Modeling microwave emissions of erg surfaces in the Sahara Desert

Sand seas (ergs) of the Sahara are the most dynamic parts of the desert. Aeolian erosion, transportation, and deposition continue to reshape the surface of the ergs. The large-scale features (dunes) of these bedforms reflect the characteristics of the sand and the long-term wind. Radiometric emissions from the ergs have strong dependence on the surface geometry. We model the erg surface as composed of tilted rough facets. Each facet is characterized by a tilt distribution dependent upon the surface roughness of the facet. The radiometric temperature (T(b)) of ergs is then the weighted sum of the T(b) from all the facets. We use dual-polarization T(b) measurements at 19 and 37 GHz from the Special Sensor Microwave Imager aboard the Defense Meteorological Satellite Program and the Tropical Rainfall Measuring Mission Microwave Imager to analyze the radiometric response of erg surfaces and compare them to the model results. The azimuth angle (φ) modulation of T(b) is caused by the surface geometrical characteristics. It is found that longitudinal and transverse dune fields are differentiable based on their polarization difference (ΔT(b)) φ-modulation, which reflects type and orientation of dune facets. ΔT(b) measurements at 19 and 37 GHz provide consistent results. The magnitude of ΔT(b) at 37 GHz is lower than at 19 GHz due to higher attenuation. The analysis of ΔT(b) over dry sand provides a unique insight into radiometric emission over ergs

Evaluación de la degradación de la tierra usando la entropía de shannon sobre imágenes polarimétricas en desiertos costeros Patagónicos

En esta investigación se focalizó en la Entropía de Shannon (ES) para la caracterización de imágenes polarimétricas de apertura sintética. Este parámetro analiza la contribución de la información por pixeles individuales para toda la imagen en la evaluación de la degradación de la tierra en imágenes ALOS PALSAR. Escenas de polarización dual y cuádruple fueron adquiridas bajo el proyecto SAOCOM (Satélite Argentino de Observación con Microondas) en 2010 y 2011, del desierto costero noreste patagónico, Argentina. Los mapas fueron verificados con información de alta verosimilitud para la misma área de estudio. Los resultados muestran que la ES puede describir y precisar las características de las imágenes de manera obvia, de tal manera que representa un valor de referencia para la detección de la degradación de la tierra y la extracción de las características de los diferentes estados y transiciones.We focus on Shannon Entropy (SE) for the characterization of polarimetric Synthetic Aperture Radar (PolSAR) images. This approach analyzes the information contribution made by individual pixels to the whole image for assessment of land degradation in the information content of ALOS PALSAR images. Additionally, the performance of other polarization parameters, and polarization decomposition is illustrated and discussed. Dual-Pol and Quad-Pol scenes have been acquired under the SAOCOM (Satélite Argentino de Observación con Microondas, Spanish for Argentine Microwaves Observation Satellite) project in 2010 and 2011, from northeastern Patagonian coastal desert, Argentina. The accuracy of the SE map was assessed using a set of ground observations based on remotely sensed data that have higher accuracy. The results show that the SE can describe and determine the image features more obviously in the study area, so that it represents an important reference value for land degradation detection and land status characteristics extraction .Fil: del Valle, Hector Francisco. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Nacional Patagonico; ArgentinaFil: Hardtke, Leonardo Andrés. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Nacional Patagonico; ArgentinaFil: Blanco, Paula Daniela. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Nacional Patagonico; ArgentinaFil: Sione, Walter Fabian. Universidad Autónoma de Entre Ríos. Fac de Ciencia y Tecnologia. Centro Regional de Geomatica; Argentina. Universidad Nacional de Luján; Argentin

Analysis of scatterometer observations of Saharan Ergs using a simple rough facet model

The Sahara desert includes large expanses of sand dunes called ergs. These dunes are formed and constantly reshaped by prevailing winds. Previous study shows that Saharan ergs exhibit significant radar backscatter (sigmadeg) modulation with azimuth angle (Phi). We use sigmadeg measurements observed at various incidence angles (thetas) and Phi from the NASA scatterometer (NSCAT), the Seawinds scatterometer aboard QuikSCAT (QS-CAT), the ERS scatterometer (ESCAT) and the Tropical Rain Monitoring Mission\u27s Precipitation Radar (TRMM-PR) to model the sigmadeg response from sand dunes. Sand dunes are modeled as a composite of tilted rough facets and small ripples. The dune fields are modeled as composed of many simple dunes. The sigmadeg measured by the scatterometer from (thetas, Phi) look direction is the sum of the returns from all the rough facets in the footprint. The model is applied to linear and transverse dunes with rough facets and Gaussian tilt distributions. The model results in a sigmadeg response similar to the NSCAT and ESCAT observations over areas of known dune types in the Sahara. This analysis gives a unique insight into scattering by large scale sand bedforms