A dual servo system for a seeker actuator comprises a seeker head gimballed on a platform and driven in the pitch and yaw senses by respective fast, i.e. high bandwidth low ambit, servo sub-systems, the platform being pivotally mounted with respect to a base and driven in the pitch and yaw senses by respective slow, i.e. low bandwidth high ambit, servo systems. The pivotal mounting between the platform and the base is configured to provide a spherical bearing of relatively large area (typically about 85 solid angle) to provide good resistance to high acceleration loads.