Re: [gpsd-users] Enable PPS Echo

[Gary E. Miller]

Yo Reto!

On Tue, 5 Apr 2016 16:17:04 +0000
Reto Zbinden ST <address@hidden> wrote:

> I'm running Debian with gpsd and ntpd on an older ALIX board as the
> time source for my local network. This setup is working well for
> quite some time and the clock is kept to within a few microseconds of
> the 1PPS reference.

Cool.

> But if the time is compared to some reliable public stratum 1 servers
> (e.g. metas.ch), it seems that the local clock is always a few
> milliseconds behind these servers.

I think you have that backwards.  Trust your PPS.

Your measurement of the stratum 1 servers is off by a few milliseconds.
Stop worrying about the length of your PPS wire (1 nano Sec per 9 inches)
and more about your network latency and asymmetry.

One example of a likely error, if you are on a cable modem then you
upload speed is much slower than you download speed.  Even if you speeds
start off symmetric, you are likely downloading more data than uploading and
thereby making your link seem asymmetric.  This confused the ntpd
network protocol which assumes symmetric link speeds.

Also think of all the buffering in you host, your switch, your router, your
ISPs router, etc. etc.

> But I
> couldn't figure out how the echo function can be activated.

Few GPS have this, and when they do it is very model specific.

You best bet is to borrow a portable calibrated and traceable time
source and compare it to your PPS.

RGDS
GARY
---------------------------------------------------------------------------
Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
        address@hidden  Tel:+1 541 382 8588

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