Effective SMOS PSF and Antenna Correlations

Given the interferometric characteristics of SMOS, SMOS snapshots are full-polarization brightness temperatures rendered on a hexagonal grid (the so-called synthetic antenna). In fact, what the instrument actually measures are the cross-correlations of all pairs of receivers, from which a visibility function can be derived. The vector of visibilities is linearly related to brightness temperatures TB by means of a reconstruction matrix G. Due to the imperfect knowledge of the matrix G, the difficulties to invert such a big matrix together with some aliasing effects, spurious spatial correlations on brightness temperature snapshots are induced. BEC team is investigating the scope of such correlations. The shape of the found correlations reveals a clear geometrical pattern.

X polarization correlation pattern

a) X polarization correlation pattern. Correlation of the central point with the rest of the synthesized scene

X polarization correlation pattern

b) Real part of XY polarization correlation pattern. Correlation of the central point with the rest of the synthesized scene

X polarization correlation pattern

c) X polarization correlation pattern. Correlation of a point located in the edge of AF-FOV with the rest of the synthesized scene

X polarization correlation pattern

d) X polarization correlation pattern. Correlation of a point beyond the horizon with the rest of the synthesized scene

Figure 1.- Correlation patterns for different polarizations and antenna points. Patterns corresponding to polarization Y and to the imaginary part of cross polarization are similar to those corresponding to X polarization (figures 1.a, 1.c and 1.d) and real part of cross polarization respectively (figure 1.b).

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5+ years of fine-scale soil moisture estimates available!!

At Barcelona Expert Center (BEC) we are able to provide a Level 4 (L4) Surface Soil Moisture (SSM) product with 1 km spatial resolution that meets the requirements of land hydrology applications. To do so, we use a downscaling method that combines highly-accurate, but low-resolution, SMOS radiometric information with high resolution, but low sensitivity, visible-to-infrared imagery to SSM across spatial scales. A sample L4 SSM map from September 1, 2014 (6 AM) is shown in Figure 1.

Fig. 1. SMOS-BEC L4 product from September 1, 2014 (6 AM).

This downscaling approach was first presented in [1] along with results of its application to a set of SMOS images acquired during the commissioning phase over the Oznet network, South-East Australia. Using reprocessed SMOS data obtained with the latest L1 and L2 processors, we have further developed and validated this technique; we now use SMOS polarimetric and multi-angular information in the downscaling method, which results in improved fine-scale soil moisture estimates [2].
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Nodal sampling: removing tails and ripples from SMOS Brightness Temperatures

Since the beginning of SMOS mission, one of the problems that has strongly affected the quality of the retrieval of SSS from SMOS Brightness Temperatures (BT) is the presence of large human-generated Radio Frequency Interference (RFI) sources, as shown in the following figure:

Image acquired over a coastal area in Europe; several strong RFI sources and their trails are very noticeable

Image acquired over a coastal area in Europe; several strong RFI sources and the associated tails are very noticeable

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A review of microwave interferometric radiometry in remote sensing

Radio Science has recently published “Microwave interferometric radiometry in remote sensing: An invited historical review” by M. Martín-Neira, D. M. LeVine, Y. Kerr, N. Skou, M. Peichl, A. Camps, I. Corbella, M. Hallikainen, J. Font, J. Wu, S. Mecklenburg, and M. Drusch. The paper (Radio Science, volume 49, issue 6, pages 415–449, June 2014, DOI: 10.1002/2013RS005230) is led by Manuel Martín-Neira, the SMOS instrument (MIRAS) principal engineer, and is co-authored by three SMOS-BEC members: Adriano Camps, Ignasi Corbella and Jordi Font. We copy below the paper’s abstract:

The launch of the Soil Moisture and Ocean Salinity (SMOS) mission on 2 November 2009 marked a milestone in remote sensing for it was the first time a radiometer capable of acquiring wide field of view images at every single snapshot, a unique feature of the synthetic aperture technique, made it to space. The technology behind such an achievement was developed, thanks to the effort of a community of researchers and engineers in different groups around the world. It was only because of their joint work that SMOS finally became a reality. The fact that the European Space Agency, together with CNES (Centre National d’Etudes Spatiales) and CDTI (Centro para el Desarrollo Tecnológico e Industrial), managed to get the project through should be considered a merit and a reward for that entire community. This paper is an invited historical review that, within a very limited number of pages, tries to provide insight into some of the developments which, one way or another, are imprinted in the name of SMOS.

David and Goliath

This image of the first ESA ground tests of a MIRAS demonstrator was selected for the cover of the Radio Science issue. The online version of the paper can be seen at http://onlinelibrary.wiley.com/doi/10.1002/2013RS005230/full


Inter-comparison of SMOS and Aquarius observations

Passive microwave remote sensing at L-band is considered to be the most suitable technique to measure soil moisture and ocean salinity from space. The ESA’s SMOS and the NASA’s Aquarius/SAC-D are the two first satellite missions, carrying L-band radiometers on-board, measuring the global Earth’s surface as brightness temperatures (TB). The two radiometers have important differences in the architecture of the instruments as well as in their operation principles. In order to verify the continuity and the consistency of the data over the entire dynamic range of observations, a comparison between one year of SMOS and Aquarius measured TB has been performed over key regions over land (Amazon rainforest and Sahara desert), ice (Dome-C in Antarctica) and sea (South Pacific ocean).

Click here to observe selected regions in Google Earth.

A global view of the comparison is shown in Fig. 1, which displays the annual mean of the two radiometers for the three Aquarius incidence angles (inner 29.36º, middle 38.49º and outer 46.29º beams). In South Pacific, Dome-C and Sahara, higher incidence angles imply lower TB at horizontal polarization and higher TB at vertical polarization. However, in the Amazon, the TB variation with incidence angle and polarization is not clear due to the vegetation scattering. As expected, there is a small difference between polarizations (TBV-TBH) for vegetation-covered soils.


Fig. 1 Aquarius TB vs. SMOS TB at (a) horizontal and (b) vertical polarizations.

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