The subsurface properties of icy moons in the outer solar system are significant to investigations of the formation, evolution, and habitability of these worlds. The NASA Europa Clipper and ESA JUpiter ICy moons Explorer (JUICE) missions will arrive in the Jovian system in the 2030’s and use radar sounding instruments to probe the ice shells of Jupiter’s moons Europa, Ganymede, and Callisto.  Recent flybys of Ganymede and Europa by the Juno spacecraft provided an unprecedented opportunity to study these worlds in advance of these missions and inform interpretation of future radar observations. Measurements of surface brightness temperature by the Juno Microwave Radiometer (MWR) instrument, acquired during these flybys, have been used to obtain constraints on the subsurface properties of these ice shells, including the ice shell thickness. Because surface brightness temperature is a measure of depth-integrated microwave emission, reflection/transmission, and attenuation, there are different combinations of ice shell properties (e.g., thickness, temperature profile, concentration of conductivity-enhancing impurities) that could produce an equivalent surface brightness temperature. In this work we explore how unknown subsurface properties can introduce ambiguities in model-based retrievals of ice shell thickness from passive radiometer observations of surface brightness temperature. We present a novel approach for combining passive microwave radiometer and active radar sounding observations to obtain improved constraints on the subsurface properties of ocean world ice shells.