[time-nuts] Re: Project Great

[Lux, Jim]

On 11/28/21 2:11 AM, Andy Talbot wrote:
> I would imagine there are already several caesium clocks on board the ISS,
> anyway.
> Don't forget there is a velocity component in relativistic time shift, as
> well as gravitational, so using a moving platform like an aircraft or the
> ISS complicated things a lot
>
> Andy
> www.g4jnt.com
>
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.

As a practical matter, there's not a lot of "infrastructure" on ISS, 
i.e. there's no "house 10 MHz" - the experiments tend to be self 
contained.  When I was working on SCaNTestbed, which launched to ISS in 
2012, there wasn't even an onboard real-time GNSS time/position feed. We 
had a software GPS receiver as part of the testbed.  What you would get 
is a "playback" of ground predicts done by GSFC Flight Dynamics for 
position and time over MIL-STD-1553 as part of Broadcast Ancillary 
Data.  And knowing precisely where you are on ISS is tricky anyway - 
it's the size of a football stadium and flexes and moves on the scale of 
meters. The BAD data was for some presumed "center of mass", as I recall.