{"id":2954,"date":"2023-11-30T18:50:58","date_gmt":"2023-11-30T17:50:58","guid":{"rendered":"https:\/\/www.ridgesolutions.ie\/?p=2954"},"modified":"2023-11-30T19:39:51","modified_gmt":"2023-11-30T18:39:51","slug":"ntp-with-gps-and-pps-working-without-network-clock","status":"publish","type":"post","link":"https:\/\/www.ridgesolutions.ie\/index.php\/2023\/11\/30\/ntp-with-gps-and-pps-working-without-network-clock\/","title":{"rendered":"NTP with GPS and PPS working without network clock"},"content":{"rendered":"

Is it possible to have a PPS disciplined NTP server without a network reference clock or network connection?<\/p>\n

While doing some camera synchronisation and timestamp testing, I setup a Raspberry Pi NTP server with PPS using an Adafruit<\/a> GPS hat so that I could sync my cameras’ time to NTP, trigger them using the PPS signal, generate UTC timestamps and check the timestamps against the expected UTC second boundaries.<\/p>\n

As part of this the question once again arose – Can I configure an NTP server with GPS time and PPS without having to configure a network reference clock along with the GPS and PPS sources?\u00a0 This was important as my test setup didn’t have access to an external network, and yet I wanted typical NTP offsets of < 100us.<\/p>\n

Well with quite a bit of messing I finally got it working, at this stage there are lots of pages<\/a> that detail how to setup a Pi as an NTP server with PPS, some mention that you must have a network reference and some mention that you don’t – I had to do a lot of digging and a lot of messing to get to the bottom of things.<\/p>\n

Amusingly, NTP configuration is something that ChatGPT is simply terrible at – don’t use it for this, it will just waste your time!<\/p>\n

I had lots of problems, but the 2 most persistent was:<\/p>\n

    \n
  1. The PPS source being marked as a false ticker (with an x to its left in the ntpq -p output)<\/li>\n
  2. The PPS source not being referenced at all, “when<\/strong>” would remain at “-“<\/li>\n<\/ol>\n

    For ages, I lurched between problems 1 and 2…<\/p>\n

    Eventually I got it working for my setup.\u00a0 The trick to get it working for me is that:<\/p>\n

      \n
    • I needed 3 reference clocks in my setup for the PPS source to be used: PPS time, GPS time and a 3rd reference, normally this is a network NTP server but I found that configuring the local clock (127.127.1.0) worked instead.<\/li>\n
    • I needed to be careful with my stratum settings, what worked for me was to put the local clock at stratum 15, GPS at stratum 10 and PPS at stratum 0.<\/li>\n
    • Marked the PPS source as prefer <\/strong>(and not the GPS source as is often recommended?)
      \n<\/strong><\/li>\n<\/ul>\n

      My working configuration:<\/p>\n

      tos mindist 0.500\r\n\r\n# Local Clock\r\nserver 127.127.1.0\r\nfudge 127.127.1.0 stratum 15\r\n\r\n# GPS\/NMEA Source from Hat\r\nserver 127.127.28.0 minpoll 4 maxpoll 4 prefer iburst\r\nfudge 127.127.28.0 time1 0.000 refid GPS stratum 10\r\n\r\n# PPS Source, stratum 0 and prefer\r\nserver 127.127.22.0 minpoll 4 maxpoll 4\r\nfudge 127.127.22.0 refid PPS stratum 0 flag3 1 flag4 1 prefer\r\n<\/pre>\n
      I deliberately left the time offset for the GPS source at 0.0 rather than trying to chase down the time packet offset so that I could see an “honest” offset value in the ntpq output:<\/p>\n
      \u00a0remote refid st t when poll reach delay offset jitter\r\n==============================================================================\r\nLOCAL(0) .LOCL. 15 l 58m 64 0 0.000 0.000 0.000\r\n*SHM(0) .GPS. 10 l 16 16 377 0.000 -106.45 1.145\r\noPPS(0) .PPS. 0 l 15 16 377 0.000 -0.001 0.001\r\n<\/pre>\n","protected":false},"excerpt":{"rendered":"
      Is it possible to have a PPS disciplined NTP server without a network reference clock or network connection? While doing some camera synchronisation and timestamp testing, I setup a Raspberry Pi NTP server with PPS using an Adafruit GPS hat so that I could sync my cameras’ time to NTP, trigger them using the PPS […]<\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[301],"tags":[295,358,373],"class_list":["post-2954","post","type-post","status-publish","format-standard","hentry","category-by-kevin-godden","tag-gps","tag-ntp","tag-pps"],"aioseo_notices":[],"aioseo_head":"\n\t\t\n\t\n\t\n\t\n\t\n\t\n\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t