A tracker system for determining orientation of an object relative to a moveable platform, comprising: an inertial sensor arrangement associated with the object and arranged to output rate data indicative of a rate of change in orientation of the object in inertial space; a non-inertial tracker system associated with the object, arranged to determine and to output orientation data indicative of orientation of the object relative to the moveable platform; an input for receiving orientation data indicative of orientation of the moveable platform in inertial space; a conversion component (20, 25) for receiving orientation data output by the non-inertial tracker system for converting a measure of orientation of the object relative to the moveable platform into a measure of orientation of the object in inertial space using corresponding data indicative of orientation of the moveable platform in inertial space and said tracker system being characterized by the conversion component being arranged to convert two or more successive measures of orientation of the object in inertial space into an estimate of the rate of change in orientation of the object in inertial space; and further characterized by comprising a corrector component (30, 35, 40) arranged to calculate corrections to rate data output by the inertial sensor arrangement, using a plurality of substantially synchronised combinations of rate data output by the inertial sensor arrangement and rate data output by the conversion component and to apply the calculated corrections to a current measure of rate data by the inertial sensor arrangement; and a predictor component (50, 55, 60) arranged to calculate, by integrating the corrected rate data output by the corrector component and by forward prediction, an orientation of the object in inertial space at a predetermined prediction time relative to a time point of validity of the corrected rate data. an inertial sensor arrangement associated with the object and arranged to output rate data indicative of a rate of change in orientation of the object in inertial space; a non-inertial tracker system associated with the object, arranged to determine and to output orientation data indicative of orientation of the object relative to the moveable platform; an input for receiving orientation data indicative of orientation of the moveable platform in inertial space; a conversion component (20, 25) for receiving orientation data output by the non-inertial tracker system for converting a measure of orientation of the object relative to the moveable platform into a measure of orientation of the object in inertial space using corresponding data indicative of orientation of the moveable platform in inertial space and said tracker system being characterized by the conversion component being arranged to convert two or more successive measures of orientation of the object in inertial space into an estimate of the rate of change in orientation of the object in inertial space; and further characterized by comprising a corrector component (30, 35, 40) arranged to calculate corrections to rate data output by the inertial sensor arrangement, using a plurality of substantially synchronised combinations of rate data output by the inertial sensor arrangement and rate data output by the conversion component and to apply the calculated corrections to a current measure of rate data by the inertial sensor arrangement; and a predictor component (50, 55, 60) arranged to calculate, by integrating the corrected rate data output by the corrector component and by forward prediction, an orientation of the object in inertial space at a predetermined prediction time relative to a time point of validity of the corrected rate data. a corrector component (30, 35, 40) arranged to calculate corrections to rate data output by the inertial sensor arrangement, using a plurality of substantially synchronised combinations of rate data output by the inertial sensor arrangement and rate data output by the conversion component and to apply the calculated corrections to a current measure of rate data by the inertial sensor arrangement; and a predictor component (50, 55, 60) arranged to calculate, by integrating the corrected rate data output by the corrector component and by forward prediction, an orientation of the object in inertial space at a predetermined prediction time relative to a time point of validity of the corrected rate data.