this paper, requiring the detection of "natural scale" mentioned in section 7. The second, dropouts and shadows caused by blockage of the beam, and gradients caused by reflections of a finite--width beam by nearby structures can be handled quite effectively by a two--tier architecture. Another possible problem, their extremely noisy nature, is already well handled by our algorithm's robustness. We avoid speckle by setting our minimum scale of working above the speckle scale; 8x8 or 4x4 are the only plausible choices. 7.5 Tram--lines