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In this paper the problem of designing a decentralized robust controller for a plant describing a drag-free satellite is addressed. From recent experiences in drag-free control design we first derive an uncertain plant set representative of many drag-free missions with non spherical test masses. A performance requirement is imposed on the absolute acceleration of the test mass along a measurement axis. The main performance requirement is first broken down into requirements on the uncertain closed loop behavior of the system. The fulfillment of this new set of requirements guarantees robust achievement of the overall system goal. Then optimal single-input-single-output controllers are designed that robustly achieve the desired level of performance. The method proposed allows to properly account for the uncertainties in the design plant retaining the decentralized structure of the controller suggested by the peculiar features of the design plant.