Optical position pick-off in zero-drag satellite

Claim 1

In a zero drag satellite having a proof mass shielded from all effects except gravity, the improvement of an optical proof mass position pick-off comprising a flat screen in fixed relationship to said satellite, a mutliplicity of photodetectors equally spaced on said flat screen, and a light source in fixed relationship to said satellite providing a light beam, said proof mass initially interposed in a null position between said flat screen and said light source with said proof mass interrupting said light beam to cast a circular shadow on said flat screen, the periphery of which passes through the center of each of said photodetectors with a shift in the proof mass in any transverse direction causing the shadow to cover more of certain of said photodetectors and less of other, and the shift of the proof mass in an axial direction causing the shadow to cover more of all of said photodetectors or less of all of said photodetectors, the unbalance of the photodetectors outputs being used for activation purposes to change the direction of the zero-drag satellite until the proof mass returns to the null position.