An extrinsic-calibration system includes at least one target and a computer. Each target includes three flat surfaces that are mutually nonparallel and a corner at which the three surfaces intersect. The computer is programmed to estimate a first set of relative positions of the corner in a first coordinate frame from a first sensor, the first set of relative positions corresponding one-to-one to a set of absolute positions of the at least one target; estimate a second set of relative positions of the corner in a second coordinate frame from a second sensor, the second set of relative positions corresponding one-to-one to the set of absolute positions; and estimate a rigid transformation between the first coordinate frame and the second coordinate frame based on the first set of relative positions and the second set of relative positions.