feeding NMEA time to SHM 0 and 1 PPS connected to /dev/pps0
refclock SHM 0 offset 0.122500000 delay 0.2 refid NMEA precision 1e-9 trust
refclock PPS /dev/pps0 lock NMEA

restart chrony

sudo systemctl start chrony

watch the PPS drift against the clock via:

sudo ppstest /dev/pps0

or via gpsmon (look at the PPS delta)

gpsmon

and watch your chrony source effectiveness via

for (( ; ; )) do chronyc sources; sleep 5; done

or via:

 sudo ppstest /dev/pps0

test your clock against another NTP server (-q means query only, -p 1 means only 1 poll so you don't flood the NTP server)

sudo ntpdate -p 1 -q ns1.anu.edu.au

supplementary

overlay rootfs

Once you have all the configuration established it is time to boot the system so rootfs resides within an overlayfs.

On later pis this can be done by running raspi-config using the performance options. I prefer to keep the /boot outside the overlayfs so I can modify boot options (carefully).

On previous models running buster or earlier, then the overlay mode needs to be scripted.

See overlay rootfs

bibliography

using ntp

Note: you must turn off the dhcp sourcing of ntp servers when you are running your own time servers, because dhcp will replace your /etc/ntp.conf contents with its own sources from the switch!!!! (Somewhere on my system a box is promulgating the wrong address for one of my Stratum-1 sources and ruining my network time coordination.)

You might want to skip NTP completely because it doesn't handle the intial starting conditions of outlying clocks. Use chrony instead, because I believe it is more resilliant. If you have a wandering clock, or you have disabled time-updates (because you want your refclock to be a master), then you can try the following and see what I mean. It's really hard to beat 1,200nS +/-428nS precision, which is what I am seeing with my pi running Chrony and a u-blox 6M GPS module I purchased from China for $14.00 (only one module out of 5 was broken with a shorted antenna section that blew up an unmarked rf amplifier chip).

If you want to use NTP then you have been warned ...

obtain ntpdate

sudo apt install ntpdate ntp

build a version of ntp that knows about pps

sudo apt-get build-dep ntp
sudo apt-get install pps-tools ubuntu-dev-tools
sudo apt-get source ntp

create the ntp package and install it
- cd <The ntp version directory something like ntp-4.2.6.p5+dfsg >

sudo dpkg-buildpackage -uc -us -nc

install it via

sudo dpkg -i ../ntp*.deb

or force install it if it has dependencies that are broken (be careful)

sudo dpkg -force-all -i ../ntp*.deb

To avoid that apt doesn't run with the ATOM clock support, use

sudo echo ntp hold  | dpkg --set-selections

source the leap-seconds.list from a reputable supply

cd /usr/share/zoneinfo
sudo wget https://www.ietf.org/timezones/data/leap-seconds.list

now configure ntp.conf

configuring other system ntp servers to use ntp.arising.com.au

You need to modify /etc/ntp.config in your other systems if you want to use the local Stratum-1 time source, instead of the www pools

vi /etc/ntp.config and comment out the pool and replace with the server ntp.arising.com.au

# 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: <http://www.pool.ntp.org/join.html>
#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

server ntp.arising.com.au

reboot and verify that all is well

ntpq -p

     remote           refid      st t when poll reach   delay   offset  jitter
==============================================================================
*ntp.arising.com .PPS1.           1 u   24  256  377    0.442    0.114   0.118

and check timedatectl

timdatectl

overclocking

hardware

parts


parts	supplier	image
raspberry pi	raspberry pi - Core Electronics
pi box	RS Components
pi plug-pack	RS components
HAT scratch board	RS components
Neo6 GPS module	ALiExpress
Neo7 GPS	ebay