A system comprising: 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, wherein the first communication path has a higher communication latency than the second communication path. 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, wherein the first communication path has a higher communication latency than the second communication path. 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, wherein the first communication path has a higher communication latency than the second communication path. <EOS>