| Re: [chrony-users] gpsd, pps and chrony |
[ Thread Index |
Date Index
| More chrony.tuxfamily.org/chrony-users Archives
]
On Tue, 5 Apr 2011, Ed W wrote:
Hi
On 05/04/2011 12:44, Miroslav Lichvar wrote:
(They also have a 20hz GPS with decent sensitivity and a GPS/GLONASS
hybrid board...)
And exactly what would a 20Hz buy you?
20 times more samples could improve the stddev of the calculated mean up to
factor sqrt(20). It would also allow shorter polling interval.
Just to be clear - I think the GPS modules are quite accurate as
standard, but the 20Hz version is one model, the version with highly
accurate PPS is a separate model and not known to put out a 20Hz PPS signal.
However, I agree that in principle given that the largest error should
be the pulse edge detection (and random measuring delay) then you
potentially have yourself an error which is largely gaussian and
additional datapoints are beneficial
I think the smearing of the pulse edge by various capacitors, which is a
systematic effect is a bigger source of edge detection error. The software
delay in the task switch from the current calculation to the interrupt
servicing is a systematic error, and the reading of the clock is a systematic
error. More data does not decrease systematic errors, and my suspicion is that
they dominate. Now at what level (in terms of length of time) they dominate, I
am not sure. One needs to have some way of triggering the interrupt that can
be timed by the same computer that then reads the interrupt.
On the parallel port that is easy, but if the hardware limits the time scale
due to the above type delays, that is harder.
Bill, is the GPS sawtooth not at a level of around the 10s of ns?
Cheers
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.
--
William G. Unruh | Canadian Institute for| Tel: +1(604)822-3273
Physics&Astronomy | Advanced Research | Fax: +1(604)822-5324
UBC, Vancouver,BC | Program in Cosmology | unruh@xxxxxxxxxxxxxx
Canada V6T 1Z1 | and Gravity | www.theory.physics.ubc.ca/
---
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.