Alessandra Monerris

Land Geophysical Parameters Retrieval Using the Interference Pattern GNSS-R Technique

In the past years, the scientific community has placed a special interest in remotely sensing soil moisture and vegetation parameters. Radiometry and radar techniques have been widely used for years. Global Navigation Satellite Systems opportunity signals Reflected (GNSS-R) over the earths surface are younger, but they have already shown their potential to perform these observations. This paper presents a GNSS-R technique, based on Global Positioning System (GPS) measurements, that allows the retrieval of several geophysical parameters from land surfaces. This technique measures the power of the interference signal between the direct GPS signal and the reflected one after scattering over the land, so it is called Interference Pattern Technique (IPT). This paper presents the results obtained after applying the IPT for topography, soil moisture, and vegetation height retrievals over vegetation-covered soils.

The emissivity of foam-covered water surface at L-band: theoretical modeling and experimental results from the FROG 2003 field experiment

Sea surface salinity can be measured by microwave radiometry at L-band (1400-1427 MHz). This frequency is a compromise between sensitivity to the salinity, small atmospheric perturbation, and reasonable pixel resolution. The description of the ocean emission depends on two main factors: (1) the sea water permittivity, which is a function of salinity, temperature, and frequency, and (2) the sea surface state, which depends on the wind-induced wave spectrum, swell, and rain-induced roughness spectrum, and by the foam coverage and its emissivity. This study presents a simplified two-layer emission model for foam-covered water and the results of a controlled experiment to measure the foam emissivity as a function of salinity, foam thickness, incidence angle, and polarization. Experimental results are presented, and then compared to the two-layer foam emission model with the measured foam parameters used as input model parameters. At 37 psu salt water the foam-induced emissivity increase is /spl sim/0.007 per millimeter of foam thickness (extrapolated to nadir), increasing with increasing incidence angles at vertical polarization, and decreasing with increasing incidence angles at horizontal polarization.

SMOS REFLEX 2003: L-band emissivity characterization of vineyards

The goal of the Soil Moisture and Ocean Salinity mission over land is to infer surface soil moisture from multiangular L-band radiometric measurements. As the canopy affects the microwave emission of land, it is necessary to characterize different vegetation layers. This paper presents the Reference Pixel L-Band Experiment (REFLEX), carried out in June-July 2003 at the Vale/spl grave/ncia Anchor Station, Spain, to study the effects of grapevines on the soil emission and on the soil moisture retrieval. A wide range of soil moisture (SM), from saturated to completely dry soil, was measured with the Universitat Polite/spl grave/cnica de Catalunyas L-band Automatic Radiometer (LAURA). Concurrently with the radiometric measurements, the gravimetric soil moisture, temperature, and roughness were measured, and the vines were fully characterized. The opacity and albedo of the vineyard have been estimated and found to be independent on the polarization. The /spl tau/--/spl omega/ model has been used to retrieve the SM and the vegetation parameters, obtaining a good accuracy for incidence angles up to 55/spl deg/. Algorithms with a three-parameter optimization (SM, albedo albedo, and opacity) exhibit a better performance than those with one-parameter optimization (SM).

L-band dielectric properties of different soil types collected during the mouse 2004 field experiment

The brightness temperature measured from land is related to the soil moisture through the dielectric constant, which depends, among other parameters, on soil composition and porosity. The aim of MOUSE campaign was to establish the brightness temperature dependence on the soil type and moisture content (1). For this study and for the retrieval of the soil moisture content from radiometric measurements, it is necessary to use accurate models for the soil complex dielectric constant. This paper presents a technique to measure the complex dielectric constant using a microwave stripline setup, and its application to the soil samples measured during MOUSE 2004 campaign. The measured dielectric constants of each soil type versus volumetric soil moisture (0% to 40%) are compared with the values obtained using models already appeared in the literature. Keywords-Soil moisture; dielectric constant.

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.

GNSS-R Delay-Doppler Maps over land: Preliminary results of the GRAJO field experiment

Within the ESAs SMOS CAL/VAL activities, the GPS and Radiometric Joint Observations (GRAJO) field experiment was conducted. Apart from contributing to the SMOS CAL/VAL, the main purpose of the GRAJO experiment was to study the synergy of L-band radiometry and GNSS Reflectometry for soil moisture retrieval. Long-term experiments under controlled conditions. During one of these intensive short term experiments, the griPAU instrument (a Delay-Doppler Map GNSS-R receiver) was set up. The first results derived from the griPAU measurements are presented.

Application of Neural Networks to Soil Moisture Retrievals from L-Band Radiometric Data

Many algorithms for retrieving geophysical variables are based on optimal estimation approaches, which can be time consuming specially if a large amount of data is to be processed. On its part, neural networks provide results almost in real time, but their use is still not generalised for remote sensing applications. In this work, a set of neural networks was trained with simulations using numerical land emission models and tested using L-band radiometric data of bare soils acquired during the T-REX and MOUSE field experiments. Soil moisture retrieved by the neural networks was then compared to ground-truth data.

Topography effects on the L-band emissivity of soils: TuRTLE 2006 field experiment

The impact of topography on soil emissivity at L-band is not well known. In order to provide data to assess this issue, the Topography effects on RadiomeTry at L-band Experiment (TuRTLE) 2006 was carried out in a mountainous area about 50 km North of Barcelona (Spain). Radiometric measurements covering the mountain slope, and up to the sky were acquired. Concurrently, ground-truth and meteorological data were registered. Radiometric measurements have been compared to the emissivity obtained by simulation using a facet model which considers the high resolution digital elevation model and land cover map of the area. Polarization mixing due to surface tilting and integration over the antenna pattern have also been included in the simulator, and results agree with the radiometric measurements. The largest discrepancies occur for an almost bare soil at large local incidence angles and H-polarization, close to the radiometer, which suggests that further modeling work is still needed.

Soil moisture retrieval errors using l-band radiometry induced by the soil type variability

The soil dielectric permittivity relates the soil water content to the brightness temperature. Thus, the precise estimation of the dielectric permittivity will lead to a better soil moisture retrieval. This paper presents a study which analyses the errors induced in the soil moisture retrieval due to the variability associated to soil type. Data sets over six different soil types were acquired during the MOUSE 2004 experiment, performed at the Joint Research Centre (JRC) outdoor test facility, Ispra, Italy. The radiometric data were processed using both laboratory measurements of soil dielectric permittivity and those computed using other semi-empirical models available in the literature. The soil texture was measured by the JRC, and soil height profiles were acquired by a laser profiler. Soil moisture retrievals using standard models lead to retrieval errors of 5.6% and 6.2%, while when the measured dielectric constant was used the errors were of the order of 5.1%. Errors associated to the modelling of the roughness effect and those due to interference signals are believed to be responsible of some scattering in the retrieved data.

SMOS' brightness temperatures validation: First results after the commisioning phase

Soil Moisture and Ocean Salinity (SMOS) mission is the second of European Space Agencys (ESA) Living Planet Programme Earth Explorer Opportunity Missions. SMOSs objective is to provide global and frequent Soil Moisture and Sea Surface Salinity maps. The single payload embarked on SMOS is the Microwave Imaging Radiometer by Aperture Synthesis (MIRAS), it is a 2D interferometric radiometer operating at the protected L-band with a nominal frequency of 1413.5 MHz. Since SMOS is the first 2D interferometric radiometers put in orbit so far, the characterization of the interferometrically measured brightness temperatures is an attractive topic for the scientific community. This study is focused on the estimation of the systematic antenna-based pattern in the measured brightness temperatures. Two improvements to the currently used method (Ocean Target Transformation) are proposed: • The elimination of the use of any forward model in the estimation of the bias. • The homogenization of the geophysical parameters distribution within the SMOS Field of View. Ocean Target Transformation is introduced in section 2, the proposed model-free methodology is described in section 3, while the effect of homogenizing the geophysical parameters distribution inside the FOV is assessed in section 4. The main conclusions are presented in section 5.