Re: My Computer Drifts!


jdow
 

On 20220604 22:30:59, David J Taylor via groups.io wrote:
On 04/06/2022 20:03, jdow wrote:
My configuration here involves a Linux router that does a lot of other work and
a largish set of PCs around the house, mostly here in the computer room. They
range from PLCs to a rather hypertrophied gaming machine use for multiple VMs,
development, and the like. (Visual Studio is THE preeminent game in the known
universe.)

When speaking of accuracy you have to deal with a pot load of different entry
points for errors. The ones NTP monitors and considers in its design are delay,
offset, and jitter. NTP, as embodied in the canonical Univ of U of Delaware
version as amended up to date, measures the path delay in such a way as to get
a reasonable estimate of path delays to multiple sources. The path to and the
path from the server may be slightly different. You can see that in the offset
value. NTP makes a best estimate for time and publishes the time offsets. It
also passes this information along to the consumers. Offset here varies from
-5.3ms to 5.9ms. The chosen clock and the secondary are coming in at + and - 2
ms. The chosen clock is .NIST.. The secondary clock is .GPS. I could setup a
pure GPS configuration here if I hooked up the PPS from my Trimble to the
router. But, it's not broke so I don't fix it. The final figure, jitter, gives
a good estimate of the link quality between the consumer and the hosts. The two
good ones are coming in at under 6 ms of jitter. All these numbers vary with
time of day. For my current needs this is good enough. If I needed better I'd
have to go to a formal PPS hookup. (None of the USB crap, either. They do look
like fun to play with, though.)

For my main PC I am using a very old tool I have had for a long time. Mr.
Horsley created an open sores partial implementation of ntp that I use to tweak
my PC's clock in 1ms steps - since at least early XP days through today. I have
it logging clock corrections. Most are 0 ms with the occasional 1ms correction.
I ran Symmetricom's ntp for awhile. But I went back to Tom's tool. It ain't
broke and I have a much older view on the urgency to make perfect than I used
to have.

(I am one of the persons responsible for the GPS phase 2B satellite segment's
extremely accurate intentional inaccuracy for whatever worth that may be.)

If ntp accuracy is insufficient at least one more protocol exists that refines
to microseconds or finer. It is/was used by radio astronomers to get them in
the ballpark.

{^_^}

Certainly tying the PPS into the router would increase performance.  Typically
here I see up to 02 ms jitter from the internal servers, and under 2 ms from
the pool servers (UK).  They all show an offset of 2-3 ms, but this with a
cable modem link that's 200 Mbps down, 20 Mbps up, hence gross asymmetry.

During the development of Windows NTP (by Dave Hart) I used to monitor much
more that I do now.  The offset graphs are enough.

As you know, reference NTP alters the click rate rather than stepping the
clock.  Since Windows 8.1 NTP uses the more accurate time instructions which
are in 100 nanosecond units, and this has improved things a lot.

Thanks for contributing to the Selective Availability work - perhaps one of the
factors which led to the Galileo constellation which has meant more satellites
for everyone!

I never mentioned that potential to the AF guy. But, then, that was way off in the future when he was only a little way off in the future with his thinking. Col. Parkinson was a different story. I still hold that grudge. I suggested some techniques for the prelaunch test and calibration receiver that he and his consultants decided were not suitable. Then when the contract for the next test receivers came out those consultants used my ideas to compete against my company, Rockwell at the time. That seemed REALLY unethical to me.

The precision timer use has me vaguely inclined to use the latest ntp toy since I suspect it can diddle that clock timer to increase potential accuracy tremendously - if I can get the PPS interrupt working without (huge) delays. (with huge in terms of nanoseconds.) But my platter has a bunch of other stuff first. Phase tracking signals on HF at night has some potential to be interesting for propagation calculations.)

{^_-}

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