An electro-optical ranging system for taking rapid measurements of the distance to a moving target object, such as a rotating log whose cross-sectional configuration is to be determined prior to the performance of a processing operation thereon. An electro-optical transceiver illuminates a small portion of the target object with a narrow beam of light modulated by a reference R-F signal. Light reflected from the target object is received by a telescope in the transceiver and focused upon a photodetector for demodulation to generate a detected R-F signal. The distance to the object is then determined by measuring the relative phase of the reference and detected signals. First, a preliminary measurement is made by adjusting the relative phase of the two R-F signals, using a calibrated, digitally-controlled phase shifter, until the output of a comparator circuit is as nearly null as possible without exceeding a digital input word to the phase shifter corresponding to a distance less than or equal to the target object. The average amplitude of the comparator output signal corresponding to the aforementioned preliminary digital word is determined, and the average comparator output corresponding to the preliminary digital word as modified by the arithmetic addition of one bit thereto is next determined. The actual distance to the surface of the target object is then calculated by interpolating between the two resulting digital words to find an equivalent digital word corresponding to a null phase-comparator output. Parallax error is minimized and the signal-to-noise ratio of the detected signal is maximized by transmitting and receiving the light along a coaxial path. Errors due to changes in the D-C offset of the comparator output are eliminated by A-C coupling the comparator's input and output utilizing a chopper stabilization circuit. Automatic calibration of the system is accomplished by an electro-mechanical shutter, which upon command cuts off the transmitted beam to zero the transceiver, and an electro-mechanical mirror which upon command deflects the transmitted beam through a fiber-optic tube of known optical length to the photo-detector for enabling a calibration factor to be calculated. Control of the operation of the transceiver and calculation of the measured distance to the object is accomplished by a digital computer.