Rob Seaman wrote:
Perhaps I might expand on some of Bill Thompson's statements in the
context of the great convenience factor of using the current UTC standard.


Each instrument team commands its own instrument directly.  Time-tagging is 
done via UTC.

UTC - or any future civil time scale - provides a common clock to tie together 
both scientific and logistical requirements between a multiplicity of teams and 
team members.  It is often the case that different instruments on a spacecraft 
are operated by different teams.  Those teams have to address scientific 
concerns - they also have to interoperate with FedEx and with each other.

One other thing I should have mentioned is that we also coordinate our
spacecraft observations with other missions and ground-based observatories.  UTC
is a convenient interoperability standard for the scientists, even if it gets
translated by the commanding software into TAI or SCLK.


For both missions, one has to deal with a significant light travel
time, much larger than the required time accuracy.

Much of the discussion to date has implicitly assumed that civil time
here on Earth can simply be transported across the Solar System or the
Galaxy as needed - that it is trivial to correct for distant locales.
On the scale of tenths of seconds, this is likely true for spacecraft
bound to the Sun.  One questions if this is achievable - or desirable -
at the level of nanoseconds, or perhaps even of microseconds or
milliseconds, for very many scientific or utility purposes for Solar
System travel.

There's definitely a limit to which the accuracy of a spacecraft clock can be
known, whether or not that knowledge is folded back to the spacecraft.
Currently, I believe that limit is mainly technological, based on how well one
can calibrate the innate delays within the electronics.  My recollection of
seeing presentations of the time drift data for SOHO is that the measurements
are good to the millisecond level.

The process of taking this into account is essentially the same,
no matter where you are in the solar system, and no matter whether you
feed the results back to the spacecraft, or simply take it into
account on the ground.

And more to the point - why should such projects have to be responsive
to such alien requirements?  Presumably there were any number of factors
involved in determining the timekeeping choices of these space projects
as a response to various use cases.  Is there any particular reason that
they should be forced to put their system clock on the ground rather
than on the spacecraft simply to meet the one-size-fits-all expectations
of others?

One thing I don't want to leave is the impression that I was criticizing the
earlier post from Randy Kaelber.  I merely wanted to share a slightly different
real-world use-case.

Bill Thompson

William Thompson
NASA Goddard Space Flight Center
Code 612.1
Greenbelt, MD  20771


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