Illustration of Aquarius/SAC-D above Earth. Credit: NASA.
On June 10, 2011, NASA and Argentina’s CONAE (ComisiÌ_n Nacional de Actividades Espaciales or National Commission for Space Activities) launched the Aquarius/SAC-D satellite. Though NASA and other space agencies have launched dozens of Earth observation satellites over the years, the Aquarius instrument fills an important gap in Earth observation data: the measurement of ocean salinity.
Instruments are already in orbit to measure most relevant ocean parameters such as sea surface temperature (SST), sea level, and surface wind, according to NASA. These data are essential to characterize our oceans and study how they cycle and change. However, measurements of sea surface salinity (SSS)—also extremely helpful for understanding our oceans and climate—have never been performed on a large scale, with the only measurements coming sporadically from individual boats and buoys. Aquarius takes SSS measurements over the entirety of Earth’s surface once every seven days.
Though relatively stable over the past few hundred million years, ocean salinity varies among oceanic regions. Ocean salinity is affected by the rates of evaporation and precipitation in localized regions, so SSS measurements can be used to infer the rate and processes of the water cycle in those regions. Global year-to-year and season-to-season measurements of SSS also will help scientists better understand ocean circulation and climate.
Salinity measurements by Aquarius are taken with a complementary set of three passive radiometers and a radar scatterometer, according to NASA. Changes in salt concentration affect the conductivity of seawater, which, in turn, changes the amount of thermal emission from the ocean’s surface. These fluctuations in thermal emission can be detected at microwave frequencies, the frequency band at which the set of threeåÊ passive radiometers collect data. All three radiometers are identical, and they are arranged on the instrument for maximum surface coverage, which reduces the time needed to get complete data collection coverage of the Earth’s surface. The radar scatterometer takes measurements of the ocean surface’s roughness and allows for correction of data from the passive radiometers.
NASA’s responsibility in the mission was primarily the Aquarius instrument, while CONAE supplied complementary instruments and the spacecraft for launch and operations. The Aquarius/SAC-D mission was the fourth installment in CONAE’s SAC series of satellite missions. The first mission served as a testing ground for various instruments, while the second focused on the study of solar physics and astrophysics. The third, fourth, and fifth satellite missions are all focused on Earth observation.