A method of measuring the vapor-liquid boundary surrounding a supercavitational high speed underwater vehicle includes the steps of arranging a sensor on a surface of the vehicle. The sensor has a transmit source for emitting an optical signal and a plurality of optical detectors for receiving a reflected optical signal. The reflected optical signal is detected with one of the optical detectors, and a duration of time between the emitting of the optical signal and the receiving of the reflected optical signal is measured. The method determines a separation distance from the transmit source to the receiving optical detector. The duration of time and the separation distance are combined to compute a boundary distance.