The measurement of surface velocity as the rate of displacement of correlated patterns of speckle, or noise, in successive radar-interferometric images of the glacier. See insar, radar. The speckle originates from large numbers of statistically independent scatterers in the scene. Small 'chips' or windows from a later image are matched to a similar chip from an earlier image. Measured from the earlier chip, the distance to and bearing of the later chip that exhibits the greatest correlation with the earlier chip is taken to be the vector displacement that has accumulated between the dates of the images. Speckle tracking (e. G. Gray et al. 2001) is less precise than interferometric measurement of velocity but, relying only on image intensity rather than on both the intensity and the phase of the complex radar signal, is more robust. See feature tracking.