An active apparatus for modifying aerodynamic properties of a vehicle, encompassing: a frame; a movable portion movable relative to the frame along a motion path; a transceiver to emit a transceiver signal and has a specified transceiver position relative to the frame; an antenna to receive the transceiver signal and arranged on the movable portion, an antenna signal, induced by a predetermined transceiver signal, of the antenna exhibiting a predetermined functional dependence dependent on a position of the movable portion along the motion path and on the transceiver position; a signal transmitting unit to emit a unit signal that is associatable with the antenna and carries information regarding the antenna signal, the transceiver configured to receive the unit signal; and a signal evaluation unit, the signal evaluation unit configured to determine the position of the movable portion along the motion path on the basis of the unit signal, the functional dependence, and the transceiver position.