[LinuxPPS] Experience with current linuxPPS kernel patch.

[Miroslav Lichvar]

On Thu, Mar 04, 2010 at 01:47:31PM +0100, Bernhard Schiffner wrote:
> Am Donnerstag, 4. März 2010 12:51:38 schrieb Miroslav Lichvar:
> > 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.).

I've read somewhere that a limit for GPS receivers is around 150ns,
unless it has its own OXCO where it can go to low nanoseconds.

If the high jitter was caused by too low frequency of the GPS chip or
bad implementation, maybe it would be visible in offset distribution
plot as an abnormality?

For last 120000 samples I'm seeing this:
http://fedorapeople.org/~mlichvar/tmp/18xlvc_errdist.png

> 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.

Unfortunately I don't have access to such equipment.

> 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.)

It's only half of the sky. gpsd reports that 9 satellites are visible
and 8 are used. In the specs the 1us accuracy actually seems like an
upper bound:

1us accuracy for all conditions in which the GPS 18x LVC or GPS
18x-5Hz has reported a valid and accurate position fix for at least
the previous 4 seconds.

-- 
Miroslav Lichvar