[time-nuts] ISS NTP operation problems.

[Magnus Danielson]

Hal,

On 2021-01-08 16:17, 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?
>
> 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.

I only partly agree. NTP likes symmetric delays, but also static delays.
It can to some limited degree handle a bit of jitter, but the basic
two-way time-transfer calculation of NTP, and for that matter other
time-transfer systems, actually assumes that the basic delay is fairly
static. Further, it assumes that the clocks compared is fairly static.

Frequency drift errors will easily convert over to detected time error,
just as asymmetry.

A shift in delay will cause the t1, t2 measure and then t3, t4 measure
to measure over different delays as they are measured at different
times, and thus different delays. As you calculate the TE value you will
have a leakage there because of the time mismatch.

So, there is secondary effects showing up for various reasons. Those are
not a problem normally, but if you take NTP (or other) technology out of
their environment, things like that may cause interesting problems. It
may or may not be a problem, but one should be cautious enough to check
on them. Some can be ignored, some can be compensated.

Oh, and it is not NTP specific, PTP would have the same problem. The big
difference is that PTP provide a delay compensation scheme for
intermediary equipment, and if you know what you do, you can play some
fun tricks.

I have not had to adapt NTP, PTP or the time-transfer system I design on
day-time to these conditions, but I would know where to look if I had
to. The analysis is not that hard.

Cheers,
Magnus