thanks for the link Enrique, do you know anything about the cameras?

The trouble with using other types of sensors is that they aren't passive i.e. they actually put something (sound or light) into the area they are heading. So if two machine of the same type came acros each other they would interfere.

Cameras can look passively, they don't interfere with each other. Much better.

Distance measurement comes from doing it one of two ways:-

1) Use two cameras, and you can calculate the distance based on the diference (like your brain does)

2) Use a parabolic mirror, and a single camera, it's a bit complicated to explain here, but have a look at cvs.anu.edu.au/andy/beye/beyehome.html if you're interested.