I have an application that requires accurate, reliable and continuous position feedback, in one dimension, range up to 30 meters.  I need the sensor to give me absolute distance from a base point to the target.  It's also going to be outdoors and in a high traffic area, so it'll definitely get dirty, oily, bumped, lots of electrical noise etc...   Laser seems to be about the only thing I can find that might work to that range, but I doubt I can keep it aligned and clean very long.  I'm wondering about gps, or a local position system (I've heard about them for keeping track of items/people in a large plant), but can't seem to find anything off the shelf for that purpose.  Anyone have any suggestions?