Active sonar for under-ice conditions

Active sonar apparatus, especially adapted for detecting submarine targets under ice, includes means for providing first and second pulsed, underwater acoustic signals respectfully having a high frequency, fH, and a low frequency, fL. The high frequency, fH, is selected for causing the signal to be reflected at the submerged, water-ice interface and and the low frequency, fL, selected for enabling the signal to be propagated through the ice with no significant reflection at the water-ice interface while then causing the signal to be reflected at the above-water, air-ice interface. Included are means for separately receiving and processing the reflected signals from the first and second acoustic signals and for detecting possible target reflections (which may include ice reflections) as a function of range and angle cells. Further included are signal comparing means for comparing range and angle cells of the two reflected signals, a probable target detection being established when possible target detections for both reflected signals are indicated in corresponding cells. The high and low frequency signals may be separately generated or may be provided as interference signals by a parametric transponder.