[time-nuts] ISS NTP operation problems.

[Jim Palfreyman]

Is this a new problem or has it being happening since day 1?

Jim Palfreyman


On Fri, 8 Jan 2021 at 5:42 pm, Steven Sommars <stevesommarsntp at gmail.com>
wrote:

> 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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