Geometry of the local (primed coordinates) and reference (unprimed coordinates) frames. The elevation angle in the reference frame is given by θ. The tilt angle (θt) at the air-snow interface (X′ axis) is along the axis of the receiving antenna's main beam center. The elevation angle in the local frame is given by θ′. The receiving antenna's height above the tilted air-snow interface (shown in green) is given by h - (t1 + t2), where h is the height of antenna above the frozen-soil surface (m), t1 is the snow layer thickness (m), and t2 is the prairie grass layer thickness (m) as given in .