With conventional pulse-echo ranging approaches it is diffcult to obtain un accurate range measurement needed by a proximity ultrasonic sensor system owing to the limited bandwidth of the transducer. A continuous-wave multifreouencv ranging method is presented to accurately measure the proximity distance for an ultrasonic distance sensor system A novel time delay estimator is derived based on a nonlinear least-squares-fitting criterion for the case where the received signal amplitude is non-negative owing to the acoustically hard reflection To minimise the highly oscillatory cost function an effcient two-stage estimation algorithm is proposed Numerical results show that the estimation algorithm can approach the Cram6r-Rao bound as the signal-to-noise ratio increases.