[ntp:questions] Realistic Performance Expectation for GPS PPS fed ntpd jitter

William Unruh unruh at invalid.ca
Thu Oct 8 00:13:47 UTC 2020

On 2020-10-07, Andreas Mattheiss <please.post at publicly.invalid> wrote:
> Hello,
> I wonder what's a realistic ballpark for the jitter I can expect when
> feeding a GPS PPS into ntpd?

around a microsecond or less. 
> My setup is: a cheap u-blox NEO6M has its PPS output connected to a MAX232
> level shifter, connected to a true serial port on my desktop PC. I use it

Not really needed since almost all serial cards will handle TTL level
with not problem. 

> at the same time for daily work, so no designated ntp server. The GPS

You need to set the seconds somehow. Are you doing that via nmea from
the GPS? 

> antenna is sitting inside, close to the window, with a building opposite -
> suboptimal view of the sky. I do feed RTMS corrections to the GPS though
> (ionospheric corrections etc.). The GPS is set to "stationary" mode, so
> it's probably set up as good as it gets.
> The jitter values I get do, sorry, jitter. I guess it's a lot to do with
> the poor view to the sky, mainly. PPS from GPS is claimed to be accurate
> to say 10's of ns; my jiitter values are around 10 micro s approximately,
> on avarege, for a well settled system set up as described. They do zoom
> up, occasionally, to 60 micro s though. I would assume that it's
> unrealistic to expect ns accuracy from ntpd on a non-designated system.

It sounds to me like you are running from the wrong edge of the signal.
Is your level shifter also and inverter? 
> What do you think, is my 10 micro jitter within experience? While I am
No.  I get using chronyd   (report from chronyc)
MS Name/IP address         Stratum Poll Reach LastRx Last sample
#* PPS0                          0   4   377    24  +2035ns[+2458ns] +/-  776ns

Name/IP Address            NP  NR  Span  Frequency  Freq Skew  Offset  Std Dev
PPS0                       24  11   367     +0.000      0.016     +1ns  1972ns

Of course chrony has long been know to give much better control of the
clock than does ntpd

> writing this I just realised a post from David Taylor
> (<qgmgcu$2ad$1 at dont-email.me>) showing 
>       remote           refid      st t when poll reach   delay   offset
> jitter
> o127.127.22.1    .uPPS.           0 l    -   16  377    0.000    0.026
> 0.013
> Pretty much what I get.
> Thanks, regards
> Andreas

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