Non-conventional interferometry
Radar Interferometry

Although water is, in the general case, a "non-target", we can envisage observing it either at an extremely low angle of incidence so as to almost face it, or using it as a radar mirror that cooperates with other types of target.

The first idea forms the basis of a new generation of altimeters that produce interferometric images of water surfaces rather than altitude profiles, and can therefore operate not only on the open ocean, but also in coastal areas and inland on lakes...