[LinuxPPS] still some offset and jitter though using pps

[Nicola Berndt]

Nicola Berndt schrieb:
> Hi,
> 
> I (finally!) sucessfully got linux-pps to work together with ntpd. Very 
> good so far!
> 
> Now I have some issues, I'd like to solve:
> 
> I have a u-blox EVK-5T evaluation kit 
> (http://www.u-blox.com/products/supportproducts_ublox5.html) wich 
> outputs a pps-signal via rs232 ack as well as standard nmea via usb.
> Both are seen and read by ntpd.
> When I start up ntpd -g -X it synchronizes pretty quickly but leaves me 
> with some offset and jitter. The offset is in the 20 ms range and the 
> jitter varies from 0.2 - 1.5 ms.
> 
> Given the pps-timesource I don't understand the offset. Why doesn't ntpd
> correct this error when setting the time for the first time (I ran -g 
> and -X)? I can see it getting smaller and again bigger wich is very 
> confusing.
> 
> My goal is to build a box that has accurate time in the least possible 
> time after startup, because we will use it to slave a couple of devices 
> built into an airplane. For that reason there is no way for us to run a 
> 24/7 server, since we fly on demand and not regularly.
> 
> With best regards,
> ../nico berndt
> 


Hi,

I figured the -X option was not what I wanted, so I dropped it.
Then I ran a simple test, to see what's going on: I manually set the 
pc-clock wrong for 3 minutes and had ntpd set the time and watched with 
watch -n1 ntpq -p. When the time was set initially, I had a 0.000 ms 
offset for about two seconds wich then jumped to 75.something ms and 
/grew/ over 125 ms.

I totally don't know what to make out of this...
I really hope, some of you do.

With best regards,
../nico berndt