A first position of a satellite is calculated at a first time in dependence on received orbit data corresponding to an orbit path of the satellite. Anan orbit path of the satellite is modeled from the first position at the first time to a second time to determine a second position of the satellite at the second time. A third position of the satellite is then calculated at the second time in dependence on the received orbit data. The second position and third position are compared to determine a validity of the orbit data.