A method, an apparatus and a computer program for providing assistance data for satellite based positioning of mobile stations operating in a first cell of a cellular radio communication system utilizing ranging signals transmitted by satellites. A sequence of assistance data for positioning of a mobile station operating in the first cell is generated (306) by including in said sequence assistance data associated with individual satellites in a set of at least four satellites. At least one of said set of satellites and the order in which assistance data associated with individual satellites occurs in said sequence of assistance data is selected (305) utilizing control data associated with the first cell and reflecting local obstructions in the first cell or its immediate surroundings affecting reception of said ranging signals in the first cell. The invention also includes a method, an apparatus and a computer program for updating (311) said control data utilizing information derived from positionings of mobile stations in the first cell.