[time-nuts] ISS NTP operation problems.

[Steven Sommars]

At the end of November a question
<https://lists.ntp.org/pipermail/questions/2020-November/041883.html>was
posed to the ntp.org list concerning NTP problems on the International
Space Station.
With William's permission I'm following up on the time-nuts list.

I analyzed NTP/IP packet captures provided by William and reported on
external visible behaviors.  The much simplified diagram is
        2 x Ground NTP(strat 1)  ---------------   ISS NTP server(strat 2)
-----(gigastor)------- ISS NTP clients(strat 3)
There is a ~600-700 msec RTT between the ground NTP servers and the ISS NTP
server.

As William describes, the clients are having trouble synchronizing with the
stratum 2 ISS NTP server.
This server is not working well.   Here is the UTC time offset seen from an
external packet capture device (Gigastor)
[The Gigastor is only approximately sync'd to UTC, hence the 4 second
offset]
[image: image.png]
The y-axis scale is seconds.
Note also that after each step the initial error is about -500 ppm.  When
the server is in alarm the error stays at -500ppm.
When not in alarm the server's clock frequency is changing in the wrong
direction with a resulting error of up to ~ -1500ppm.
Could this be an example of the Integral Windup problem mentioned recently
by PHK and Magnus?
Have others seen this behavior in NTP?

Getting additional diagnostic information from the ISS is quite difficult.
 Even simple changes (e.g., remove ntp.drift) require much planning.

Steve Sommars
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