[gpsd-dev] Draft Stratum 1 Microserver HOWTO is up

[Gary E. Miller]

Yo Hal!

On Sat, 21 May 2016 03:11:45 -0700
Hal Murray <hmurray at megapathdsl.net> wrote:

> That doesn't make sense to me.  It's partly true, but the other part
> is important enough that you can't stop there, but you did.  Here is
> a dump of that area.

For a Howto you pick one way, a simple way.  SHM() is it.

> refclock 20 with fudge to automagially use the PPS has the hard to
> monitor problem.

Yup.

> The combination of refclocks 20 and 22 have 2 slots for their two
> hats.  But the PPS slot needs help to number the seconds.  It gets
> that from a driver marked prefer.  Normally, you can mark the serial
> driver as prefer.

So now we have prefer on a crappy NMEA clock, Ugh.

> The results from SHM and JSON are identical.

Uh, no.  Unless you use the undocumented +128 mode in the JSON it duplicates
the problems in refclock 20.  It intentionally duplicated the problems 
in refclock 20.  Until the undocumented mode is made the one mode the
JSON refclock is unuseable.


> Note that we need fudge factors for use with both ntpd refclocks and
> gpsd.  They may be different.

Never seen that case for a HAT like object.  Some of the USB and Serial
fudges are already builtin to gpsd, maybe they should be shouwn to the
user.  Maybe the KPPS in gpsd takes a few microSec off the PPS delay and
similar jitter from PPS.

Worrying about the fudge factor on the PPS is giling the lily.

> Some table of HATs should include a column for the chip and a link to
> the data sheet.

Or a quick tutorial on howto find the valid fudge for the NMEA from
comparing it to the PPS.

> Do you want this document to cover the GR-701W?

Nope, a GR-701W tutorial could be much more general, and of course yeilds
seriously inferior results.

> # uBlox, 701W, USB with PPS
> if [ ! -e /dev/pps1 ]; then
>   ln -s /dev/ttyUSB0 /dev/gps1
>   # setserial /dev/ttyUSB0 low_latency
>   ldattach 18 /dev/ttyUSB0 & # gets /dev/pps1
>   sleep 1
> fi

gpsd already does the ldattach.  Another reason to use gpsd instead of
the poor native ntp refclocks.

> That gets pps1 and uses gps1 because it's running on a system that
> already has a HAT which is using gps0 and pps0.

Maybe an appendix, to show the user how much better the HAT is over the 
GR-701W.

> PPS over USB will be more accurate with a fudge factor.

Gack, no.  The USB delay is much less than the sample interval on the
USB 1.1 bus.

RGDS
GARY
---------------------------------------------------------------------------
Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
	gem at rellim.com  Tel:+1 541 382 8588
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