Actually neither. It never checks the ground. It uses a laser guidance system to verify the aircraft position. Using this confirm centerline, distance, etc. The captain accepts responsibility for ensuring no equipment is over the safety lines when entering the gate. Not a bad system really, but of course it means we all have to make damn sure everything is behind every safety line at every gate. Even the empty ones because they can now go live without anyone being there to move the equipment. Now try this with most gates setup literally wingtip to wingtip with no extra room. I don't recall the number, but there is an SAI lesson on it if you really are interested in knowing more.
