The 658kg (1,450 lb) Soil Moisture and Ocean Salinity (SMOS) satellite launched by the European Space Agency (ESA) this week carried a payload composed of a single instrument – the Microwave Imaging Radiometer using Aperture Synthesis (MIRAS). The SMOS is the first ever satellite designed both to map sea surface salinity and to monitor soil moisture on a global scale and the unique radiometer it carries will enable passive surveying of the water cycle between oceans, the atmosphere and land thereby playing a key role in the monitoring of global climate change.

MIRAS is an interferometer that connects together 69 antenna-receivers mounted on three deployable arms, which unfold to form a large three-pointed star shape that measures eight meters (26-feet) across. Each of the 69 receivers, called LICEFs, measures radiation emitted from the Earth’s surface within the “L-band”, around 1.4 GHz. This frequency provides the best sensitivity to variations in moisture in the soil and changes in the salinity of the surface waters of the oceans. In addition, this frequency is not affected too much by the weather, atmosphere and vegetation cover.

"The data collected by SMOS will complement measurements already performed on the ground and at sea to monitor water exchanges on a global scale,” said Volker Liebig, ESA’s Director of Earth Observation Programmes. “Since these exchanges – most of which occur in remote areas – directly affect the weather, they are of paramount importance to meteorologists. Moreover, salinity is one of the drivers for the Thermohaline Circulation, the large network of currents that steers heat exchanges within the oceans on a global scale, and its survey has long been awaited by climatologists who try to predict the long-term effects of today’s climate change," Liebig added.

Achieving the spacial resolution required by the data users, would usually only work with a huge antenna – which would be too big to be carried by a satellite. To overcome this challenge, the SMOS mission has borrowed techniques used in radio astronomy. Like the Very Large Array in New Mexico, US, that combines 27 radio telescopes deployed on a Y-shaped track that can be extended up to 35 km the MIRAS instrument also forms a Y-shape when its arms are deployed. Through a process of interferometry, which is the technique of diagnosing the properties of two or more waves by studying the pattern of interference created by their superposition, the 69 small antenna receivers mimic a much larger antenna.

SMOS is the second satellite launched under the Earth Explorer program conducted by ESA to foster the acquisition of new environmental data for the science community. It follows the Gravity and steady-state Ocean Circulation Explorer (GOCE) launched in March 2009 that is measuring the minute differences in the Earth’s gravity field around the globe, and more Earth Explorers are already undergoing preparation. Cryosat-2, which will measure the thickness of the ice sheets, is due for launch in February 2010. It will be followed in 2011 by ADM-Aeolus to study atmospheric dynamics, and the Swarm mission to monitor the weakening of the Earth’s magnetic field.