A method for validating received positional data in vehicle surveillance applications wherein vehicles transmit positional data indicating their own position to surrounding vehicles. A a radio direction finding antenna arrangement of a receiving unit receives a signal carrying positional data indicating an alleged position of a vehicle, transmitted from a radio source. The bearing from the receiving unit to the radio source is estimated utilizing the radio direction finding antenna arrangement and the received signal. The distance between the receiving unit and the radio source is estimated based on the time of flight for a signal travelling there between at known speed. An estimated position of the radio source is calculated based on the estimated bearing and the estimated distance. A deviation value indicating the deviation/coincidence between the alleged position of a vehicle is determined according to the received positional data and the estimated position of the radio source.