On 5/26/21 6:19 PM, Lux, Jim wrote:
On 5/26/21 4:45 PM, Ronald Held wrote:
What is the main reference time on the ISS? Is it a primary frequency
standard? How is it updated?
Others may weigh in with better info, but as of a few years ago, there
wasn't anything like a "house 10 MHz" - most things were timed off the
MIL-STD-1553B bus if they needed timing. There is Ethernet, but I
don't know if there's an NTP server, or whether they've implemented
anything like PTP. For the most part, if you need good timing for
your experiment, you fly your own reference (a GPSDO or a Rb or
something).
Typically, you don't even know where you are in 3D space that
accurately - there's a Broadcast Ancillary Data (BAD) stream with XYZ
and rates, but it's generated as a predict by Goddard's Flight
Dynamics Facility. For that matter, the entire structure (which is as
large as a football stadium) flexes quite a bit (meters), so if you're
looking for meter or nanosecond accuracy, you would need to ask
"relative to where".
https://www.nasa.gov/mission_pages/station/research/quick-start-guide
https://www.nasa.gov/pdf/190373main_TP-2007-214768.pdf
http://large.stanford.edu/courses/2014/ph240/clark1/docs/np-2010-09-682-hq.pdf
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As a general thing, and as a time-nut, I am continually disappointed
that space-flight folks don't pay enough attention to good timing
on-board. They're used to single spacecraft, a free running oscillator,
and figuring it out on the ground.
If we're ever to do multispacecraft autonomy away from earth, it's
something that needs to be figured out.
Todd Ely and the Deep Space Atomic Clock are doing a lot, but it's not
common place yet, and Todd's got to do a lot of selling.
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