Sonar ice thickness measurement comprises transmitting an AM and subsequent CW sonic radiant energy signals (13,30) from a high frequency sonar transducer (11) in the water (12) toward the sonic signal reflecting ice cap (19). Reflection (30ʹ) of the CW signal (30) from the water-ice interface (18) and reflection (15ʹ) from the water-air interface (16) of the modulating frequency (15), generated by non-linear of parametric effects in the water (12) by the modulated transmit signal (13), causes the CW signal (30) to be modulated to generate side bands at the modulating frequency on each side of the carrier frequency to provide an FM high frequency signal (27), received by the transducer (11). Detection of the FM signal (27) determines the time of arrival of the echo of said detected FM signal (27) relative to the detection of the reflected modulated signal (14) from the water-ice interface (18). The time difference (TT) between the detected signals (14ʹ,27ʹ) and an assumed value of the speed (CICE) of sound through the ice (19) give the thickness (D) of the ice cap (19).