Invention relates to a novel method of and system for ranging between an acoustic source carried on an unmanned or autonomous undersea mobile vehicle (UUV or AUV) and preferably a plurality of hydrophone receivers remotely deployed from the vehicle in predetermined patterns, generally suspended from sonobuoys equipped with above-the-sea relay radio transmitting antennas, and with time synchronization provided amongst the source and the receivers, wherein the time delay from the transmissions of the source is measured by utilizing special signal processing, enabling range to be measured in close to real time by determining the product of the sound velocity and the measured time delay, and with the process continually and periodically being repeated throughout the duration of the vehicle run. Given the range, the system may then be used to measure the acoustical properties of and/or receiver system performance in the sea or other water body, such as transmission or propagation loss TL, channel impulse response, bottom geoacoustic properties, source level, receiver sensitivity calibration, sonar operator readiness and sonar receiver performance and the like. Further, in situ measured data can be assimilated with models to enable more accurate prediction of the ocean environment than could be obtained from either individually.