Abstract: An absolute gravity inline measuring apparatus includes an interferometer arrangement, a test mass tracking system, and a superspring mechanism. The interferometer arrangement includes first and second beam splitters for respective splitting and recombining a laser light beam and upper and lower cornercube retroreflectors facing toward one another and spaced above and below the first beam splitter so as to reflect one split portion of the laser beam in vertically-aligned upper gravity measuring and lower motion compensating portions of a variable leg of the interferometer light beam path. The test mass tracking system includes a cart vertically movably in an evacuated elongated housing and releasably supporting a test mass in a drag-free enclosure. The test mass incorporates the upper retroreflector of the variable leg of the interferometer light beam path.