Gravity measurements by the Galileo spacecraft suggests that Europa is surrounded by an outer ice layer some 80-170 km thick. Energy dissipated in Europa's interior resulting from a slightly eccentric orbit is sufficient to melt some of the ice to liquid water. A planned Europa orbiter spacecraft will present the first opportunity to identify conclusively the presence of a liquid ocean below the icy outer lithosphere. If the orbiter is equipped with a radar sounding instrument designed to probe the ice layer covering the moon, radio frequency signals can penetrate many km into the icy lithosphere and be reflected from an ice/water interface at depth.
The design of the instrument and its data processing approach is dependent on expectations of the scattering mechanisms reflecting radar energy back to the instrument. Generally a radar sounding instrument is deployed on a sled in contact with the surface, or on low-flying aircraft to minimize corrupting reflections from surface scatterers. Here it is necessary to optimize antenna design to attenuate off-angle returns in order to isolate the weak echo from the desired region at depth. Signal processing techniques further reduce stray signals. We present a design that should provide visibility through the icy outer crust to layers of liquid water at depths of up to 20 km, perhaps more if the ice is exceedingly pure.