[LinuxPPS] Experience with current linuxPPS kernel patch.

[Bernhard Schiffner]

Am Donnerstag, 4. März 2010 12:51:38 schrieb Miroslav Lichvar:
> On Wed, Mar 03, 2010 at 08:03:55PM -0700, clemens at dwf.com wrote:
> > I still have the feeling that something is not quite right (but mabe
> > elsewhere in
> > the kernel code) since the accuracy is just about 1us.  That is, the
> > 'fuzz' of the timestamps around the mean is just about 1us.  I would
> > expect a factor of x10 better than that, and would not be surprised if
> > there is not still some bad ns <-> us conversions in the kernel.
> 
> What is your PPS source?
> 
> I'm using GPS 18x LVC and I'm seeing about 1us dispersion too. But the
> specs say it's accurate only to 1 us, so I'm not sure if I can expect
> anything better from it.
> 
It's difficult to state, what's the reason.
Most receivers use a programmable counter to form the PPS-signal and keep it 
aligned to the sky-segment. (Divide by x normaly and by x+/-1 in case of a 
correction). Maximum possible accuracy here is one cycle of the GPS-"CPU"-
Clock. In my case this is about 10 MHz resulting in a "regular" 100 ns jitter 
of PPS. But this is a case where developers paid attention on these details.
It's possible to be far worse (use of 16 bit counters only etc.).

The only way to state, what's kernel and what's GPS is to compare pulses by 
scope or counter to a _good_ local quartz.

Other question: How close to (a half sphere's) 2Pi angle does your GPS antenna 
see the sky?
(If it's only 1Pi as from a normal window, 1µs is very good.)


Bernhard