Science Meetings

From SMOS to Aquarius and Beyond
Kerr, Y., Cabot, F., Leroux, D., Rougé, B., and Albitar, A. (15-Apr-13)

he SMOS (Soil Moisture and Ocean Salinity) satellite was successfully launched in November 2009. This ESA led mission for Earth Observation is dedicated to provide soil moisture over continental surface (with an accuracy goal of 0.04 m3/m3) and ocean salinity. These two geophysical features are important as they control the energy balance between the surface and the atmosphere. Their knowledge at a global scale is of interest for climatic and weather researches in particular in improving models forecasts. In June 2011, The Aquarius mission, on board SAC-D was successfully launched. Tailored to monitoring sea surface salinity this NASA led mission differs on several aspects from SMOS. Finally, between the SMAP mission to be launched and the SMOS NEXT mission in early phase A stage there are number of venues to be considered to go towards the establishment of long term data records. The purpose of this communication is to present the SMOS mission results after more than three years in orbit as well as to describe how it compares to similar Aquarius' so as to try to identify the advantages and drawbacks of the two approaches. A special attention will be devoted to land related products and possibly Cryopsheric results.


The SMOS instrument measures the passive microwave emission of the Earth surface at a frequency of 1,4 GHz (L-band). It has been demonstrated that this frequency is well adapted to monitor surface soil moisture (first 5 cm) and sea surface salinity [2]. The instrument is an interferometer and provides brightness temperatures with an average resolution of 40 km, at several angle and dual polarizations. It means that a point at the surface is seen several times with different incidence angles. Data are acquired at two times in a day at 6 am and 18 pm (local time) and insure a complete coverage of the Earth surface in 3 days. This concept has some similarities with subsequent systems either in orbit (Aquarius) or to be launched soon (SMAP).

However Aquarius on top of the fact that it uses a real antenna, has an active system coupled to the passive one, has some differences in terms of spatio temporal sampling (100 km and 8 day coverage of the earth) and sensitivity (at least an order of magnitude better for Aquarius). The Calibration scheme is also different. SMAP bears some similarities with Aquarius the trade offs are more oriented towards land applications.

Over land SMOS the level 2 algorithm delivers data with a fairly adequate accuracy as long as the perturbing RFI are not too strong. Soil moisture has been monitored continuously with extensive cal val exercises over areas well covered with ground measurements. Droughts and floods have been monitored, etc. Now the focus is on higher level products including obviously level 3 but also drought indices and Root zone soil moisture. Similarly lots of interesting results have been obtained over forested areas, marshes and organic rich soils as well as over frozen or snow covered soils. In parallel other retrievals approaches are being tested (empirical, neural networks) as well as approaches to infer high resolution results (dis-aggregation) soil moisture maps leading to use in irrigation monitoring and management. Aquarius land products are more in infancy as it is not the main science goal but nevertheless some results are beginning to emerge. The spatio temporal is there a handicap but an efficient use of active sensor information coupled with a better RFI processing may prove to have some advantages.

Next SMOS Aquarius and SMAP were designed a few years ago to meet the challenges then. Now, with the increasing resolution of NWP models, with the needs for risk assessment or water management a much better spatial resolution must be sought after typically an order of magnitude. The SMOS design cannot do that as it is optimized for the concept and if gains can be made, they would be small and at a significant cost. Real aperture antennas would become intractable. So as to solve this issue, dis-agregation techniques have been tested. They work up to a factor 5-8 but not for all cases and with a degradation of the signal. Consequently, though very promising, they do not fit the bill completely.

Having an in depth knowledge of SMOS it was relatively easy to find another technical approach solving this issue and we have now a design which enables to gain a factor 10 in terms of spatial resolution (i.e., 4 km spatial resolution, very comparable to SMAP after dis-aggregation) with a system still tractable in space.

Using dis-aggregation techniques could bring the SM field within the km range resolution which would make such a mission a really innovating tool.

This presentation shows in SMOS products and results and compares with Aquarius.