A method, system, and computer program product for determining the location of a portable data processing system is provided. Fastening of the chinstrap secures the helmet in the downward direction and tightens the movable element against the nape of the neck, thereby securing the helmet in the area of the occipital lobe. A tensioned focus mask, having a plurality of spaced-apart first conductive lines, is located adjacent to an effective picture area of the screen. A phase switch is provided in each conductive path.