Abstract: An offshore GNSS reference station apparatus comprises: a processing resource and an underwater acoustic communications unit. The underwater acoustic communications unit comprises an underwater acoustic transducer, the underwater acoustic communications unit being arranged to cooperate with the processing resource in order to calculate a position. The apparatus also comprises a GNSS antenna site and an orientation determination unit operably coupled to the processing resource. The orientation determination unit is arranged to determine an orientation associated with the GNSS antenna site and the acoustic transducer of the underwater acoustic communications unit.

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.