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Re: [gpsd-dev] Disturbing numbers

From: Andy Walls
Subject: Re: [gpsd-dev] Disturbing numbers
Date: Sun, 03 Nov 2013 19:57:09 -0500
User-agent: K-9 Mail for Android


Not feeling contradicted at all. :)

I was too lazy to go through the gory detail of how offset, jitter, frequency error, and wander might evolve as time went on. And all in a way that wouldn't lose the reader.

I like your explanation.


Greg Troxel <address@hidden> wrote:

Andy Walls <address@hidden> writes:

Sounds reasonable for startup. I wouldn't expect it at steady/stable

ntpd prefers to slew clocks into alignment.

system clock frequency will change, and ntpd will have to adjust it, a
bit until your computer is thermally stable.

Do keep in mind, that compared to a decent PPS, it is the ntpd
disciplined system clock which is drifting.

All true, and I don't mean to contradict Andy

What I'd expect is that because the frequency error estimate that NTP
uses ("drift") isn't right when you first start NTP, there will be a
phase error that persists while the frequency is estimated. So if your
clock is a litle slow, then it will keep getting behind, and the
positive offset will cause NTP to adjust the phase forward and also
increase the frequency offset error. After a day or so or maybe less
the frequency estimate will be very close and there won't be a
persistent offset. This is all normal.

Sent from my Android phone with K-9 Mail. Please excuse my brevity.
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