[LinuxPPS] still some offset and jitter though using pps

[Nicola Berndt]

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