Abstract: A method of obtaining data for use by a receiver of a satellite positioning system or a GNSS comprises deriving the data remotely from the receiver by a server (200), using meteorological information and a regional or global three dimensional map of grid points from which it computes tropospherical delays by ray tracing through the refractivity field derived from atmospheric measurements of pressure, temperature and water data content, such measurements being available from meteorological bodies. When used to enhance position determined by a user receiver that includes a non-meteorological, climate based model (130) giving zenith delays and means (130?) to map them to particular inclinations, the server also includes a copy of such non-meteorological model (230) and provides its ray traced delay values as zenith delays.