70 research outputs found

    Modelling soil moisture at SMOS scale by use of a SVAT model over the Valencia Anchor Station

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    16 páginas, 9 figuras, 5 tablas.The main goal of the SMOS (Soil Moisture and Ocean Salinity) mission is to deliver global fields of surface soil moisture and sea surface salinity using L-band (1.4 GHz) radiometry. Within the context of the Science preparation for SMOS, the Valencia Anchor Station (VAS) experimental site, in Spain, was chosen to be one of the main test sites in Europe for Calibration/Validation (Cal/Val) activities. In this framework, the paper presents an approach consisting in accurately simulating a whole SMOS pixel by representing the spatial and temporal heterogeneity of the soil moisture fields over the wide VAS surface (50x50 km(2)). Ground and meteorological measurements over the area are used as the input of a Soil-Vegetation-Atmosphere-Transfer (SVAT) model, SURFEX (Externalized Surface) - module ISBA (Interactions between Soil-Biosphere-Atmosphere) to simulate the spatial and temporal distribution of surface soil moisture. The calibration as well as the validation of the ISBA model are performed using in situ soil moisture measurements. It is shown that a good consistency is reached when point comparisons between simulated and in situ soil moisture measurements are made. Actually, an important challenge in remote sensing approaches concerns product validation. In order to obtain an representative soil moisture mapping over the Valencia Anchor Station (50x50 km(2) area), a spatialization method is applied. For verification, a comparison between the simulated spatialized soil moisture and remote sensing data from the Advanced Microwave Scanning Radiometer on Earth observing System (AMSR-E) and from the European Remote Sensing Satellites (ERS-SCAT) is performed. Despite the fact that AMSR-E surface soil moisture product is not reproducing accurately the absolute values, it provides trustworthy information on surface soil moisture temporal variability. However, during the vegetation growing season the signal is perturbed. By using the polarization ratio a better agreement is obtained. ERS-SCAT soil moisture products are also used to be compared with the simulated spatialized soil moisture. However, the lack of soil moisture data from the ERS-SCAT sensor over the area (45 observations for one year) prevented capturing the soil moisture variability.The authors wish to thank the European Space Agency (ESA), the Centre National d’Etudes Spatiales (CNES), the Centre National de la Recherche Scientifique – Institut National des Sciences de l’Univers (CNRS- INSU SIC) and the French National Programme TOSCA (Terre, Oc´eans, Surfaces Continentales et Atmosph´ere) for supporting this work. We also wish to thank the NASA National Snow and Ice Data Center (NSIDC) for providing AMSR-E data as well as the Institute for Photogrammetry and Remote Sensing, Vienna University of Technology, Vienna, Austria for providing the ERS-SCAT data. We thank also the Centre National de Recherches Météorologiques and Jean Christophe Calvet (CNRM) - Météo-France for the SURFEX model. The authors wish to thank also the Spanish Agency for Meteorology (AEMet) and to the Jucar River Basin Authority (CHJ) for the meteorological data. Edited by: N. Verhoest The publication of this article is financed by CNRS-INSU.Peer reviewe

    Ultra-precise manufacturing of aspherical and freeform mirrors for high resolution telescopes

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    Optical freeforms are increasingly gaining interest for optical systems like telescopes and spectrometers. This is a key topic of discussions for many years; however, the manufacturing process of freeform optics remains a challenging task whose complexity derives from the missing symmetry in freeform surfaces. Ultra-precise manufacturing with diamond tools is an appropriate method to realize optical freeforms. Aspherical off axis mirrors machined similar to freeform or classical freeform mirrors like anamorphic mirrors can be fabricated in a deterministic process by using reference structures and correction loops. Diamond machining offers an excellent technology to meet the requirements regarding small values of surface deviation and low tolerances of position accuracy. Nevertheless, the typical micro-roughness of approximately 5 nm rms and the periodic turning structure set the limitation for diamond machined surfaces. The surfaces fulfill requirements for application in the Near Infrared (NIR) and Infrared (IR) spectral ranges, respectively. For smoothing the periodic structure, the diamond turning is combined with post polishing techniques like MRF (Magnetorheological Finishing) or computer assisted polishing. Therefore, the aluminum mirror has to be coated with amorphous nickel-phosphorous or silicon. Thus, the specification of applications in the visible (VIS) spectral range is reached. This process chain is interesting for a growing number of multi- and hyperspectral imaging devices such as telescopes and spectrometers based on all reflective metal optics. The paper summarizes the fabrication of an optical bench for a high resolution IR telescope, discusses the results of post polishing mirrors for VIS telescopes, and shows an efficient and easy snap-together alignment strategy. The optical function of the TMA demonstrator built is an afocal imaging for a Limb-Sounder Instrument with a magnification of 4.5:1. Besides the design and manufacturing approach, the snap-together integration of the optical bench is presented, too. The presentation is finished with a forecast of a freeform IR telescope based on anamorphic mirrors

    MERTIS: Optics manufacturing and verification

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    The MERTIS reflective infrared optics can be beneficial implemented as diamond turned aluminium mirrors coated with a thin gold layer. The cutting processes allow the manufacturing of both, the optical surface and mechanical interfaces, in tight tolerances. This is one of the major advantages of metal optics and was consequently used for the MERTIS sensor head optics. This paper describes the entire process chain of the MERTIS spectrometer optics including the manufacturing methods for the mirrors and for the spherical grating, the coating with sputtered gold for infrared reflectivity as well as the alignment and the verification of the spectrometer optics

    Development and fabrication of a hyperspectral, mirror based IR-telescope with ultra-precise manufacturing and mounting techniques for a snap-together system assembly

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    We report on an ultra-precise manufacturing method of a hyperspectral, mirror based IR-Telescope for applications in the Mid-wavelength infrared (MWIR). The proposed method simplifies the otherwise time consuming system alignment by the use of a snap-together assembly technique, that can be used for rotationally symmetric designs such as Korsch or Three Mirror Anastigmatic (TMA) telescope designs. The proposed technology is based on diamond machining of at least two mirror surfaces on one common substrate in one and the same machine setup. A novel hybrid manufacturing approach, which is a combination of diamond turning and diamond milling is used to manufacture fiducials and mounting planes that reduce the adjustment expenditure significantly. Reference elements and interfaces on the substrates are the basis for a precise metrology of the shape and the position of the optical surfaces as well as for the final assembly of the optical bench. The system integration into a hexapod framework is also based on precisely diamond machined stop surfaces to define the air distance and tilt between the mirrors. The presented method is a novel manufacturing and mounting technology for IR-telescope assemblies with diffraction limited optical performance in the MWIR
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