Abstract: Apparatus has at least one processor and at least one memory having computer-readable code stored thereon which when executed controls the at least one processor: to store a model with parameters for extending ephemeris data for a first satellite; to receive first ephemeris data from the first satellite, the first ephemeris data relating to clock error data and/or orbit data for the first satellite at a first time; to receive second ephemeris data from the first satellite, the second ephemeris data relating to clock error data and/or orbit data for the first satellite at a second time; to use the model to predict clock error data and/or orbit data for the first satellite at the second time; to calculate a measure of deviation between the predicted clock error data and/or orbit data for the first satellite at the second time and the clock error data and/or orbit data for the first satellite at the second time as described in the second ephemeris data; to use the measure of deviation to determine whether there