Re: [gpsd-dev] RFC 2783

[Gary E. Miller]

Yo Hal!

On Tue, 10 Mar 2015 12:57:54 -0700
Hal Murray <address@hidden> wrote:

> 
> > In Eric's writeup "Introduction to Time Service", he lists the 1PPS
> > via USB as having a deviation of up to 1ms vs. 1PPS via Serial at
> > only up to 5uSec. The USB GPS's I have will have THAT much latency
> > on the PPS? That's a factor of 200, USB vs. Serial!  If so, I may
> > shy away from using the PPS on the USB GPS and get another unit
> > from adafruit... 
> 
> USB is polled.  For old/slow USB, the polling interval is 1 ms.  I
> think it's 1/4 ms for newer, but all the GPS over USB that I've seen
> are old/slow.

No, USB 1.1 is scheduled, at least on the PL2303.  1024 slots.  So we
see in practice about +/1 600 uSec on the GR-601W.

That may seem poor, but is way better than I get with just NTP
over my local GigE net.

> If you are lucky, the "average" of several samples is 1/2 that.  If
> you are unlucky, the USB clock will be synchronized (or close to
> synchronized) to the PPS clock and all your samples will be at one
> end or the other of the sampling interval.

Yup.

> Geek alert.  The term is Hanging bridges.
>   http://www.leapsecond.com/pages/m12/sawtooth.htm

Yup, seen that, on paper, and in practice.

RGDS
GARY
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Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97701
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