Abstract: In the present invention, the altitude is introduced in the positioning determination. Preferably a three-dimensional position determination is performed. The altitude is according to the present invention introduced as additional information in terms of pre-determined geographical altitude data (40). The position determining can take place either in the mobile unit (20) or in any suitable node (10:1) in the wireless communication system (1). A provisional two-dimensional position can also be provided, which is subsequently corrected to a three-dimensional position by use of pre-determined altitude data. The position determination is based on arrival time measurements (t1–t3) of signals (30:1–30:3) between the mobile unit (20) and base stations (10:1–10:3) of the system (1). Both downlink and uplink signals are possible to use. Round trip time measurements can be used to further reduce the need for several hearable base stations (10:1–10:3).