Abstract: A position measurement apparatus and methodology is provided. Embodiments include a probe including an acoustic signal source, an optical signal source, and a probe processor for driving the signal sources such that the signals of the acoustic source and the optical source have a known temporal relationship to each other. A receiver receives the optical and acoustic signals, and a processor communicates with and controls the probe and the receiver, and processes data from the receiver. The acoustic signal source is a sinusoidally varying acoustic energy source, and the acoustic signal received by the receiver comprises a sinusoidal signal. The processor correlates the received sinusoidal signal to a mathematical reference sinusoidal signal, and determines a specific cycle and a specific phase as a time of flight measurement point based on the correlation.