[time-nuts] ISS NTP operation problems.

[Lux, Jim]

On 1/8/21 7:17 AM, Hal Murray wrote:
> jim at 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.