On Jan 1, 2006, at 3:29 AM, Poul-Henning Kamp wrote:
> http://lheawww.gsfc.nasa.gov/users/ebisawa/ASCAATTITUDE/
This describes a system for "attitude determination", i.e., for
pointing the ASCA X-ray telescope at celestial objects. It appears
there were several bugs in time handling. They get full marks for
diagnosing, documenting, and publicizing the resulting issues. These
include issues of data characterization as well as of spacecraft
operations.
As someone already pointed out, the fundamental issue was the
adoption of UTC as a standard when TAI, for instance, might have been
more appropriate. UTC conveys Earth orientation information - one
might wonder why it would be the obvious choice for a platform not
located on Earth. The blind adoption of any standard is a bad
thing. Adopting American standards instead of metric, or worse yet,
adopting both, can be fatal to a project. Give them an inch and
they'll take a metre.
Posit for the moment that this project had completely avoided the use
of UTC. Since the mission required maintaining attitude with respect
to the celestial sphere and solar system objects, requirements for
handling DUT1 (or the equivalent) would have remained. Having
implemented an astrometric software package (others here are much
more experienced), I can assure you that such software is no less
subject to the creation of obscure bugs than any other system. The
bugs that were realized as reflections of poor leap second handling
might well have instead appeared in other ways. UTC is a handy tool
for conveying both UT1 and TAI in one concise package. Depending on
the project, various assumptions and approximations may be
appropriate. It sounds like inappropriate assumptions were made. Oh
well.
Time handling bugs typically appear in the interfaces between systems
that make contradictory assumptions. The quoted page's description
of two bugs that originally cancelled out later reasserting
themselves with a maintenance update is classic for such things.
Fixing one bug reveals others that were previously masked.
Bugs don't result from standards, they are the responsibility of
engineers. The existence of a bug certainly isn't an argument for
ignoring underlying facts of nature.
The ASCA mission ended in 2000 as the result of a geomagnetic storm
(another pesky fact of nature keyed to Earth orientation) and the
spacecraft burned up on reentry in 2001. (Earth orientation is
certainly a matter of public interest as to where de-orbiting
spacecraft might impact.) However, we have yet to hear of any
significant issues encountered during the 2005 leap second by current
(appropriate or inappropriate) users of UTC.
Rob Seaman
National Optical Astronomy Observatory
Received on Tue Jan 03 2006 - 08:45:11 PST