On Fri, Jan 8, 2021 at 11:40 AM Lux, Jim <j...@luxfamily.com> wrote: > > On 1/8/21 6:59 AM, Magnus Danielson wrote: > > 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. > > I don't know where the ground NTP server is, but if it's at HOSC at > Marshall Spaceflight Center, there's a fairly high performance dedicated > link to White Sands and Guam with fairly consistent latency. > > > (HOSC - Huntsville Operations Support Center, also the POIC - Payload > Operations and Integration Center) > > > >> 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. > > Indeed - after all, they also support VoIP and teleconferencing via the > Ku-band link and there's a whole raft of QoS rules and constraints. The > scheduling of the Ku-band link is pretty complex, because it needs a > high gain antenna on a gimbal that's on ISS, and there's a whole host of > constraints when there are visiting vehicles, etc. > > > > >> 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. > > That is probably harder than it seems. There's a lot of isolation among > systems on ISS - partly for safety, partly from history, partly from > institutional inertia. My payload on ISS (SCaN Testbed) had a > MIL-STD-1553 connection and a unidirectional Ethernet connection (out of > payload only). There's multiple GNSS receivers on ISS, but not all are > visible to an arbitrary payload - their output might get packaged up as > telemetry and store/forward sent to the ground via episodic > transmissions on the Ku-band system. One of the experiments on my > payload was to actually try to measure the time and position offsets > between our radio(which had S-band Tx/Rx and GPS receiver) and the > various time sources on the Station.
I must admit that I'm very surprised that GPS receivers worked and were able to compute a fix. The majority of GPS receiver chipsets have altitude and speed limits built in, both because of assumptions/discarding pathological results, but also because of ITAR and similar regulations. Were these special / licensed receivers which didn't have the "Erk, I think I'm on an ICBM" logic? W > > It is exceedingly unlikely that there is a 1pps signal available for > distribution on board - it's just not something that someone would have > written a requirement for. The folks designing Station are not time-nuts > - the idea of a "house frequency/time standard" would not have occurred > to them, except perhaps in the context of a limited subsystem. > > The best bet is probably hooking into the "Broadcast Ancillary Data" > (BAD) which does get fed to a lot of subsystems and experiments on > Station in various forms. It has current (predicted) position and time > (Flight Dynamics Facility at GSFC calculates where ISS is going to be, > that gets uplinked, and then broadcast across Station) with some sort of > time hacks. > > Station (writ large, not just the part in space) is kind of an unusual > place to work - think of it as a village or small town of several > thousands of people, each with a specialization and some knowledge of > what their neighbor does, but very few with details about the whole > thing. And because it's a thriving, but isolated, community, they speak > a different language. And the overall architecture was determined in the > 1970s and has been substantially modified over the years since then, but > still has a lot of ties back to "the way it was done". I used to > liken trying to find out stuff to being dropped on the edge of a small > town in France, and you don't know French, but you do know some Spanish, > and you have to find the person you're looking for by asking questions > and being handed off from one person to the next. Once you're "in the > system" you can get stuff done pretty easily, but oh wow, if you are > new, or trying to do something "different" it can be quite the > adventure. I'm glad I did it. I'd rather not do it again. > > > > > > > _______________________________________________ > 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. -- The computing scientist’s main challenge is not to get confused by the complexities of his own making. -- E. W. Dijkstra _______________________________________________ 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.