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.
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