The underwater construction in port and harbor is shifting from human strength to mechanization for efficiency improvement and the safety improvement. The underwater positioning technology influences the accuracy of construction. An existing underwater positioning technology was made for taking a position such as the dive boats and ROV, which moved the wide range. Therefore, this technology can not be used for the underwater construction.
　We developed the underwater positioning system, which can measure the position of underwater construction machine by accuracy of ±10cm or less. The underwater construction machine usually moves within 20m in radius from barge. This system is made up of 3 responders and 4 receivers. The main measurement errors of a past system are time and sound wave velocity.We excluded these and achieved making to high accuracy. M-sequence was used to improve the accuracy of the time measurement. The average speed of sound in water was obtained as a solution of the simultaneous equations.
　The following results were obtained by the simulation and the water tank experiment.
1) The following errors are included in the measurement of time in the use of M-sequence.
　-2T< measurement error<T T:Sampling time of AD converter
2) The base line of 5.9m or more is necessary to maintain the accuracy of ±lOcm, when time can be measured until lμ second.
3) When all the measurement values at time are included of the same error level , the measurement accuracy of position is not influenced.
4) The Doppler effect caused at the movement speed of the underwater construction machine does not influence on the measurement accuracy.