The acoustic position measuring systems for the unmanned research submersible are used in severe environment conditions : thruster noise and disturbances, signal reflections, etc. In this paper the detection probability of position based on statistical decision theory are examined using stochastic model for the acoustic channel, and some useful results are extracted in order to design high reliable system even though in such severe environmental conditions : 1) time window is effective, but do not need in case of high signal level. 2) redundancy of one hydrophone is equivalent to 1 dB in S /N. 3) threshold level should be higher than half of received signal.