[thelist] metric conversion

[James S. Huggins (Evolt)]

http://www.spaceref.com/news/viewpr.html?pid=43
"The 'root cause' of the loss of the [Mars Climate Orbiter] spacecraft was
the failed translation of English units into metric units in a segment of
ground-based, navigation-related mission software, as NASA has previously
announced," said Arthur Stephenson, chairman of the Mars Climate Orbiter
Mission Failure Investigation Board.




ftp://ftp.hq.nasa.gov/pub/pao/reports/1999/MCO_report.pdf
The MCO MIB has determined that the root cause for the loss of the MCO
spacecraft was
the failure to use metric units in the coding of a ground software file,
“Small Forces,”
used in trajectory models. Specifically, thruster performance data in
English units instead
of metric units was used in the software application code titled SM_FORCES
(small
forces). A file called Angular Momentum Desaturation (AMD) contained the
output data
from the SM_FORCES software. The data in the AMD file was required to be in
metric
units per existing software interface documentation, and the trajectory
modelers assumed
the data was provided in metric units per the requirements.

During the 9-month journey from Earth to Mars, propulsion maneuvers were
periodically
performed to remove angular momentum buildup in the on-board reaction wheels
(flywheels). These Angular Momentum Desaturation (AMD) events occurred 10-14
times more often than was expected by the operations navigation team. This
was because
the MCO solar array was asymmetrical relative to the spacecraft body as
compared to
Mars Global Surveyor (MGS) which had symmetrical solar arrays. This
asymmetric
effect significantly increased the Sun-induced (solar pressure-induced)
momentum
buildup on the spacecraft. The increased AMD events coupled with the fact
that the
angular momentum (impulse) data was in English, rather than metric, units,
resulted in
small errors being introduced in the trajectory estimate over the course of
the 9-month
journey. At the time of Mars insertion, the spacecraft trajectory was
approximately 170
kilometers lower than planned. As a result, MCO either was destroyed in the
atmosphere
or re-entered heliocentric space after leaving Mars’ atmosphere.

[snip]

Root Cause: Failure to use metric units in the coding of a ground software
file, “Small
Forces,” used in trajectory models

Contributing Causes:
1. Undetected mismodeling of spacecraft velocity changes
2. Navigation Team unfamiliar with spacecraft
3. Trajectory correction maneuver number 5 not performed
4. System engineering process did not adequately address
transition from development to operations
5. Inadequate communications between project elements
6. Inadequate operations Navigation Team staffing
7. Inadequate training
8. Verification and validation process did not adequately address
ground software




James S. Huggins