On 1/8/21 12:24 AM, Poul-Henning Kamp wrote:
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Steven Sommars writes:
There is a ~600-700 msec RTT between the ground NTP servers and the ISS NTP
server.
How stable is that ?
Is there a lot of sample-to-sample jitter ?
Have they clamped the poll-rate on the S2 ?
If the pathway is like the ones to/from ISS that I am familiar with,
they're using the Ku-band or S-band link through TDRSS. In both cases,
the signal has to go from White Sands (or Guam) up to TDRSS, which is in
GEO, and then back down to ISS. There are also handoffs between, say,
TDRSS W and TDRSS E, where there will be a gap in comms, and then it
will resume, with a different light time delay.
There will also be some delays in translating the IP packets in and out
of the data streams, which encapsulate IP datagrams in some other packet
form (I can't remember if they're using CCSDS AOS or something else, but
there's a fair amount of encapsulation and segmentation going on to put
the IP traffic into a virtual channel). There could be delays in the
processing at HOSC that change during a pass, depending on their
buffering strategy.
This is a propagation path that I suspect NTP is just not designed to
deal with.
And, oh yes, getting diagnostic information or changes is quite the
tedious process, taking many days, typically.
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