Abstract Since B. Franklin and F. Arago, the Oceans are known for their regulator role in the climate evolution. Beyond this simple ascertainment, it is now necessary and possible to quantify more precisely the impact of the Oceans in climate global changes. Accurate observations at a global scale and over several decades, jointly used with dynamic models, are needed to solve this problem. One dedicated technique is radar altimetry on-board satellites. Sea surface slopes and derived surface currents can thus be measured with a remarkable accuracy. The TOPEX/POSEIDON altimetry mission was optimized to fulfil this objective, this project was jointly conducted by NASA and CNES with support of the associated scientific community involved in the design, the evaluation and the analysis of the system. Two years after the launch (10 August 1992), the results are even better than expected. Sea surface heights and variations are measured at a basin-scale, with an accuracy of 1–2 centimeters over 10 days. Such performances give access to the seasonal changes due to the thermal expansion between the two hemispheres, the interannual variability and the detection and propagation of Kelvin and Rossby waves in the tropical regions. These observations can then be assimilated in numerical models to describe and predict the ocean circulation in four dimensions.