The Mars Climate Orbiter was launched by NASA on Dec 11, 1998 to study the Martian climate. On its arrival at Mars on September 23, 1999, communication was lost shortly after an orbital insertion maneuver was performed. The cause of the failure was a lower than anticipated altitude with a resulting burn-up of the orbiter. It was entirely due to human error. The error occurred because one piece of software entered the required force in pounds and a separate piece of software interpreted this as newtons. The result was $125 million dollar lesson on the importance of consistency in units.
When performing an alignment, consistency with measurement units is the key to preventing costly errors. We recently conducted a training class for a company that worked on the metric system. Our alignment systems allow for easy conversion “on the fly” between imperial and metric units, so we simply operated everything in the metric system. We then started noticing that some students were taking much longer times on their alignments than usual. It became apparent they were misinterpreting the values of the shims, which are express in “thou” and thought they represented some form of a metric value. Fortunately it was not a $125 million mistake as this can simply be a lesson to be learned in training.
When working with different units, consider all of the stakeholders involved in the project. Who will operate the tool? Who will make corrections? Who will interpret whether or not the alignment is acceptable? You will have to determine which units of measurement will be the standard for the entire project. Fortunately, if someone makes a mistake with units for corrections, they will most likely not see the alignment improving. However, if there is a mistake on units for acceptability criteria, this could be dangerous. There is a big difference between 1 mm and 1 thou! Avoid multiple conversions for the alignment process. Standardize on one set of units and remain consistent for the whole project. If there is a need to convert units during the alignment, make very sure everyone understands when this happens and why this is the case.