[Lil' Lavery]

Nobody is saying wheels on carpet don't benefit from sensors, but rather, they don't benefit from a "land speed" sensor as you were proposing. Almost every competitive team uses some form of sensors in their drivetrain, but usually just to monitoring their driven drive components. They'll mount a shaft enconder to their drive shaft or the output shaft on their gearbox, for example. They trust that the slip effects are small enough that any error they induce will not effect their gameplay.

If you want "external", field-oriented input, you don't always have to base it off the playing surface. Your GPS comment is on the right track. There have been games in the past with non-moving optical targets (such as the lights in 2004 and 2006) that can be incorporated into navigation software. While there hasn't been a game with enough stationary vision targets to triangulate your position on the whole field, using these points can certainly be helpful in many other ways.