A system includes a data processing apparatus; and a position detector to detect a position of a trackable object at successive sampling time points, the position detector being configured to provide, by a first data communication path, position data indicative of the position of the trackable object to the data processing apparatus at each sampling time point along with data indicating a time of that sampling time point according to a clock signal at the position detector, and to send synchronisation data to the data processing apparatus by a second, different, data communication path indicative of the clock signal at the position detector; in which the data processing apparatus comprises: a detector to detect a difference between a clock signal at the data processing apparatus and the clock signal at the position detector in dependence upon the synchronisation data; and a processor to perform a data processing operation in dependence upon the position indicated by the position data, the time of a sampling time point associated with that position data, and the detected difference between the clock signal at the data processing apparatus and the clock signal at the position detector.