Evaluation of RTK-OTF positioning system for free running manoeuvrability test of a model ship

A free running model test using the real-time-kinematic on-the-fly (RTK-OTF) mode in GPS was carried out in a 80-by-80 meter square basin. This positioning system consists of three GPS receivers, one radio link and an onboard personal computer for data storage. One GPS receiver was located on the shore as a base station which transmits reference data every 1.0 second and two others were on the model ship as mobile stations which produce fore and aft position data every 0.2 seconds (5 Hz) or 1.0 seconds (1 Hz) selectively. An ultrasonic positioning system was also used for comparison to discuss the accuracy and reliability of the GPS data. Fixed point observations proved that GPS 1 Hz and 5 Hz data had an accuracy of about 0.4 cm and 4.0 cm horizontal RMS respectively. On the other hand, it was observed that error of constant length measurement between the fore and aft GPS antennas which is one of indices to measure accuracy under moving conditions did not exceed ±4 cm for GPS 5 Hz positioning and ±1 cm for GPS 1 Hz positioning. Manoeuvrability characteristics such as speed, yaw rate, drift angle and yaw angle during free running tests were also calculated using fore and aft position data and are confirmed to have sufficient accuracy. Noise sources in the GPS 5 Hz data and their properties are also discussed