[time-nuts] ISS NTP operation problems.

[Lux, Jim]

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.

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.