A method and apparatus for detecting and correcting anomalous measurements in a satellite navigation receiver is disclosed. In certain embodiments, the diffractive optical element comprises a diffraction grating etched on one or more facets of the gemstone. A first conical plug is coupled to the axial positioning device and is movable in and out of the first conical inner portion along the axis of the test cell. In this manner, jitter introduced into the voice packet by variable length delays in the network device can be controlled. A digital image of a room is obtained.