1. A method of simultaneously measuring n overlay errors in a structure, the method comprising: illuminating a target with incident radiation, the target having a plurality of diffraction based overlay pads, each diffraction based overlay pad having a number m of periodic patterns, where n≧2 and m=n+1, wherein the target comprises: a first diffraction based overlay pad having a programmed shift between each pair of periodic patterns in the number m of periodic patterns; and each remaining diffraction based overlay pad having a same programmed shift for each pair of periodic patterns as a corresponding pair of periodic patterns in the first diffraction based overlay pad except for only one pair of periodic patterns that has a different programmed shift than the corresponding pair of periodic patterns in the first diffraction based overlay pad, wherein for each of the one pair of periodic patterns that has the different programmed shift the different programmed shift is a different value and each of the one pair of periodic patterns that has the different programmed shift corresponds to a different pair of periodic patterns in the first diffraction based overlay pad; detecting radiation from the target after the incident radiation interacts with the plurality of diffraction based pads; and using differential signals from the detected radiation from different pairs of the plurality of diffraction based overlay pads to simultaneously determine the n overlay errors based on the programmed shift between each pair of periodic patterns.