Tag: gps

Categories
Linux NTP Raspberry Pi

Millisecond accurate Chrony NTP with a USB GPS for $12 USD

Introduction

Building off my last NTP post (Microsecond accurate NTP with a Raspberry Pi and PPS GPS), which required a $50-60 GPS device and a Raspberry Pi (also $40+), I have successfully tested something much cheaper, that is good enough, especially for initial PPS synchronization. Good enough, in this case, is defined as +/- 10 milliseconds, which can easily be achieved using a basic USB GPS device: GT-U7. Read on for instructions on how to set up the USB GPS as a Stratum 1 NTP time server.

YouTube Video Link

https://www.youtube.com/watch?v=DVtmDFpWkEs

Microsecond PPS time vs millisecond USB time

How accurate of time do you really need? The last post showed how to get all devices on a local area network (LAN) within 0.1 milliseconds of “real” time. Do you need you equipment to be that accurate to official atomic clock time (12:03:05.0001)? Didn’t think so. Do you care if every device is on the correct second compared to official/accurate time (12:03:05)? That’s a lot more reasonable. Using a u-blox USB GPS can get you to 0.01 seconds of official. The best part about this? The required USB GPS units are almost always less than $15 and you don’t need a Raspberry Pi.

Overview

This post will show how to add a u-blox USB GPS module to NTP as a driver or chrony (timekeeping daemon) as a reference clock (using GPSd shared memory for both) and verify the accuracy is within +/- 10 milliseconds.

Materials needed

  • USB u-blox GPS (VK-172 or GT-U7), GT-U7 preferred because it has a micro-USB plug to connect to your computer. It is important to note that both of these are u-blox modules, which has a binary data format as well as a high default baudrate (57600). These two properties allow for quick transmission of each GPS message from GPS to computer.
  • 15-30 minutes

Steps

1 – Update your host machine and install packages

This tutorial is for Linux. I use Ubuntu so we utilize Aptitude (apt) for package management:

sudo apt update
sudo apt upgrade
sudo rpi-update
sudo apt install gpsd gpsd-clients python-gps chrony

2 – Modify GPSd default startup settings

In /etc/default/gpsd, change the settings to the following:

# 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.
# this could also be /dev/ttyUSB0, it is ACM0 on raspberry pi
DEVICES="/dev/ttyACM0"

# -n means start listening to GPS data without a specific listener
GPSD_OPTIONS="-n"

Reboot with sudo reboot.

3a – Chrony configuration (if using NTPd, go to 3b)

I took the default configuration, added my 10.98 servers, and more importantly, added a reference clock (refclock). Link to chrony documentation here. Arguments/parameters of this configuration file:

  • 10.98.1.198 is my microsecond accurate PPS NTP server
  • iburst means send a bunch of synchronization packets upon service start so accurate time can be determined much faster (usually a couple seconds)
  • maxpoll (and minpoll, which isn’t used in this config) is how many seconds to wait between polls, defined by 2^x where x is the number in the config. maxpoll 6 means don’t wait more than 2^6=64 seconds between polls
  • refclock is reference clock, and is the USB GPS source we are adding
    • ‘SHM 0’ means shared memory reference 0, which means it is checking with GPSd using shared memory to see what time the GPS is reporting
    • ‘refid NMEA’ means name this reference ‘NMEA’
    • ‘offset 0.000’ means don’t offset this clock source at all. We will change this later
    • ‘precision 1e-3’ means this reference is only accurate to 1e-3 (0.001) seconds, or 1 millisecond
    • ‘poll 3’ means poll this reference every 2^3 = 8 seconds
    • ‘noselect’ means don’t actually use this clock as a source. We will be measuring the delta to other known times to set the offset and make the source selectable.
pi@raspberrypi:~ $ sudo cat /etc/chrony/chrony.conf
# Welcome to the chrony configuration file. See chrony.conf(5) for more
# information about usuable directives.
#pool 2.debian.pool.ntp.org iburst
server 10.98.1.1 iburst maxpoll 6
server 10.98.1.198 iburst maxpoll 6
server 10.98.1.15 iburst

refclock SHM 0 refid NMEA offset 0.000 precision 1e-3 poll 3 noselect

Restart chrony with sudo systemctl restart chrony.

3b – NTP config

Similar to the chrony config, we need to add a reference clock (called a driver in NTP). For NTP, drivers are “servers” that start with an address of 127.127. The next two octets tell what kind of driver it is. The .28 driver is the shared memory driver, same theory as for chrony. For a full list of drivers, see the official NTP docs. To break down the server:

  • ‘server 127.127.28.0’ means use the .28 (SHM) driver
    • minpoll 4 maxpoll 4 means poll every 2^4=16 seconds
    • noselect means don’t use this for time. Similar to chrony, we will be measuring the offset to determine this value.
  • ‘fudge 127.127.28.0’ means we are going to change some properties of the listed driver
    • ‘time1 0.000’is the time offset calibration factor, in seconds
    • ‘stratum 2’ means list this source as a stratum 2 source (has to do with how close the source is to “true” time), listing it as 2 means other, higher stratum sources will be selected before this one will (assuming equal time quality)
    • ‘refid GPS’ means rename this source as ‘GPS’
server 127.127.28.0 minpoll 4 maxpoll 4 noselect
fudge 127.127.28.0 time1 0.000 stratum 2 refid GPS

Restart NTPd with sudo systemctl restart ntp.

4 – check time offset via gpsmon

Running gpsmon shows us general information about the GPS, including time offset. The output looks like the below screenshot. Of importance is the satellite count (on right, more is better, >5 is good enough for time), HDOP (horizontal dilution of precision) is a measure of how well the satellites can determine your position (lower is better, <2 works for basically all navigation purposes), and TOFF (time offset).

gpsmon showing time offset for a USB GPS

In this screenshot the TOFF is 0.081862027, which is 81.8 milliseconds off the host computer’s time. Watch this for a bit – it should hover pretty close to a certain value +/- 10ms. In my case, I’ve noticed that if there are 10 or less satellites locked on, it is around 77ms. If there are 11 or more, it is around 91ms (presumably due to more satellite information that needs to be transmitted).

5 – record statistics for a data-driven offset

If you are looking for a better offset value to put in the configuration file, we can turn on logging from either chrony or NTPd to record source information.

For chrony:

Edit /etc/chrony/chrony.conf and uncomment the line for which kinds of logging to turn on:

# Uncomment the following line to turn logging on.
log tracking measurements statistics

Then restart chrony (sudo systemctl restart chrony) and logs will start writing to /var/log/chrony (this location is defined a couple lines below the log line in chrony.conf):

pi@raspberrypi:~ $ ls /var/log/chrony
measurements.log  statistics.log  tracking.log

For NTPd (be sure to restart it after making any configuration changes):

austin@prox-3070 ~ % cat /etc/ntp.conf
# Enable this if you want statistics to be logged.
statsdir /var/log/ntpstats/

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

Wait a few minutes for some data to record (chrony synchronizes pretty quick compared to NTPd) and check the statistics file, filtered to our NMEA refid:

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

This spits out the lines that have NMEA present (the ones of interest for our USB GPS). To include the headers to show what each column is we can run

# chrony
cat /var/log/chrony/statistics.log | head -2; cat /var/log/chrony/statistics.log | grep NMEA

# ntp, there is no header info so we can omit that part of the command
cat /var/log/peerstats | grep 127.127.28.0
Screenshot showing chrony statistics for our NMEA USB GPS refclock

NTP stats don’t include header information. The column of interest is the one after the 9014 column. The columns are day, seconds past midnight, source, something, estimated offset, something, something, something. We can see the offset for this VK-172 USB GPS is somewhere around 76-77 milliseconds (0.076-0.077 seconds), which we can put in place of the 0.000 for the .28 driver for NTP and remove noselect.

austin@prox-3070 ~ % cat /var/log/ntpstats/peerstats | grep 127.127.28.0
59487 49648.536 127.127.28.0 9014 -0.078425007 0.000000000 7.938064614 0.000000060
59487 49664.536 127.127.28.0 9014 -0.079488544 0.000000000 3.938033388 0.001063537
59487 49680.536 127.127.28.0 9014 -0.079514781 0.000000000 1.938035682 0.000770810
59487 49696.536 127.127.28.0 9014 -0.079772284 0.000000000 0.938092429 0.000808697
59487 49712.536 127.127.28.0 9014 -0.079711708 0.000000000 0.438080791 0.000661032
59487 49728.536 127.127.28.0 9014 -0.075098563 0.000000000 0.188028843 0.004311958

So now we have some data showing the statistics of our NMEA USB GPS NTP source. We can copy and paste this into Excel, run data to columns, and graph the result and/or get the average to set the offset.

screenshot showing chrony/NTP statistics to determine offset

This graph is certainly suspicious (sine wave pattern and such) and if I wasn’t writing this blog post, I’d let data collect overnight to determine an offset. Since time is always of the essence, I will just take the average of the ‘est offset’ column (E), which is 7.64E-2, or 0.0763 seconds. Let’s pop this into the chrony.conf file and remove noselect:

refclock SHM 0 refid NMEA offset 0.0763 precision 1e-3 poll 3

For NTP:

Restart chrony again for the config file changes to take effect – sudo systemctl restart chrony.

6 – watch ‘chrony sources’ or ‘ntpq -pn’ to see if the USB GPS gets selected as the main time source

If you aren’t aware, Ubuntu/Debian/most Linux includes a utility to rerun a command every x seconds called watch. We can use this to watch chrony to see how it is interpreting each time source every 1 second:

# for chrony
watch -n 1 chronyc sources
watching chrony sources

In the above screenshot, we can see that chrony actually has the NMEA source selected as the primary source (denoted with the *). It has the Raspberry Pi PPS NTP GPS ready to takeover as the new primary (denoted with the +). All of the sources match quite closely (from +4749us to – 505us is around 5.2 milliseconds). The source “offset” is in the square brackets ([ and ]).

# for ntp
watch -n 1 ntpq -pn
NTP showing (via ntpq -pn) that the GPS source is 0.738 milliseconds off of the host clock. The ‘-‘ in front of the remote means this will not be selected as a valid time (presumably due to the high jitter compared to the other sources, probably also due to manually setting it to stratum 2).

7- is +/- five millseconds good enough?

For 99% of use cases, yes. You can stop here and your home network will be plenty accurate. If you want additional accuracy, you are in luck. This GPS module also outputs a PPS (pulse per second) signal! We can use this to get within 0.05 millseconds (0.00005 seconds) from official/atomic clock time.

Conclusion

In this post, we got a u-blox USB GPS set up and added it as a reference clock (refclock) to chrony and demonstrated it is clearly within 10 millisecond of official GPS time.

You could write a script to do all this for you! I should probably try this myself…

In the next post, we can add PPS signals from the GPS module to increase our time accuracy by 1000x (into the microsecond range).

A note on why having faster message transmission is better for timing

My current PPS NTP server uses chrony with NMEA messages transmitted over serial and the PPS signal fed into a GPIO pin. GPSd as a rule does minimum configuration of GPS devices. It typically defaults to 9600 baud for serial devices. A typical GPS message looks like this:

$GPGGA, 161229.487, 3723.2475, N, 12158.3416, W, 1, 07, 1.0, 9.0, M, , , , 0000*18

That message is 83 bytes long. At 9600 baud (9600 bits per second), that message takes 69.1 milliseconds to transmit. Each character/byte takes 0.833 milliseconds to transmit. That means that as the message length varies, the jitter will increase. GPS messages do vary in length, sometimes significantly, depending on what is being sent (i.e. the satellite information, $GPGSV sentences, is only transmitted every 5-10 seconds).

I opened gpsmon to get a sample of sentences – I did not notice this until now but it shows how many bytes each sentence is at the front of the sentence:

(35) $GPZDA,144410.000,30,09,2021,,*59
------------------- PPS offset: -0.000001297 ------
(83) $GPGGA,144411.000,3953.xxxx,N,10504.xxxx,W,2,6,1.19,1637.8,M,-20.9,M,0000,0000*5A
(54) $GPGSA,A,3,26,25,29,18,05,02,,,,,,,1.46,1.19,0.84*02
(71) $GPRMC,144411.000,A,3953.xxxx,N,10504.xxxx,W,2.80,39.98,300921,,,D*44
(35) $GPZDA,144411.000,30,09,2021,,*58
------------------- PPS offset: -0.000000883 ------
(83) $GPGGA,144412.000,3953.xxxx,N,10504.xxxx,W,2,7,1.11,1637.7,M,-20.9,M,0000,0000*52
(56) $GPGSA,A,3,20,26,25,29,18,05,02,,,,,,1.39,1.11,0.84*00
(70) $GPGSV,3,1,12,29,81,325,27,05,68,056,21,20,35,050,17,18,34,283,24*76
(66) $GPGSV,3,2,12,25,27,210,14,15,27,153,,13,25,117,,02,23,080,19*78
(59) $GPGSV,3,3,12,26,17,311,22,23,16,222,,12,11,184,,47,,,*42
------------------- PPS offset: -0.000000833 ------
(71) $GPRMC,144412.000,A,3953.xxxx,N,10504.xxxx,W,2.57,38.19,300921,,,D*48
(35) $GPZDA,144412.000,30,09,2021,,*5B
(83) $GPGGA,144413.000,3953.xxxx,N,10504.xxxx,W,2,7,1.11,1637.6,M,-20.9,M,0000,0000*52
(56) $GPGSA,A,3,20,26,25,29,18,05,02,,,,,,1.39,1.11,0.84*00
(71) $GPRMC,144413.000,A,3953.xxxx,N,10504.xxxx,W,2.60,36.39,300921,,,D*41
(35) $GPZDA,144413.000,30,09,2021,,*5A

These sentences range from 83 bytes to 35 bytes, a variation of (83 bytes -35 bytes)*0.833 milliseconds per byte = 39.984 milliseconds.

Compare to the u-blox binary UBX messages which seem to always be 60 bytes and transmitted at 57600 baud, which is 8.33 milliseconds to transmit the entire message.

UBX protocol messages (blanked out lines). I have no idea what part of the message is location, hopefully I got the right part blanked out.

The variance (jitter) is thus much lower and can be much more accurate as a NTP source. GPSd has no problem leaving u-blox modules at 57600 baud. This is why the USB GPS modules perform much more accurate for timekeeping than NMEA-based devices when using GPSd.

For basically every GPS module/chipset, it is possible to send it commands to enable/disable sentences (as well as increase the serial baud rate). In an ideal world for timekeeping, GPSd would disable every sentence except for time ($GPZDA), and bump up the baud rate to the highest supported level (115200, 230400, etc.). Unfortunately for us, GPSd’s default behavior is to just work with every GPS, which essentially means no configuring the GPS device.

Update 2024-01-19: RIP Dave Mills, inventor/creator of NTP – https://arstechnica.com/gadgets/2024/01/inventor-of-ntp-protocol-that-keeps-time-on-billions-of-devices-dies-at-age-8

Categories
Linux Raspberry Pi

Microsecond accurate NTP with a Raspberry Pi and PPS GPS

Raspberry Pi hooked up to PPS GPS
Raspberry Pi hooked up to PPS GPS

Introduction

Lots of acronyms in that title. If I expand them out, it says – “microsecond accurate network time protocol with a Raspberry Pi and pulse per second global positioning system [receiver]”. What it means is you can get super accurate timekeeping (1 microsecond = 0.000001 seconds) with a Raspberry Pi and a GPS receiver (the GT-U7 is less than $12) that spits out pulses every second. By following this guide, you will your very own Stratum 1 NTP server at home!

Why would you need time this accurate at home?

You don’t. There aren’t many applications for this level of timekeeping in general, and even fewer at home. But this blog is called Austin’s Nerdy Things so here we are. Using standard, default internet NTP these days will get your computers to within a 10-20 milliseconds of actual time (1 millisecond = 0.001 seconds). By default, Windows computers get time from time.windows.com. MacOS computers get time from time.apple.com. Linux devices get time from [entity].pool.ntp.org, like debian.pool.ntp.org. PPS gets you to the next SI prefix in terms of accuracy (milli -> micro), which means 1000x more accurate timekeeping.

YouTube video link

If you prefer a video version – https://www.youtube.com/watch?v=YfgX7qPeiqQ

Materials needed

Steps

0 – Update your Pi and install packages

This NTP guide assumes you have a Raspberry Pi ready to go. If you don’t, I have a 16 minute video on YouTube that goes through flashing the SD card and initial setup – https://youtu.be/u5dHvEYwr9M.

You should update your Pi to latest before basically any project. We will install some other packages as well. pps-tools help us check that the Pi is receiving PPS signals from the GPS module. We also need GPSd for the GPS decoding of both time and position. I use chrony instead of NTPd because it seems to sync faster than NTPd in most instances and also handles PPS without compiling from source (the default Raspbian NTP doesn’t do PPS) Installing chrony will remove ntpd.

sudo apt update
sudo apt upgrade
sudo rpi-update
sudo apt install pps-tools gpsd gpsd-clients python-gps chrony

1 – add GPIO and module info where needed

In /boot/config.txt, add ‘dtoverlay=pps-gpio,gpiopin=18’ to a new line. This is necessary for PPS. If you want to get the NMEA data from the serial line, you must also enable UART and set the initial baud rate.

sudo bash -c "echo '# the next 3 lines are for GPS PPS signals' >> /boot/config.txt"
sudo bash -c "echo 'dtoverlay=pps-gpio,gpiopin=18' >> /boot/config.txt"
sudo bash -c "echo 'enable_uart=1' >> /boot/config.txt"
sudo bash -c "echo 'init_uart_baud=9600' >> /boot/config.txt"

In /etc/modules, add ‘pps-gpio’ to a new line.

sudo bash -c "echo 'pps-gpio' >> /etc/modules"

Reboot

sudo reboot

2 – wire up the GPS module to the Pi

I used the Adafruit Ultimate GPS breakout. It has 9 pins but we are only interested in 5. There is also the Adafruit GPS hat which fits right on the Pi but that seems expensive for what it does (but it is significantly neater in terms of wiring).

Pin connections:

  1. GPS PPS to RPi pin 12 (GPIO 18)
  2. GPS VIN to RPi pin 2 or 4
  3. GPS GND to RPi pin 6
  4. GPS RX to RPi pin 8
  5. GPS TX to RPi pin 10
  6. see 2nd picture for a visual
Adafruit Ultimate GPS Breakout V3
We use 5 wires total. GPS PPS to pin 12 (GPIO 18), GPS VIN to pin 2 or 4, GPS GND to pin 6, GPS RX to pin 8, GPS TX to pin 10.
GPS with wires attached to the board (but not to the Pi) and the antenna. The antenna has a SMA connector, and there is another adapter that is SMA to u.fl to plug into the GPS board.

Now place your GPS antenna (if you have one) in a spot with a good view of the sky. If you don’t have an antenna, you might have to get a bit creative with how you locate your Raspberry Pi with attached GPS.

I honestly have my antenna in the basement (with only the kitchen and attic above) and I generally have 8-10 satellites locked all the time (11 as of writing). Guess that means the antenna works better than expected! Either way, better exposure to the sky will in theory work better. Pic:

My super awesome placement of the GPS antenna on top of the wood “cage” (?) I built to hold my 3d printer on top of my server rack. I guess this is a testimony for how well the GPS antenna works? It has 11 satellites locked on as of writing, with a HDOP of 0.88. The components in this rack (Brocade ICX6450-48P, 1U white box with Xeon 2678v3/96GB memory/2x480GB enterprise SSDs/4x4TB HDDs, Dell R710 with 4x4TB and other stuff) will be detailed in an upcoming post.

2.5 – free up the UART serial port for the GPS device

Run raspi-config -> 3 – Interface options -> I6 – Serial Port -> Would you like a login shell to be available over serial -> No. -> Would you like the serial port hardware to be enabled -> Yes.

Finish. Yes to reboot.

3 – check that PPS is working

First, check that the PPS module is loaded:

lsmod | grep pps

The output should be like:

pi@raspberrypi:~ $ lsmod | grep pps
pps_gpio               16384  0
pps_core               16384  1 pps_gpio

Second, check for the PPS pulses:

sudo ppstest /dev/pps0

The output should spit out a new line every second that looks something like this (your output will be a bit farther from x.000000 since it isn’t yet using the GPS PPS):

pi@pi-ntp:~ $ sudo ppstest /dev/pps0
trying PPS source "/dev/pps0"
found PPS source "/dev/pps0"
ok, found 1 source(s), now start fetching data...
source 0 - assert 1618799061.999999504, sequence: 882184 - clear  0.000000000, sequence: 0
source 0 - assert 1618799062.999999305, sequence: 882185 - clear  0.000000000, sequence: 0
source 0 - assert 1618799063.999997231, sequence: 882186 - clear  0.000000000, sequence: 0
source 0 - assert 1618799064.999996827, sequence: 882187 - clear  0.000000000, sequence: 0
source 0 - assert 1618799065.999995852, sequence: 882188 - clear  0.000000000, sequence: 0
^C

4 – change GPSd to start immediately upon boot

Edit /etc/default/gpsd and change GPSD_OPTIONS=”” to GPSD_OPTIONS=”-n” and change DEVICES=”” to DEVICES=”/dev/ttyS0 /dev/pps0″, then reboot. My full /etc/default/gpsd is below:

pi@raspberrypi:~ $ sudo cat /etc/default/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/ttyS0 /dev/pps0"

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

sudo reboot

4.5 – check GPS for good measure

To ensure your GPS has a valid position, you can run gpsmon or cgps to check satellites and such. This check also ensures GPSd is functioning as expected. If your GPS doesn’t have a position solution, you won’t get a good time signal. If GPSd isn’t working, you won’t get any updates on the screen. The top portion will show the analyzed GPS data and the bottom portion will scroll by with the raw GPS sentences from the GPS module.

gpsmon showing 10 satellites used for the position with HDOP of 0.88. This indicates a good position solution which means the time signals should be good as well. The PPS of 0.000000684 indicates the Raspberry Pi is only 684 nanoseconds off of GPS satellite time.

5 – edit config files

For chrony, add these two lines to the /etc/chrony/chrony.conf file somewhere near the rest of the server lines:

refclock SHM 0 refid NMEA offset 0.200
refclock PPS /dev/pps0 refid PPS lock NMEA

My entire /etc/chrony/chrony.conf file looks like this:

###### below this line are custom config changes #######
server 10.98.1.1 iburst minpoll 3 maxpoll 5
server time-a-b.nist.gov iburst
server time-d-b.nist.gov
server utcnist.colorado.edu
server time.windows.com
server time.apple.com

# delay determined experimentally by setting noselect then monitoring for a few hours
# 0.325 means the NMEA time sentence arrives 325 milliseconds after the PPS pulse
# the delay adjusts it forward
refclock SHM 0 delay 0.325 refid NMEA
refclock PPS /dev/pps0 refid PPS

allow 10.98.1.0/24 # my home network
###### above this line are custom config changes #######

###### below this line is standard chrony stuff #######
keyfile /etc/chrony/chrony.keys
driftfile /var/lib/chrony/chrony.drift
#log tracking measurements statistics
logdir /var/log/chrony
maxupdateskew 100.0
hwclockfile /etc/adjtime
rtcsync
makestep 1 3

Restart chrony, wait a few minutes, and verify.

sudo systemctl restart chrony

5 – verify the NTP server is using the GPS PPS

Right after a chrony restart, the sources will look like this (shown by running ‘chronyc sources’)

pi@pi-ntp:~ $ chronyc sources
210 Number of sources = 9
MS Name/IP address         Stratum Poll Reach LastRx Last sample
===============================================================================
#? NMEA                          0   4     0     -     +0ns[   +0ns] +/-    0ns
#? PPS                           0   4     0     -     +0ns[   +0ns] +/-    0ns
^? pfsense.home.fluffnet.net     0   3     3     -     +0ns[   +0ns] +/-    0ns
^? time-a-b.nist.gov             1   6     3     1  -2615us[-2615us] +/- 8218us
^? time-d-b.nist.gov             1   6     1     3  -2495us[-2495us] +/- 7943us
^? india.colorado.edu            0   6     0     -     +0ns[   +0ns] +/-    0ns
^? 13.86.101.172                 3   6     1     4  -4866us[-4866us] +/-   43ms
^? usdal4-ntp-002.aaplimg.c>     1   6     1     4  -2143us[-2143us] +/-   13ms
^? time.cloudflare.com           3   6     1     3  -3747us[-3747us] +/- 9088us

The # means locally connected source of time. The question marks mean it is still determine the status of each source.

After a couple minutes, you can check again:

pi@pi-ntp:~ $ chronyc -n sources
210 Number of sources = 9
MS Name/IP address         Stratum Poll Reach LastRx Last sample
===============================================================================
#x NMEA                          0   4   377    23    -37ms[  -37ms] +/- 1638us
#* PPS                           0   4   377    25   -175ns[ -289ns] +/-  126ns
^? 10.98.1.1                     0   5   377     -     +0ns[   +0ns] +/-    0ns
^- 132.163.96.1                  1   6   177    22  -3046us[-3046us] +/- 8233us
^- 2610:20:6f96:96::4            1   6    17    28  -2524us[-2524us] +/- 7677us
^? 128.138.140.44                1   6     3    30  -3107us[-3107us] +/- 8460us
^- 13.86.101.172                 3   6    17    28  -8233us[-8233us] +/-   47ms
^- 17.253.2.253                  1   6    17    29  -3048us[-3048us] +/-   14ms
^- 2606:4700:f1::123             3   6    17    29  -3325us[-3325us] +/- 8488us

For the S column, * means the source that is active. + means it is considered a good source and would be used if the current one is determined to be bad or is unavailable. The x shown for the NMEA source means it is a “false ticker”, which means it isn’t being used. In our case that is fine because the PPS source is active and valid. Anything else generally means it won’t be used.

In this case, chrony is using the PPS signal. The value inside the brackets is how far off chrony is from the source. It is showing that we are 289 nanoseconds off of GPS PPS time. This is very, very close to atomic clock level accuracy. The last column (after the +/-) includes latency to the NTP source as well as how far out of sync chrony thinks it is (for example, the 17.253.2.253 server is 12.5 milliseconds away one-way via ping):

pi@pi-ntp:~ $ ping -c 5 17.253.2.253
PING 17.253.2.253 (17.253.2.253) 56(84) bytes of data.
64 bytes from 17.253.2.253: icmp_seq=1 ttl=54 time=25.2 ms
64 bytes from 17.253.2.253: icmp_seq=2 ttl=54 time=27.7 ms
64 bytes from 17.253.2.253: icmp_seq=3 ttl=54 time=23.8 ms
64 bytes from 17.253.2.253: icmp_seq=4 ttl=54 time=24.4 ms
64 bytes from 17.253.2.253: icmp_seq=5 ttl=54 time=23.4 ms

--- 17.253.2.253 ping statistics ---
5 packets transmitted, 5 received, 0% packet loss, time 4007ms
rtt min/avg/max/mdev = 23.403/24.954/27.780/1.547 ms

For a full list of information about how to interpret these results, check here – https://docs.fedoraproject.org/en-US/Fedora/18/html/System_Administrators_Guide/sect-Checking_if_chrony_is_synchronized.html

6 – results

A day after the bulk of writing this post, I turned on logging via the .conf file and restarted chrony. It settled to microsecond level accuracy in 57 seconds. For the offset column, the scientific notation e-7 means the results are in nanoseconds. This means that for the 19:54:35 line, the clock is -450.9 nanoseconds off of the PPS signal. There is that e-10 line in there which says that it is 831 picoseconds off PPS (I had to look up SI prefixes to make sure pico came after nano! also I doubt the Pi can actually keep track of time that closely.). After the initial sync, there is only 1 line in the below log that is actually at the microsecond level (the others are all better than microsecond) – the 20:00:59 entry, which shows the clock is -1.183 microseconds off.

Things that affect timekeeping on the Pi

Thought I’d toss in this section for completeness (i.e. thanks for all the good info Austin but how can I make this even better?). There are a few things that affect how well the time is kept on the Pi:

  • Ambient temperature around the Pi – if you plot the freq PPM against ambient temperature, there will be a clear trend. The more stable the ambient temp, the less variation in timekeeping.
  • Load on the Pi – similar to above. Highly variable loads will make the processor work harder and easier. Harder working processor means more heat. More heat means more variability. These crystals are physical devices after all.
  • GPS reception – they actually making timing GPS chips that prefer satellites directly overhead. They have better ability to filter out multipathing and such. In general, the better the GPS reception, the better the PPS signal.

Conclusion

After running through the steps in this guide, you should now have a functional Stratum 1 NTP server running on your Raspberry Pi with microsecond level accuracy provided by a PPS GPS. This system can obtain time in the absence of any external sources other than the GPS (for example, internet time servers), and then sync up with the extremely precise GPS PPS signal. Our NTP GPS PPS clock is now within a few microseconds of actual GPS time.

Update 2024-01-19: RIP Dave Mills, inventor/creator of NTP – https://arstechnica.com/gadgets/2024/01/inventor-of-ntp-protocol-that-keeps-time-on-billions-of-devices-dies-at-age-85/

References

I read a ton on https://www.satsignal.eu/ntp/Raspberry-Pi-NTP.html and other pages on that domain over the years which has been extremely helpful in getting GPS PPS NTP going for my setup. There is a lot of background info for how/why this stuff works and many useful graphics. Another source is https://wellnowweknow.com/index.php/2019/12/27/how-to-ntp-a-raspberry-pi-4-via-gps-and-pps/ for a short and sweet (maybe too short) set of commands.