Over ice sheets a second synthetic aperture system is added to form an interferometer across the satellite track. The angle of the echo at each range may be determined, and this, together with the range, determines the elevation and across-track location of the surface. When the across-track echo direction is unambiguous, as in (B) or (C), its coherence is high. Echoes at ranges in ambiguous directions, as in (A), have low coherence, and may be masked.