Artist's impression of the Topex/Poseidon oceanography research satellite, which was launched on 10 August 1992 by an Ariane 42P rocket into a 1331-km high orbit. This satellite has resulted from the merging of two programmes, NASA's Topex (The Ocean Topography Experiment) and France's Poseidon. The satellite's primary instruments are the US C/Ku-band radar altimeter and a similar French instrument operating only in the Ku-band. Both instruments share the same dish antenna, and may determine sea height to an accuracy of about 6 cm. A microwave radiometer is used to correct altimeter data for the effects of tropospheric water vapour.