Abstract: In order to drive a unit mounted on a carrier rotatable about an axis, the drive for the unit is transmitted from a unit actuator to the unit-drive mechanism on the carrier by a shaft-driven member so arranged that its rotation relative to the carrier is unaffected by rotation of the carrier itself. In one alternative this is achieved by transmitting the output of the unit actuator through a flexible shaft which enters the carrier tangentially and is guided in the fixed structure for its adjacent end to project in a direction normal to the axis of rotation of the carrier, and in an alternative arrangement rotation transmitted to the carrier actuator is superimposed, by means of a differential gear drive arrangement, one gear of which is geared to the output shaft of the carrier drive actuator, upon the rotation of the pinion transmitting, to a ring gear coaxially rotatable about the carrier, the movement of the unit drive servomotor output shaft.