A tracking system for use in head-mounted display apparatus includes at least one emitter that emits signals; a first receiver and a second receiver that sense the emitted signals and generate sensor data, the first receiver and the second receiver being arranged on a first portion and a second portion, respectively, wherein the first portion faces a user when the head-mounted display apparatus is worn by the user, and the second portion is a part of a user-interaction controller of a head-mounted display; and a processor configured to process the generated sensor data to determine relative positions and orientations of first receiver and second receiver with respect to the emitter, and to determine a relative position and orientation of the second receiver with respect to the first receiver.