Re: Timing via serial port

[Michael J. Tubby B.Sc. MIET]

    Wire the PPS signal from your GPS through an RS232 driver to the DCD
    input of your UART and use GPSD's shared memory with NTP:
    
    root@ns0:/home/mike# ntpq
    ntpq> peers
         remote           refid      st t when poll reach  
      delay   offset  jitter
    ==============================================================================
    xSHM(0)          .GPS.            0 l    4   16  377   
      0.000  -141.19   8.240
    *SHM(1)          .PPS.           
          0 l    2   16  377    0.000   -0.002   0.024
    +ns1.thorcom.net .PPS.            1 u   14   64  377   
      0.190   -0.016   0.043
    -2a02:8010:7010: .PPS.            1 u   33   64  377  
      23.795   -1.260   0.112
    #139.143.5.30    139.143.45.169   2 u  133   64   26  
      10.569   -0.002   0.126
    #139.143.5.31    139.143.45.169   2 u   23   64  377  
      10.192   -2.181   0.122
    -ntp1.ja.net     .MRS.            1 u   31   64  377  
      15.780   -0.968   0.099
    +ntp2.ja.net     193.62.22.74     2 u   12   64  377  
      14.857   -0.149   0.044
    #swale.mc.man.ac 193.62.22.66     2 u   25   64  377  
      15.266    0.279   0.014
    -winn-dmzutil-1. 10.178.5.138     2 u   31   64  377  
      19.584   -0.504   0.044
    -ntp0.cam.ac.uk  197.205.52.7     2 u   27   64  377  
      13.692   -0.750   0.042
    -ntp1.cam.ac.uk  197.205.52.7     2 u   42   64  377  
      13.478   -0.797   1.674
    -ntp3.cam.ac.uk  197.205.52.7     2 u   38   64  377  
      13.936   -0.388   0.076
    #ntp0.cis.strath 131.188.3.222    2 u   54   64  377  
      18.535   -0.627   0.055
    -2001:67c:1560:8 131.188.3.220    2 u   51   64  377   
      8.268   -0.662   0.109
    ntpq>
    
    and the results are quite good.
    
    Mike

On 30/09/2020 13:19, Kai Harrekilde-Petersen wrote:

I've measured the delay between the 1PPS signal and the TX uart output on the NEO-M9N board with a logic analyzer. I would need to find the or re-measure the data for you tonight.

Front edge of the UART frame is the most precise. Rear edge depends too much on the length of the message.

As I recall, the latency from the PPS to the front edge increases(!) with increasing update rate. I did not check for the variation between the 1PPS and the start of the UART message.

Regards,

Kai

---

From: "O'Connor, Daniel"
Sent: Wed Sep 30 14:08:21 GMT+02:00 2020
To: Hal Murray
Cc: gpsd-dev@nongnu.org
Subject: Re: Timing via serial port

 On 30 Sep 2020, at 18:28, Hal Murray <hmurray@megapathdsl.net> wrote:
 Has anybody tested various GPS devices to see how good the timing via the 
 serial port is?
 
 Some of the NMEA sentences have sub-second time stamps.  Are any of them 
 useful?
 
 Has anybody looked at whether the front of the sentence (or burst) has better 
 timing than the end?

I would say it depends how tight your requirements are.
I setup USB serial to a GPS engine and fed NTP with it and it seemed quite good - certainly NTP was happy to use it and all the numbers look good.

I imagine it would also depend a lot on your hardware and OS as to what sort of jitter you would get.

--
Daniel O'Connor
"The nice thing about standards is that there
are so many of them to choose from."
 -- Andrew Tanenbaum

--

Michael J Tubby B.Sc. (Hons) MIET / Technical Director
Email: mike.tubby@thorcom.co.uk
Direct: +44 (0)1905 752892
Mobile: +44 (0)7973 225144

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