Re: [gpsd-users] PPS MID 52 (0x34) working?

[Alexander Carver]

On 6/4/2012 15:33, Gary E. Miller wrote:
> Yo Alexander!
>
> On Sun, 03 Jun 2012 11:58:34 -0700
> Alexander Carver<address@hidden>  wrote:
>
> > I have a question about the SiRF PPS message MID 52 and whether it's
> > actually working in gpsd 3.4 (haven't quite updated to 3.6 yet).  I'm
> > playing around with turning MID 52 on and off and have noticed that I
> > get less wander in the offset to the SHM that is sent over to ntpd.
>
> I suggest to just totally ignore the MID 52.  Its accuracy compared
> to the PPS is 10,000x worse.
>
> > With MID 52 on, my offset wanders +/-40ms.  With MID 52 off, the
> > wander is reduced to about +/-10ms.
>
> Big deal.  Your PPS should be just a few micro Sec jitter, thus the
> uselessness of the MID 52.

It's interesting that you say it's useless when gpsd's code turns it on 
at startup (unless given the -b flag):

        gpsd_report(LOG_PROG, "SiRF: Enabling PPS message...\n");
        (void)sirf_write(session, enablemid52);
        (void)usleep(3330); /* guessed settling time */

In my case the lower wander is actually a big deal because it was 
driving ntpd crazy.  The SHM offset would be all over the place and 
cause ntpd to swing the clock.

> Just be sure your NMEA is running faster than 4800, preferably 38,400
> or better.

The SiRF messages are coming in at 38,400 now.  Most of the time it's 
idle with only five messages per second and one extra message every few 
minutes (the visible list message MID 13 is the one that typically pops 
up).