Global mean sea level (GMSL)—the height of the oceans averaged over the globe is a crucial barometer of climate change because it reflects both the amount of heat being added to the oceans and the mass loss of global ice reservoirs. GMSL has been observed to be rising at an average rate of approximately 3 mm/year. The processing to provide the GMSL record mainly depends on the geophysical corrections that are applied to the sea surface height measurements and on the gridding process applied to average the along-track measurements. An important step for accurately computing the GMSL time series is the calculation of the instrument biases between the altimeter missions, both at global and regional scales. The second main step is to average the along-track altimeter data in order to derive regional and GMSL time series. The main objective of comparing altimeter and tide gauge measurements is to ensure that the altimeter instruments are not drifting over time.