[time-nuts] ISS NTP operation problems.

[Magnus Danielson]

Jim,

On 2021-01-08 17:31, Lux, Jim wrote:
> 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.
Make sense. The serialization delay scales the prioritation/scheduling
delay differently. Make sense. It's just like a modern microwave link.
Well, it is a modern microwave link, just a bit of different design
parameters.
>
> 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.

Bent pipe is good, not too much delay and delay variations.

So, you have about 1 earth radius in difference then, from when the ISS
turn up on the horizon, to where it is on the same longitude as the
TDRSS and then as it disappears over the horizon. Depending on exactly
where it is for the NTP packet to be sent to the ISS and then the reply
actually gets sent back, this delay will be different, except only when
ISS transition just over the same longitude and you have a bit of luck
in timing.

Makes sense.

Thanks for the many details.

Cheers,
Magnus