Hal, On 2021-01-08 16:17, Hal Murray wrote: > j...@luxfamily.com said: >> 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. > Is the back down direct or back through TDRSS? > > NTP likes symmetric delays. To first order, it doesn't matter how long it is > as long as it is symmetric. This may not be a first order problem. There is > a cutoff at 1 second total round trip time.
I only partly agree. NTP likes symmetric delays, but also static delays. It can to some limited degree handle a bit of jitter, but the basic two-way time-transfer calculation of NTP, and for that matter other time-transfer systems, actually assumes that the basic delay is fairly static. Further, it assumes that the clocks compared is fairly static. Frequency drift errors will easily convert over to detected time error, just as asymmetry. A shift in delay will cause the t1, t2 measure and then t3, t4 measure to measure over different delays as they are measured at different times, and thus different delays. As you calculate the TE value you will have a leakage there because of the time mismatch. So, there is secondary effects showing up for various reasons. Those are not a problem normally, but if you take NTP (or other) technology out of their environment, things like that may cause interesting problems. It may or may not be a problem, but one should be cautious enough to check on them. Some can be ignored, some can be compensated. Oh, and it is not NTP specific, PTP would have the same problem. The big difference is that PTP provide a delay compensation scheme for intermediary equipment, and if you know what you do, you can play some fun tricks. I have not had to adapt NTP, PTP or the time-transfer system I design on day-time to these conditions, but I would know where to look if I had to. The analysis is not that hard. 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.