rf may also be arranged on in the cart assembly 10 or for example in the housing 28 for functioning as a timer or u201c clock u201d for the ultrasound transponders ut 1 and ut 2 32 by knowing the distance and direction of a player at anytime and at all time during an activity for example a golf game the microcomputer 30 can keep track of the player 39 s u201c g u201d walking path u201c p u201d represented in fig7 and fig8 for short distances the microprocessor 30 provides instructions to the independent microprocessor 39 controlled and motor 24 driven wheels 37 to follow or preceded the golfer u201c g u201d almost exactly for a long distances the microcomputer may provide instructions to the wheels 37 to follow a shorter path all based upon feedback into the control algorithm from the sensors 32 and 54 precise tracking of continuous line of positions of for example golfers using this cart arrangement is accomplished by the ultrasound sensors ut 1 amp ut 2 32 in a phase differential manner the ultrasound sensor array 32 is utilized for detection and controlled avoidance of obstacles u201c o u201d nearby or crossing the cart assembly 39 s path the rf sensor arrangement 54.