Hi Jim, On 2021-01-08 15:06, Lux, Jim wrote: > On 1/8/21 12:24 AM, Poul-Henning Kamp wrote: >> -------- >> 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. > Not a trivial path. Then again, the connection to White Sands or Guam also comes into the path. > 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. > Beyond that, exactly how high priority it has in the scheduling of buffers as traversing that path, will be very relevant. > This is a propagation path that I suspect NTP is just not designed to > deal with. Well, I wonder if NTP over that path is even the best solution. Taking time off a GPS/GNSS receiver onboard the ISS would be a significant improvement. Just having the PPS would help immensly. > > And, oh yes, getting diagnostic information or changes is quite the > tedious process, taking many days, typically.
For good and bad. Cheers, Magnus _______________________________________________ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.