SMOS

The Soil Moisture and Ocean Salinity (SMOS) mission, launched on 2 November 2009, is the European Space Agency’s (ESA) second Earth Explorer Opportunity mission. The scientific objectives of the SMOS mission directly respond to the need for global observations of soil moisture and ocean salinity, two key variables used in predictive hydrological, oceanographic and atmospheric models. SMOS observations also provide information on the characterisation of ice and snow covered surfaces and the sea ice effect on ocean-atmosphere heat fluxes and dynamics, which affects large-scale processes of the Earth’s climate system.

This paper will provide an overview on the various aspects of the SMOS mission, such as

The performance of the mission after more than 6 years in orbit: The SMOS mission has been in routine operations since May 2010, following the successful completion of the 6-months commissioning phase. The paper will summarise the technical and scientific status of the mission, including the status of the RFI detection and mitigation and its effect on the data products. SMOS was the first satellite mission operating in the purely passive band 1400-1427 MHz. All emissions are prohibited in this protected band L-Band as adopted by the International Telecommunication Union (ITU) Radio-Regulations No. 5.340. Nevertheless strong interference sources have been detected worldwide but the situation is continuously improving. SMOS has so far provided very reliable instrument operations, data processing and dissemination to users. The paper will also provide a brief overview on the MIRAS instrument performance, including the instrument calibration and level 1 brightness temperature data processing. The recent developments in the level 1 processor have shown significant improvements in particular with respect to the orbital, seasonal and annual stability. In addition the spatial biases have been further reduced. The new version of the processor will make use of the full polarimetric information sensed by the instrument and provides more accurate measurements for the third and fourth Stokes parameters

An overview on the SMOS data products: SMOS provides continuously level 1 (brightness temperature) and level 2 (soil moisture and ocean salinity) to its scientific user community since summer 2010. SMOS also provides brightness temperature and a neural network based soil moisture data product (from November 2015 onwards) via the WMO’s GTS and EUMETSAT’s EUMETCast data dissemination systems to other operational agencies. This direct link into the data collection points for operational agencies has increased the uptake of SMOS data in this community and has opened up new operational applications for SMOS data. Other data products are under development, responding to the requirements of the science community in particular in the area of hydrology, climate, land use and ship routing, namely a frozen soil indicator, data products for freeze/thaw periods, sea ice thickness and vegetation water content.

Provide an update on the overall validation approach and recent activities: SMOS data products are continuously improved and approach the scientific mission objectives. Validation activities are essential to ensure high data quality. ESA in collaboration with national agencies and institutions maintains a frame for validation activities such as reference sites, ground based observations as well as campaigns. The paper will provide an update on recent activities, such as the ground based L-Band radiometer activities at DOME-C and over selected land sites.

Summarise the collaboration with other space-borne L-band passive sensors, such as NASA’s Aquarius and SMAP missions. International collaboration is ongoing both with L-Band mission counterparts (Aquarius and SMAP) but also other satellite and in-situ derived data sets for soil moisture and ocean salinity, including the working groups focusing on stratification and salinity inter-comparison/cross-calibration were established.

Provide the status of discussions on a potential SMOS follow-on mission.   

The aims of the SMOS+ Sea Ice project funded by ESA's Support To Science Elements (STSE) are 1) to further improve the retrieval of sea ice thickness from SMOS (Soil Moisture and Ocean Salinity) L-band radiometer data, 2) to combine the SMOS with CryoSat-2 data to generate a synergistic sea ice thickness product, and 3) to assess the benefit of using the new sea ice products for the initialization in forecast models.

Brightness temperatures at 1.4 GHz (L-band) measured by the SMOS Mission have been used to derive the thickness of sea ice. The retrieval method is applicable only for relatively thin ice and not during the melting period. A validation based on field data from an extensive campaign in the Barents Sea in March 2014 showed that two different SMOS sea ice thickness products reproduce thickness gradients in the ground truth measurements but substantially underestimate the thickness of deformed ice. A sea ice thickness retrieval algorithm based on a combined sea ice thermodynamic and radiative transfer model takes variations of ice temperature and ice salinity into account and corrects for the statistical thickness distribution function derived from high-resolution ice thickness measurements. Improvements to account for the vertical temperature and salinity gradients in the ice, and the effects of ice concentration and snow layer are currently under development. Spatio-temporal evolution of ice thickness in the SMOS thickness products is assessed by intercomparison to MODIS thin ice thickness charts for the Barents and Kara Seas in 2014-2015.

SMOS and CryoSat-2 provide complementary information because of their different spatio-temporal sampling and resolution, and because of the complementary uncertainty due to the fundamental difference in the radiometric and altimetric ice thickness measurement principles. The main limitations of the ice thickness retrieval depend on the emission e-folding depth and the vertical resolution of the effective radar pulse-length, respectively. It was shown that the combination of SMOS and CryoSat-2 considerably reduces the uncertainty with respect to the products derived from the single sensors. An optimal interpolation method was developed which will be used to generate a combined sea ice thickness product.

The impact of using the new sea ice product in terms of improved forecast quality is assessed for the TOPAZ4 system, an operational coupled ocean-sea ice data assimilation system in the Copernicus Marine Environment Monitoring Service (CMEMS). The forecast quality with respect to sea ice thickness and sea ice concentration will be investigated.

Measurements of salt held in surface seawater are becoming ever-more important for oceanographers and climatologists to gain a deeper understanding of ocean circulation and Earth’s water cycle. ESA’s SMOS mission is proving essential for this aim. Launched in 2009, SMOS has provided the longest continuous record (now ~5.5 years) of sea-surface salinity measurements from space. The salinity of surface seawater is controlled largely by the balance between evaporation and precipitation, but freshwater from rivers and the freezing and melting of ice also cause changes in concentrations. Along with temperature, salinity drives ocean circulation – the thermohaline circulation – which, in turn, plays a key role in the global climate. With a wealth of salinity data from SMOS now in hand complemented by measurements from the NASA–CONAE Aquarius and SMAP satellites, which use different measuring techniques. In this talk we shall provide an overview of how the SMOS mission – now celebrating five years 1/2 in orbit – is providing detailed global measurements of ocean-surface salinity. An ensemble of key ocean processes for climate and biochemistry can now be determined and monitored for the first time from space using SMOS salinity data. This includes, for example, the detailed salinity structure of tropical instability waves along the equator and the salt exchanged across major oceanic current fronts through energetic ocean rings. Occurrences of large-scale salinity anomalies in the Pacific and Indian oceans related to El Niño, La Niña and the Indian Ocean climate are also well evidenced in the five year and a half-long data. In addition, the dispersal of freshwater into the ocean from the major large tropical rivers, namely the Amazon, Orinoco, Mississipi and Congo Rivers, their impact on tropical cyclone (TC) intensification and the oceanic imprints of the intense rainfall in the Trade Wind convergence zones and under TC can now be regularly monitored to better understand the variability of the oceanic part of the global water cycle. We will present how SMOS data, along with concurrent in situ Argo ocean-profile data, other satellite observations of sea-surface temperature, sea-surface height, surface-wind stress and ocean colour, are now providing new opportunities to investigate the surface and subsurface ocean mesoscale dynamics. The talk will tentatively illustrate how this type of data synergy is the key to unlock further scientific insight and increase our knowledge of the oceanic part of the hydrologic cycle.

SMOS, a L Band radiometer using aperture synthesis to achieve a good spatial resolution, was successfully launched on November 2, 2009. It was developed and made under the leadership of the European Space Agency (ESA) as an Earth Explorer Opportunity mission. It is a joint program with the Centre National d’Etudes Spatiales (CNES) in France and the Centro para el Desarrollo Teccnologico Industrial (CDTI) in Spain. SMOS carries a single payload, an L band 2D interferometric, radiometer in the 1400-1427 MHz protected band. This wavelength penetrates well through the vegetation and the atmosphere is almost transparent enabling to infer both soil moisture and vegetation water content. SMOS achieves an unprecedented spatial resolution of 50 km at L-band maximum (43 km on average) with multi angular-dual polarized (or fully polarized) brightness temperatures over the globe and with a revisit time smaller than 3 days. SMOS has been now acquiring data for 5.5 years. The data quality exceeds what was expected, showing very good sensitivity and stability. The data is however impaired by man-made emission in the protected band, leading to degraded measurements in several areas including parts of Europe and of China. However, many different international teams are now addressing data use in various fields including hydrology. We have now acquired data over a number of significant “extreme events” such as droughts and floods giving useful information of potential applications. We are now working on the coupling with other models and or disaggregation to address soil moisture distribution over watersheds. We are also concentrating efforts on water budget and regional impacts. From all those studies, it is now possible to express the “lessons learned” and derive a possible way forward. This paper thus gives an overview of the water cycle science goals of the SMOS mission, a description of its main elements, and a taste of the first results including performances at brightness temperature as well as at geophysical parameters level and how they are being put in good use for hydrological applications.

ESA’s Soil Moisture and Ocean Salinity (SMOS) mission has been in orbit for over 6 years, and its Microwave Imaging Radiometer with Aperture Synthesis (MIRAS) in two dimensions keeps working well. The data for almost this whole period has been reprocessed with the new fully polarimetric version (v620) of the Level-1 processor which also includes refined calibration schema for the antenna losses. This reprocessing has allowed the assessment of an improved performance benchmark.

The spatial tilt existing in the images produced with the previous version of the Level-1 processor, in the two main polarizations X and Y, has been considerably decreased, removing the negative trend at low incidence angles and reducing the overall standard deviation of the spatial ripples. The expected improvement in the third and fourth Stokes, after correcting the use of the cross-polar antenna patterns, has been confirmed, enabling accurate retrieval of the Faraday rotation angle, total electron content in the ionosphere and the start of the development of fully polarimetric retrieval schemes at Level-2. The mitigation of the side lobes of the Sun and the Radio Frequency Interference (RFI) sources in the images remains being a challenge, although a much more precise Sun and RFI flagging strategy has been implemented, allowing for the removal of the affected data with as little impact as possible in the overall number of observations. Also new image reconstruction techniques from which the instrument response could be better estimated are currently under evaluation. Further effort is being directed towards a more accurate modelling of the Sun and the galactic glint.

In terms of bias, the new version of the Level-1 processor produces slightly warmer ocean images resulting in an increased average deviation with respect to the geophysical models. The portioning of this positive bias into instrumental and forward model contributions is under investigation. In any case, the consistency between SMOS and the forward ocean model, as well as with NASA's Aquarius mission is at an acceptable level of only a fraction of a Kelvin.

The freshening of waters around continental masses, known as land-sea contamination, due to imperfect visibility amplitude calibration and side lobes of the impulse response of MIRAS, will be improved in the next version of the processor. Two parallel lines of investigation have been followed: the first one consists of an empirical correction based on the accumulated record of brightness temperatures since launch, showing very promising results against models and in situ measurements; the second approach comes from an improvement in the calibration of the amplitude of the visibilities, in particular, from a better estimation of the correlation efficiency coefficients.

Regarding temporal variations, the long term drift exhibited by the previous processor version has been significantly mitigated thanks to a better calibration of the antenna losses and the use of only the most accurate Noise Injection Radiometer (NIR) out of the 3 units available in MIRAS. These improvements have also reduced the orbital and seasonal variations, although residual drifts still remain, in particular during October (which might be due to galactic glint) and the eclipse season. External calibration manoeuvers follow, since October 2014, a different strategy and are scheduled so that the Sun shines at low elevation over the antenna horizon, keeping it warm. This new strategy should result in more accurate retrieval of the antenna losses which are one of the main drives for the MIRAS calibration.

The SMOS Calibration and Level-1 Processor team continues working on new image reconstruction algorithms taking into account land-sea distribution, a more accurate thermal model of the instrument, the determination of antenna pattern mis-modelling, the Sun correction, and a the better handling of Radio Frequency Interference effects. A simpler mode of operation of MIRAS, called ALL-LICEF is being assessed, with the hope that it could bring a more stable behavior at orbital and seasonal scales. An overview of the results and the progress achieved in both calibration and image reconstruction will be presented in this contribution.