M. Vall-llossera

MIRAS end-to-end calibration: application to SMOS L1 processor

End-to-end calibration of the Microwave Imaging Radiometer by Aperture Synthesis (MIRAS) radiometer refers to processing the measured raw data up to dual-polarization brightness temperature maps over the earths surface, which is the level 1 product of the Soil Moisture and Ocean Salinity (SMOS) mission. The process starts with a self-correction of comparators offset and quadrature error and is followed by the calibration procedure itself. This one is based on periodically injecting correlated and uncorrelated noise to all receivers in order to measure their relevant parameters, which are then used to correct the raw data. This can deal with most of the errors associated with the receivers but does not correct for antenna errors, which must be included in the image reconstruction algorithm. Relative S-parameters of the noise injection network and of the input switch are needed as additional data, whereas the whole process is independent of the exact value of the noise source power and of the distribution network physical temperature. On the other hand, the approach relies on having at least one very well-calibrated reference receiver, which is implemented as a noise injection radiometer. The result is the calibrated visibility function, which is inverted by the image reconstruction algorithm to get the brightness temperature as a function of the director cosines at the antenna reference plane. The final step is a coordinate rotation to obtain the horizontal and vertical brightness temperature maps over the earth. The procedures presented are validated using a complete SMOS simulator previously developed by the authors.

Analysis of correlation and total power radiometer front-ends using noise waves

A complete and systematic noise analysis of radiometer front-ends, including both total power and correlation measurements, is presented. The procedure uses the concepts of noise waves and S-parameters, widely used in microwave systems design and takes into account full noise characterization of receivers including mismatch effects. The general formulation is compatible with known total power radiometer analysis and is specially appropriate in correlation radiometers for which the effect of nonideal components, such as input isolators, is analyzed. Along with numerical simulations, simple formulas are given to compute the measured visibility in nonideal conditions. The analysis is validated using experimental results consisting of correlation measurements of four receivers placed inside an anechoic chamber. Good agreement between theoretical predictions and experimental data is observed.

Surface Topography and Mixed Pixel Effects on the Simulated L-band Brightness Temperatures

The impact of topography and mixed pixels on L-band radiometric observations over land has not been properly quantified so far. With this purpose, simulations have been performed with an upgraded version of the Soil Moisture and Ocean Salinity (SMOS) End-to-end Performance Simulator (SEPS). The brightness temperature (TB) generator of SEPS has been improved to include a 100 m-resolution land cover map (21 uses) and a 30 m-resolution digital elevation map of the Catalonian Region (NE Spain). The high resolution TB generator allows to assess the errors in the soil moisture retrieval algorithms due to limited spatial resolution, and to set the basis for the development of pixel disaggregation techniques. Variation of the local incidence angle as seen from SMOS, shadowing, and atmospheric effects (up- and down-welling radiation) due to surface topography have been analyzed. Results are compared to TB values computed under the assumption of an ellipsoidal Earth

Review of the CALIMAS Team Contributions to European Space Agency’s Soil Moisture and Ocean Salinity Mission Calibration and Validation

This work summarizes the activities carried out by the SMOS (Soil Moisture and Ocean Salinity) Barcelona Expert Center (SMOS-BEC) team in conjunction with the CIALE/Universidad de Salamanca team, within the framework of the European Space Agency (ESA) CALIMAS project in preparation for the SMOS mission and during its first year of operation. Under these activities several studies were performed, ranging from Level 1 (calibration and image reconstruction) to Level 4 (land pixel disaggregation techniques, by means of data fusion with higher resolution data from optical/infrared sensors). Validation of SMOS salinity products by means of surface drifters developed ad-hoc, and soil moisture products over the REMEDHUS site (Zamora, Spain) are also presented. Results of other preparatory activities carried out to improve the performance of eventual SMOS follow-on missions are presented, including GNSS-R to infer the sea state correction needed for improved ocean salinity retrievals and land surface parameters. Results from CALIMAS show a satisfactory performance of the MIRAS instrument, the accuracy and efficiency of the algorithms implemented in the ground data processors, and explore the limits of spatial resolution of soil moisture products using data fusion, as well as the feasibility of GNSS-R techniques for sea state determination and soil moisture monitoring.

Soil moisture downscaling activities at the REMEDHUS Cal/Val site and its application to SMOS

The European Space Agency (ESA) Soil Moisture and Ocean Salinity (SMOS) mission will provide accurate global surface soil moisture estimates. However, due to technological limitations, the spatial resolution of SMOS observations is limited to 40-km. This resolution is adequate for many global applications, but restricts the use of the data in regional studies over land, where a resolution of 1–10 km is needed. Different downscaling experiments have been carried out at the REMEDHUS Cal/Val site within the GPS and RAdiometric Joint Observations one-year field experiment (GRAJO) to explore the possibility of enhancing the spatial resolution of SMOS-derived soil moisture estimates. This paper investigates the use of visible/infrared remote sensing data to improve the spatial resolution of passive microwave soil moisture estimates, and presents an approach to down-scale airborne observations acquired with the UPC Airborne RadIomEter at L-band (ARIEL) using LANDSAT imagery. Although a full validation of the method was not possible, preliminary results indicate that with this approach it is feasible to improve the spatial and radiometric resolution of the soil moisture estimates. The comparison with ground-based soil moisture demonstrates the strength of the link between visible/infrared satellite data and soil moisture status, and the potential of adapting this downscaling technique to SMOS-derived soil moisture estimates.

Soil moisture and vegetation height retrieval using GNSS-R techniques

Global Navigation Satellite Signals Reflections (GNSS-R) techniques are currently being used for remote sensing purposes retrieving geophysical parameters over different types of surfaces. Over the ocean, sea state information can be retrieved to improve the ocean salinity retrieval. Furthermore, over land these techniques can be used to retrieve soil moisture. This paper presents the theoretical and experimental results of using GNSS-R to retrieve soil moisture when vegetation is present. The particular technique being applied in this study is the Interference Pattern Technique (IPT) that measures the interference pattern of the GPS direct and reflected signals, after reflecting over the surface.

Impact and compensation of diffuse sun scattering in 2D aperture synthesis radiometers imagery

 The SMOS (Soil Moisture and Ocean Salinity) Mission was selected in May 1999 by the European Space Agency to provide global and frequent soil moisture and sea surface salinity maps. SMOS has a sun-synchronous polar dawn-dusk orbit and its single payload is MIRAS (Microwave Imaging Radiometer by Aperture Synthesis): a Lband 2D aperture synthesis interferometric radiometer. The array boresight is tilted 32.5o upwards in the orbital plane to maximize the angular excursion of the observed pixels. This study describes the practical implementation of the Sun effects (direct and diffuse scattering models) into the SMOS End-to-end Performance Simulator, a technique to cancel these effects in the brightness temperature image generated by the instrument, and the residual errors that can be expected.

Soil Moisture Retrieval Using L-band Radiometry: Dependence on Soil Type and Moisture Profiles

The Monitoring Underground Soil Experiment (MOUSE) 2004 was conducted at the Joint Research Centre, Ispra (Italy) in May-June 2004. During MOUSE 2004, L-band radiometric observations at vertical and horizontal polarizations of six different bare fields at five incidence angles from 25deg to 65deg were acquired. Soil moisture and temperature were registered at various depths, and surface roughness was measured with a laser profiler. The radiometric data set was processed using a laboratory-derived expression for the dielectric constant and results were compared with those obtained using dielectric constant values computed with semi-empirical models (Wang & Schmugge and Dobson et al.). Soil moisture values retrieved from the measurements using various dielectric constant models were then compared to the actual soil moisture measured at different depths (0, 5, 10, and 15 cm) to get an indication of the goodness of each model for each soil type, and the depth at which the soil moisture is being remotely sensed for each type

Sea surface emissivity observations at L-band: first preliminary results of the WInd and Salinity Experiment WISE-2000

This paper presents the first measurements processed from the data acquired with the L-band AUtomatic RAdiometer (LAURA) during the WInd and Salinity Experiment (WISE-2000). Experimental results are compared to a sea surface emissivity model developed by the Polytechnic University of Catalonia (UPC). The sensitivity of the brightness temperatures at vertical and horizontal polarizations to wind speed are discussed, as well as the weak azimuthal signature found.

Reliability analysis in aperture synthesis interferometric radiometers: Application to L band Microwave Imaging Radiometer with Aperture Synthesis instrument

The Microwave Imaging Radiometer with Aperture Synthesis (MIRAS) instrument will be the first radiometer using aperture synthesis techniques for Earth observation. It will be boarded in the Soil Moisture and Ocean Salinity (SMOS) Earth Explorer Opportunity Mission of the European Space Agency and launched in 2005. The configuration under study in the MIRAS Demonstrator Pilot Project is a Y-shaped array with 27 dual-polarization L band antennas in each arm, spaced 0.89 wavelengths. In addition to these 81 antennas there are 3 additional ones between the arms for phase restoration and redundancy purposes and an extra one at the center of the Y array that is connected to a noise injection radiometer. The digitized in-phase and quadrature outputs of each receiver are multiplexed in groups of four and optically transmitted to the hub where the complex cross correlations are computed. In this configuration there are 85 antennas-receiving channels and 21 multiplexers. The objectives of this paper are twofold: (1) the study of the performance degradation of Y-shaped aperture synthesis interferometric radiometers in case of single or multiple subsystem failures and (2) a reliability analysis at subsystem level.