Foreword to the Special Issue on Status of Aquarius/SAC-D Mission Calibration/Validation and Retrieval Algorithms

The papers in this special section focus on the Aquarius/SAC-D observatory, a partnership between the USA Space Agency, NASA, and the Argentine Space Agency, CONAE (Comisi??n Nacional de Actividades Espaciales). Aquarius/SAC-D observatory was launched on June 10, 2011 from Vandenberg Air Force Base, Vandenberg, CA, USA. The observatory included several instruments and two of them are related to the primary goal of the mission. The primary goal of the mission was mapping ocean surface salinity, and mission parameters (e.g., the orbit and repeat cycle) were tuned to this objective. From the NASA perspective, this was a pathfinder mission to demonstrate the technology of remote sensing of sea urface salinity from space and to provide data for understanding the role of salinity in Earth???s water cycle, and Aquarius was the primary instrument. But the mission was international in scope and also fit the needs of the Argentine plans for space applications.