rpi3 clock drift

[Ross Alexander]

On Fri, 29 Nov 2019, Peter Jeremy wrote:

> On 2019-Nov-27 22:20:50 -0700, Ross Alexander <rwa at athabascau.ca> wrote:
>> An Adafruit "ultimate gps hat" (typical commercial hyperbole, sigh.)
>> It's about $65 after you add the 40 pin molex connector, the
>> micro-whatever to SMA cable, and the external amplified patch antenna.
>
> You can get a uBlox 7 or 8 module with a patch antenna and PPS output
> for O($10).  It should be possible to wire the serial data to the PL011
> UART and preserve the serial console UART but you'd to write a driver.

Agreed.  The hat spares you the wiring and mounting challenge.  The
s/w issues are why I'm running the GPS box on Debian.  I do plan to go
back and try it again on that same h/w using 12-stable at some point.
I recall that some of the uBlox parts are designed for timing apps
rather than navigation - they would be the ones to go with.

> I have a metal roof and find putting the GPS module with a patch
> antenna near a window (or even near an external wall) is sufficient.

I have an identical (pi-2, ult hat, external patch antenna, debian)
system running at work where I can get only a view of half the sky and
it has significantly more jitter.  A broader sky view gives more
confidence in the position domain (more birds, and they are more
spread out), which translates into lower time jitter.  I'm arguing
from a very small sample size here, of course.  I would love to
have a larger population to test.

BTW, another *significant* source of jitter is the brand and age of
the sd/mmc card used.  As they age, the write speed decreases and
block write latency gets less uniform; this shows up as system clock
jitter in the loopstats.  After a few years (3 or 4), the box becomes
a complete falseticker and you need to replace the sd/mmc card.

regards,
Ross

============================================================================
Ross Alexander, (780) 675-6823 desk / (780) 689-0749 cell, rwa at athabascau.ca
                         54.71593 N 113.30835 W

     Order is simply a thin, perilous condition
     we try to impose on the basic reality of chaos.

        -- William Gaddis, _J R_
--
This communication is intended for the use of the recipient to whom it is addressed, and may contain confidential, personal, and or privileged information. Please contact us immediately if you are not the intended recipient of this communication, and do not copy, distribute, or take action relying on it. Any communications received in error, or subsequent reply, should be deleted or destroyed.
---