[Jon Stratis]

Your best bet might be an accelerometer. Use it to sense the direction of gravity - if your on the defense, you'll likely be tilted in one direction or another (except for that split second your at the top of the defense driving over it).

A photoelectric sensor might work, too - shine a light down, and measure how much is returned. You might find there's a big enough different between carpet and the defense, but that would definitely require some on-field calibration.