[Thumbgps-devel] Project clarification

[Eric S. Raymond]

Ron Frazier (NTP) <timekeepingntplist at c3energy.com>:
> I'd like to get a further minor clarification on the project goal.
> Do you want your time reading to be within plus or minus 1 ms of
> UTC, for a total range of 2 ms?  Or do you want it to be within plus
> or minus 500 us of UTC, for a total range of 1 ms?

The latter. Total range of 2ms.

This is believed to be achievable because the data sheet for the ZTI
Z050 (the only existing USB thumb GPS with PPS) promises 1ms in the way
you seem to mean it.  

Their claim is credible given that PPS is guaranteed accurate within
50ns. The default USB polling rate of 125Hz implies polling latency of
8msec, but it's possible to jack that up by a factor of 8 within the
USB spec, and I'm certain that's what ZTI's timing software is doing.
That would produce a polling interval of 1ms, neatly matching their
accuracy claim.  The additional 50ns jitter (6 magnitudes down) and
the handful of us required for processing on the host side (3
magnitudes down) would vanish compared to this.

If I'm misunderstanding ZTI's accuracy claim, it's possible we might
get the tighter 500us range.  We'll test.

Either figure will do for Dave's intended use, which is network
delay tomography where the feature size is likely to be about 10
milliseconds.
-- 
		<a href="http://www.catb.org/~esr/">Eric S. Raymond</a>