On 1/8/21 7:17 AM, 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?
Through TDRSS both ways.
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.
But the time delay isn't symmetric. The link isn't symmetric (data rate
wise) and the traffic is different. Think of it as like a cable modem -
skinny up, fat down, for the most part. Lots of scheduled (in a "days
ahead" sense) traffic interspersed with "real time" traffic. In usual
space comm architecture, the uplink and downlink are totally separate
systems, operated by different groups of people.
The actual TDRSS links, by the way, are bent pipe translators - A signal
is modulated with the data on the ground, sent up through TDRSS, and
it's demodulated on ISS (or vice versa). So the "light time delay" for
that part of the system is entirely determined by orbital mechanics for
a link that's somewhere around 85,000 km +/- several thousand km.
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