Abstract: An apparatus and method for detection of icing onset and ice thickness upon an accretion surface utilizing ultrasonic echo ranging techniques, including propagation of ultrasonic waves through a buffer block. A portion of the wave energy is reflected by reference reflection means and another portion of the wave energy is propagated to the ice accretion surface and to a reflecting interface. The reflecting interface is represented either by the accretion surface in absence of icing, or by a thin ice layer at the icing onset, or by the ice/air interface of an ice layer accreted upon the accretion surface. Reflected waves are transduced to electrical signals. Relative signal amplitudes and time delays provide measures of particular icing conditions upon the accretion surface, and are appropriately resolved into calibrated signals indicating icing onset, ice thickness, and ice accretion rate.