1. A method of simultaneously measuring n overlay errors in a structure with a number m of periodic patterns, where n≧2 and m=n+1, 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 the same number m of periodic patterns as the structure and wherein each diffraction based overlay pad includes a programmed shift between each pair of periodic patterns; detecting radiation from the target after the incident radiation interacts with the plurality of diffraction based pads; and using the detected radiation from each 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.