Icy moons of giant planets are now known to have liquid water oceans beneath their icy mantles. Deep radio probing is the only chance to get immediate proof of that now. In the presented talk, issues of deep radio probing of icy moons are addressed. Previously developed theoretical background of Martian GPR exploration is reviewed. A new type instrument concept -- a passive radio wave sounder -- is introduced. Utilization of Jovian natural radio emissions as a sounding wave allows to create a low mass and weight radio instrument exceeding active radars in sounding capabilities. Comparative analysis of both active and passive instrument types and simulation results supporting feasibility of the new instrument are presented. [mehr]