Aquarius is a proposed satellite mission that will measure sea surface salinity. The salinity of the ocean surface reflects the input of fresh water from precipitation, melting of ice and river runoff; the loss of water through evaporation; and the mixing and circulation of surface water with the deep ocean water below. The Aquarius program will address NASA Earth Science Enterprise goals concerning the global cycling of water and the response of the ocean circulation to climate change. Aquarius scientific objectives will examine the cycling of water to the ocean's surface, the response of the ocean circulation to buoyancy forcing, and the impact of buoyancy forcing on the ocean's thermal feedback to the climate. Ocean buoyancy is a roughly equal function of sea water temperature and salinity. Currently, only the global sea surface temperature is measured with adequate accuracy and spatial/temporal resolution. Global sea surface salinity measurements will resolve this problem, and are also expected to improve modeling of ocean surface solubility chemistry, thereby improving estimates of the air-sea exchange of CO2.

In order to meet these science objectives, the NASA Salinity Sea Ice Working Group has recommended that a sea surface salinity mission should provide measurements that are accurate at least 0.2 practical salinity units (psu) accuracy, at 100-200 km spatial scale, and with a 7-30 day global update rate, depending on the science goal. Aquarius will meet these measurement requirements by using a real aperture, dual-polarized L band radiometer-radar system designed to achieve the required radiometric temperature measurement accuracy. A 3 m antenna at ~600km altitude in a sun-synchronous orbit and 300 km swath will provide the desired 100 km resolution global coverage every 8 days.

Salinity is measured from space by measuring the sea surface radiometric brightness temperature at long wavelengths, by adjusting for the combined effects of sea surface temperature and roughness, and by correcting for other background sources of radiation. This can be done because salinity affects the dielectric constant of sea water; hence, the radiometric emission from the sea surface to space. The first radiometric observations of salinity from space used an L-band radiometer on Skylab in the mid 1970s. Since then, numerous aircraft missions have addressed the technical details of building the highly accurate L-band radiometer required to accurately measure sea surface salinity. Based on these developments, the state of technology is now sufficient to produce an accurate L-band radiometer-radar system that can be coupled with a planned global array of in situ salinity observations to provide the needed level of measurement accuracy.