Principle of elevational speckle decorrelation. The in-plane motion between scans A and B (translation in the x and y directions, roll around the plane normal) is readily determined using conventional 2D image registration techniques. This leaves three degrees of freedom: translation in the elevational direction, tilt (rotation about x) and yaw (rotation about y). Consider corresponding patches in scans A and B (the shaded ellipses). Because of the imperfect elevational focusing, the contents of the patches depend on scatterers within overlapping resolution cells (the hollow ellipsoids) and are therefore correlated. The correlation coefficient depends on the degree of overlap and hence the elevational separation. It follows that, given a suitable decorrelation curve, a measured correlation ρ
1 can be used to look up the corresponding separation d
1. Repeating this process for three (or more) non-collinear patches determines the out-of-plane separation, tilt and yaw of A relative to B.