[Holtzman]

One sensor that I would like to see in the Kit of parts is an ultra sonic range finder. You could use it to measure the distance from the player station wall, or the distance to some other field features, and it wouldn't depend on wheel slip like optical encoders or hall effect sensors would. They would give you a certain degree of absolute positioning on the field, and not depend on perfect starting orientation.

They're not nearly as expensive as I would have expected either. Hobby Engineering has a selection of these sensors, and a number of other goodies on their website.
Check it out.
http://www.hobbyengineering.com/Sect...l?src=overture