Soil moisture retrieval from space: The Soil Moisture and Ocean Salinity (SMOS) mission

Microwave radiometry at low frequencies (L-band: 1.4 GHz, 21 cm) is an esta blished technique for estimating surface soil moisture and sea surface sali nity with a suitable sensitivity. However, from space, large antennas (seve ral meters) are required to achieve an adequate spatial resolution at L-ban d. So as to reduce the problem of putting into orbit a large filled antenna , the possibility of using antenna synthesis methods has been investigated. Such a system, relying on a deployable structure, has now proved to be fea sible and has led to the Soil Moisture and Ocean Salinity (SMOS) mission, w hich is described in this paper. The main objective of the SMOS mission is to deliver key variables of the land surfaces (soil moisture fields), and o f ocean surfaces (sea surface salinity fields). The SMOS mission is based o n a dual polarized L-band radiometer using aperture synthesis (two-dimensio nal [2-D] interferometer) so as to achieve a ground resolution of 50 km at the swath edges coupled with multiangular acquisitions. The radiometer will enable frequent and global coverage of the globe and deliver surface soil moisture fields over land and sea surface salinity over the oceans. The SMO S mission was proposed to the European Space Agency (ESA) in the framework of the Earth Explorer Opportunity Missions. It was selected for a tentative launch in 2005. The goal of this paper is to present the main aspects of t he baseline mission(1) and describe how soil moisture will be retrieved fro m SMOS data.