RTK-GPS is requested to give a real time performance. The GPS receiver used here is a Trimble 4000SSi. It calculates its own position for about 2 s on a synchronized mode, by comparing the carrier phase data measured at the mobile station with CMR data transmitted from the GPS-based control station. The calculated result is outputted with the delay of 2.0-2.5 s, therefore if the data transmission delay exceeds 2 s, no result is outputted.