[chrony-users] How to plot statistics to show tracking accuracy?

[Ed W <lists@xxxxxxxxxxxxxx>]

Hi, I'm getting very decent results with chrony and a usb gps.  
According to the statistics the std dev is around 1ms, which I might 
hope is slightly too high and that as low as 0.5ms is feasible?  I think 
that this might indicate that we can do better measuring the arrival 
time of the $ in the NMEA sentence, but now I need to collect some 
evidence for that

So my theory is that gpsd is measuring the timestamp of the end of the 
ZDA sentence (and working that back to get the initial timestamp) and 
this gives an additional 0.5-1ms jitter that might not be there if we 
measured the timestamp of the initial $.  If so then I might expect to 
see a jitter distribution which is positively skewed (every time the 
last character drops into a later 1ms read)

Can someone give some guidance on the correct statistics to plot to 
investigate this further?  I'm a little unsure on how to interpret the 
stats in /var/log/statistics.log also /var/log/tracking.log seems 
interesting?


On a related note, did anyone ever try and improve the time accuracy of 
USB serial reading by noting that serial jitter is lower than USB jitter 
and trying to interpolate character arrival time vs USB timestamps to 
try and figure out a kind of subsample accuracy? ie every time you read 
from the USB, note the timestamp and how many characters read - given 
that the usb freq isn't the same as the serial freq we might be able to 
improve our estimate of the exact time the intial $ character was emitted?


Thanks

Ed W


--
To unsubscribe email chrony-users-request@xxxxxxxxxxxxxxxxxxxx 
with "unsubscribe" in the subject.
For help email chrony-users-request@xxxxxxxxxxxxxxxxxxxx 
with "help" in the subject.
Trouble?  Email listmaster@xxxxxxxxxxxxxxxxxxxx.