-----Jim wrote-----

I wouldn't actually think there's a Cs on ISS.  What purpose would it serve?  
We as time-nuts think "of course you'd have a precise source of time", but 
really, there's not much need for timing on ISS on a scale smaller than 
seconds, if that.  NTP to timestamp files, for instance.

You are probaly right about the actual situation. However there is the ESA ACES 
project idling since years without being launched yet:
"ACES is an ESA ultra-stable clock experiment, a time and frequency mission to 
be flown on the Columbus module of the ISS (International Space Station), in 
support of fundamental physics tests. The mission objectives are both 
scientific and technological and is of great interest to two main scientific 
communities:
• The Time and Frequency (T&F) community; which aims to use ACES as a tool for 
high precision Time and Frequency metrology
• The Fundamental Physics community; which will benefit from the use of ACES 
data for accurate tests of general relativity.

See https://earth.esa.int/web/eoportal/satellite-missions/i/iss-aces

The ACES development was initiated in the 1990s. However, the decision to 
complete the development of the project has been achieved only at the ESA 
council at Ministerial Level of November 2008.
The launch was planned in 2018, but,  as said, the clock ensemble (which BTW 
includes two AXTAL OCXO 100 MHz) is still sitting on the test bench and waiting 
and waiting.

Best regards
Bernd

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