A method of measuring a level of a material in a tank. A first secondary reference (FSR) signal is generated when a reference pulse is transmitted into the tank and a second secondary reference signal (SSR) signal is generated when an echo signal responsive to the reference pulse is received. A plurality of FSR signals and a plurality of SSR signals are stored. The plurality of FSR signals and plurality of SSR signals are transferred to a correlator block which calculates a plurality of correlation function values (CFVs) that have magnitudes reflecting a degree of matching between pairs of the SSR signals with their associated FSR signals. Using a maximum CFV result a time interval (Tm) is calculated between the FSR signal and SSR signal associated with the maximum CFV result, and the level of material in the tank is determined from Tm.