The document summarizes the Aquarius/SAC-D mission to measure global sea surface salinity from space using an L-band radiometer and scatterometer. The mission aims to improve understanding of interactions between ocean circulation, the water cycle, and climate. It will provide global salinity measurements every 7 days at 150km resolution to study links between salinity, rainfall, evaporation and ocean density. The observatory carries instruments from Argentina and the US to also measure soil moisture, atmospheric water vapor, sea ice and other variables.