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Michael E Schaepman - One of the best experts on this subject based on the ideXlab platform.
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USING MERIS ON ENVISAT FOR LAND COVER MAPPING
2020Co-Authors: Jan G. P. W. Clevers, Michael E Schaepman, R. Zurita Milla, Harm BartholomeusAbstract:This paper describes the results of a study towards the use of MERIS for land cover mapping at a national scale, using the Netherlands as a test site. Full resolution MERIS data of February 18 th 2003, April 16 th 2003, June 16 th 2003 and July 14 th 2003 were used in this study. The Dutch land use database LGN was used as a reference by aggregating the database from 25 m to 300 m. Results showed very good geometric characteristics of the MERIS images used in this study. Major land cover classes could be differentiated by their spectral signatures. For land cover classification best results in terms of classification accuracies were obtained for the image of July 14 th , 2003. The merit of MERIS lies primarily in the improved spatial resolution of the full resolution mode (300 m) when compared to lowresolution sensors. The use of multitemporal data analysis was also explored.
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Multitemporal unmixing of MERIS FR data
2020Co-Authors: R. Zurita Milla, Jan G. P. W. Clevers, Michael E Schaepman, Luis Gómez-chova, Gustavo Camps-vallsAbstract:The possibilities of using MERIS full resolution (FR) data to extract sub-pixel land cover composition over The Netherlands are explored in this paper. More precisely, the use of MERIS FR time series is explored in this paper since it should facilitate the discrimination of spectrally similar land cover types because of their seasonal variations. The main steps of the methodology used to extract sub-pixel information can be summarized as follows. First, a set of seven MERIS FR Level 1b images that covered the period February to December 2003 were selected. Second, the images were projected into the Dutch national coordinate system. Special attention was paid to this process in order to account for the orbital differences of each MERIS acquisition. Third, a cloud screening algorithm was applied to all MERIS images. Next, the MERIS level 1b TOA radiances were converted into surface reflectance. After that, the latest version of the Dutch land use database (LGN5) was used to support the selection of the endmembers from the MERIS images. Finally, a constrained linear unmixing algorithm was applied to each of the MERIS scenes and to the multi-temporal dataset. The results were validated both at sub-pixel and per-pixel scales using the LGN5 as a reference. The paper concludes by describing the potential and limitations of the selected approach to extract sub-pixel land cover information over heterogeneous and frequently clouded areas. * Corresponding author. Raul.Zurita-Milla@wur.nl.
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MONITORING VEGETATION DYNAMICS USING MERIS FUSED IMAGES
2020Co-Authors: R. Zurita Milla, Jan G. P. W. Clevers, G Kaiser, Werner Schneider, Michael E SchaepmanAbstract:The MEdium Resolution Imaging Spectrometer (MERIS) can be used to monitor vegetation dynamics at regional to global scales. However, the spatial resolutions provided by this sensor (300 or 1200 m) might not be appropriate to monitor fragmented landscapes. This is why the synergistic use of MERIS full resolution (300 m) and a high spatial resolution land use/land cover database (25 m) is studied in this paper. An unmixing-based data fusion approach was used to produce images that have the spectral and temporal resolutions provided by MERIS and a Landsat-like spatial resolution. The central part of The Netherlands was selected to illustrate this approach. Seven MERIS full resolution and one Landsat TM image were available over this area too. The radiometric characteristics of the fused images were evaluated at 25 and at 300 m. After this quantitative quality assessment, the best fused images were used to compute MTCI and MGVI profiles for the main land cover types present in the study area.
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Multitemporal validation of an unmixing-based MERIS cloud screening algorithm
2020Co-Authors: Luis Gómez-chova, Jan G. P. W. Clevers, Michael E Schaepman, R. Zurita Milla, Gustavo Camps-valls, Luis Guanter, J. Calpe, Jose MorenoAbstract:The operational use of MERIS images can be hampered by the presence of clouds because this instrument works in the visible and near-infrared part of the electromagnetic spectrum. This work presents a cloud screening algorithm that takes advantage of the high spectral and radiometric resolutions of MERIS and the specific location of some of its bands to increase the cloud detection accuracy. In order to validate the proposed algorithm we set up a real multitemporal land cover mapping application over cloudy areas. A temporal series of MERIS FR images acquired over The Netherlands was used to derive sub-pixel land cover composition by means of linear unmixing techniques.
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CLOUD SCREENING AND MULTITEMPORAL UNMIXING OF MERIS FR DATA
2020Co-Authors: Luis Gómez-chova, Jan G. P. W. Clevers, Michael E Schaepman, R. Zurita Milla, Gustavo Camps-valls, Luis Guanter, J. Calpe, Jose MorenoAbstract:The operational use of MERIS images can be hampered by the presence of clouds. This work presents a cloud screening algorithm that takes advantage of the high spectral and radiometric resolutions of MERIS and the specific location of some of its bands to increase the cloud detection accuracy. Moreover, the proposed algorithm provides a per-pixel probabilistic map of cloud abundance rather than a binary cloud presence flag. In order to test the proposed algorithm we propose a cloud screening validation method based on temporal series. In addition, we evaluate the impact of the cloud screening in a multitemporal unmixing application, where a temporal series of MERIS FR images acquired over The Netherlands is used to derive sub-pixel land cover composition by means of linear unmixing techniques.
J P Burrows - One of the best experts on this subject based on the ideXlab platform.
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retrieval of aerosol optical properties using MERIS observations algorithm and some first results
Remote Sensing of Environment, 2017Co-Authors: V V Rozanov, J P Burrows, M Vountas, R C Levy, Wolfhardt LotzAbstract:Abstract The MEdium Resolution Imaging Spectrometer (MERIS) instrument on board European Space Agency (ESA) Envisat made measurements from 2002 to 2012. Although MERIS was limited in spectral coverage, accurate Aerosol Optical Thickness (AOT) from MERIS data are retrieved by using appropriate additional information. We introduce a new AOT retrieval algorithm for MERIS over land surfaces, referred to as eXtensible Bremen AErosol Retrieval (XBAER). XBAER is similar to the “dark-target” (DT) retrieval algorithm used for Moderate-resolution Imaging Spectroradiometer (MODIS), in that it uses a lookup table (LUT) to match to satellite-observed reflectance and derive the AOT. Instead of a global parameterization of surface spectral reflectance, XBAER uses a set of spectral coefficients to prescribe surface properties. In this manner, XBAER is not limited to dark surfaces (vegetation) and retrieves AOT over bright surface (desert, semiarid, and urban areas). Preliminary validation of the MERIS-derived AOT and the ground-based Aerosol Robotic Network (AERONET) measurements yield good agreement, the resulting regression equation is y = (0.92x ± 0.07) + (0.05 ± 0.01) and Pearson correlation coefficient of R = 0.78. Global monthly means of AOT have been compared from XBAER, MODIS and other satellite-derived datasets.
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The Intercomparison of Top-of-Atmosphere Reflectivity Measured by MERIS and SCIAMACHY in the Spectral Range of 443–865 nm
IEEE Geoscience and Remote Sensing Letters, 2007Co-Authors: A. A. Kokhanovsky, W. Von Hoyningen-huene, K Bramstedt, J P BurrowsAbstract:This letter is aimed at better understanding of Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY) reflectance radiometric calibration errors using the Medium Resolution Imaging Spectrometer (MERIS) onboard ENVISAT. Earlier investigations showed that the SCIAMACHY calibration error can reach 20% in the visible bands, which prevents aerosol retrievals using the SCIAMACHY data. Recent improvements of the SCIAMACHY calibration are discussed. It is found that the differences in reflectances for the wavelengths 443, 560, 665, 754, and 865 nm between MERIS and improved Processor 6 SCIAMACHY data are close to the MERIS radiometric calibration error, which is below 4%
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the semianalytical cloud retrieval algorithm for sciamachy ii the application to MERIS and sciamachy data
Atmospheric Chemistry and Physics, 2006Co-Authors: A. A. Kokhanovsky, W Von Hoyningenhuene, V V Rozanov, S Noel, Konstantin Gerilowski, H Bovensmann, K Bramstedt, M Buchwitz, J P BurrowsAbstract:The SemiAnalytical CloUd Retrieval Algorithm (SACURA) is applied to the SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY) data. In particular, we derive simultaneously cloud optical thickness (COT) and cloud top height (CTH), using SCIAMACHY measurements in the visible (442 nm, COT) and in the oxygen A-band (755–775 nm, CTH). Some of the results obtained are compared with those derived from the Medium Resolution Imaging Spectrometer (MERIS), which has better spatial resolution and observes almost the same scene as SCIAMACHY. The same cloud algorithm is applied to both MERIS and SCIAMACHY data. In addition, we perform the vicarious calibration of SCIAMACHY at the wavelength 442 nm, using MERIS measurements at the same wavelength. Differences in the retrieved COT for the same cloud field obtained using MERIS and SCIAMACHY measurements are discussed.
Jurgen Fischer - One of the best experts on this subject based on the ideXlab platform.
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1d var retrieval of daytime total columnar water vapour from MERIS measurements
Atmospheric Measurement Techniques, 2011Co-Authors: R Lindstrot, Ralf Bennartz, Rene Preusker, Hannes Diedrich, Lionel Doppler, Jurgen FischerAbstract:Abstract. A new scheme for the retrieval of total columnar water vapour from measurements of MERIS (Medium Resolution Imaging Spectrometer) on ENVISAT (ENVIronmental SATellite) is presented. The algorithm is based on a fast forward model of the water vapour transmittance around 900nm, including a correction for atmospheric scattering and the influence of the temperature- and pressure-profile on the water vapour absorption lines. It provides the water vapour column amount for cloud-free scenes above land and ocean at a spatial resolution of 0.25 km × 0.3 km and 1 km × 1.2 km, depending on whether applied to the "full resolution" or the operational "reduced resolution" mode of MERIS. Uncertainties are provided on a pixel-by-pixel basis, taking into account all relevant sources of error. An extensive validation against various sources of ground-based reference data reveals a high accuracy of MERIS water vapour above land (root mean square deviations between 1.4 mm and 3.7 mm), apart from a wet bias of MERIS between 5 and 10% that is found in all comparison studies. This wet bias might be caused by spectroscopic uncertainties, such as the description of the water vapour continuum. Above ocean the accuracy is reduced, due to the uncertainty introduced by the unknown atmospheric scattering. Consequently, an increased root mean square deviation of ≥5 mm was found by comparing MERIS total columnar water vapour above ocean against SSM/I and ENVISAT MWR data. An increased wet bias of 2–3 mm is found over ocean, potentially due to a not properly working atmospheric correction scheme.
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ALBEDOMAP: MERIS land surface albedo retrieval using data fusion with MODIS BRDF and its validation using contemporaneous EO and in situ data products
2007 IEEE International Geoscience and Remote Sensing Symposium, 2007Co-Authors: Janpeter Muller, Rene Preusker, Jurgen Fischer, Marco Zuhlke, Carsten Brockmann, P RegnerAbstract:A representative albedo is required by ESA for the improved retrieval of atmospheric products, such as water vapour, from the ENVISAT-MERIS instrument. This albedo is required at 13 of the 15 spectral channels of MERIS. An algorithm is described which has been applied to the generation of a global albedo for these 13 spectral bands of MERIS over 16-day time periods at a resolution of 0.05deg for the time period from June 2002 to December 2006. Inputs to this algorithm include improved cloud and snow detection, aerosol correction using the MERIS data itself and derived Spectral surface Directional Reflectances, SDRs (Schroder et ah, Preusker et al, see, http://envisat.esa. int/workshops/MERIS_aatsr2005/). Owing to the lack of sufficient directional samples for most of the Earth's land surface, even for a monthly compositing time period, direct inversion of BRDF parameters for MERIS are not possible worldwide. Instead the 16-day Collection 4 MODIS BRDFs at 0.05deg resolution (derived using either full inversions or magnitude inversions from the MOD43 values) were employed in a magnitude inversion scheme for the 4 common MERIS (490plusmn5 [b3], 560plusmn5 [b5], 665plusmn5 [b7], 865plusmn10 [bl3]) bands with MODIS (459-479 {b3}, 545-565 {b4}, 620-670 {bl}, 841-8766 {b2}). The impact of applying Terra-only compared to Terra+Aqua-combined are here compared and presented. In keeping with previously reported results, there are larger numbers of full MODIS inversions (and consequently better MERIS albedos) for the combined BRDF but the total number of MODIS BRDF retrievals remains stubbornly similar. Spectral interpolation to the remaining 9 MERIS bands and to 3 broadband regions (0.4-0.7 mum, 0.7-3 mum, 0.4-3 mum) is then performed using polynomial look-up tables derived by Dr Shunlin Liang (University of Maryland). As spectral albedo data is required on monthly time-steps for most applications, a simple- minded weighting function based on the fractional time-period of each 16-day time period within a month was adopted for creating monthly products from 16-day products at 10km and 0.1deg. Global products on a 16-day time-step are being generated for the whole time period using both Terra-only and, where available, Terra+Aqua-combined Collection 4 MODlS-BRDFs. These EO products have been compared against contemporaneous MODIS spectral albedos (over 16-days) which have been gap-filled (Moody et al., 2005) as well as Collection 5 products, where available. Satellite intercomparisons have also been performed for monthly products of the monthly MERIS spectral albedo products for common bands against M1SR level-3 and POLDER level-3 products as well as for a subset of M1SR level-2 instantaneous products and the results will be shown here. Finally, inter-comparisons are shown of these EO products against ground-based albedometer measurements for a site in Finland will be presented as representative of one of several worldwide sites available through the CA VE taking into account, where necessary, issues associated with land cover heterogeneity.
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atmospheric correction algorithm for MERIS above case 2 waters
Journal of remote sensing, 2007Co-Authors: Th Schroeder, Jurgen Fischer, I Behnert, M Schaale, Roland DoerfferAbstract:The development and validation of an atmospheric correction algorithm designed for the Medium Resolution Imaging Spectrometer (MERIS) with special emphasis on case-2 waters is described. The algorithm is based on inverse modelling of radiative transfer (RT) calculations using artificial neural network (ANN) techniques. The presented correction scheme is implemented as a direct inversion of spectral top-of-atmosphere (TOA) radiances into spectral remote sensing reflectances at the bottom-of-atmosphere (BOA), with additional output of the aerosol optical thickness (AOT) at four wavelengths for validation purposes. The inversion algorithm was applied to 13 MERIS Level1b data tracks of 2002-2003, covering the optically complex waters of the North and Baltic Sea region. A validation of the retrieved AOTs was performed with coincident in situ automatic sun-sky scanning radiometer measurements of the Aerosol Robotic Network (AERONET) from Helgoland Island located in the German Bight. The accuracy of the derived reflectances was validated with concurrent ship-borne reflectance measurements of the SIMBADA hand-held field radiometer. Compared to the MERIS Level2 standard reflectance product generated by the processor versions 3.55, 4.06 and 6.3, the results of the proposed algorithm show a significant improvement in accuracy, especially in the blue part of the spectrum, where the MERIS Level2 reflectances result in errors up to 122% compared to only 19% with the proposed algorithm. The overall mean errors within the spectral range of 412.5-708.75 nm are calculated to be 46.2% and 18.9% for the MERIS Level2 product and the presented algorithm, respectively.
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MERIS in flight spectral calibration
International Journal of Remote Sensing, 2007Co-Authors: Steven Delwart, R. Santer, Rene Preusker, Ludovic Bourg, Didier Ramon, Jurgen FischerAbstract:This paper will describe the spectral calibration activities conducted during the MERIS commissioning phase and during operation since orbit 12000. MERIS is a medium resolution (300-1200m) push-broom imaging spectrometer covering the spectral domain 390-1040nm with 15 bands, programmable in position and width down to steps of 1.25nm. The onboard spectral calibration hardware is based on the use of an Erbium doped-diffuser panel presenting well-defined absorption peaks. In the spectral calibration mode, MERIS is configured with narrow bands centred on an Erbium absorption feature (two are used). The first orbit, the instrument is calibrated by viewing the "white" radiometric diffuser plate and the following orbit the "pink" Erbium diffuser plate is deployed. This method allows each of the MERIS detectors involved to be characterized in wavelength. The Fraunhofer absorption lines were used to complement these results by providing additional measurements in the violet and near infrared part of the spectrum. For this method, MERIS was configured both for Earth and diffuser observations and acquired data for only a limited number of orbits. This procedure was repeated for different band settings covering a number of Fraunhofer absorption lines. Finally, using Oxygen (O2A) absorption Earth observation data, two different approaches were developed, one based on the retrieval of surface pressure and one based on the shape of the O2A absorption band. Both methods were developed for clear sky land observations, but their performances are improved over bright land targets. Both methods agree to within an accuracy of 0.02 nm. The results from the different methods are analyzed in order to propose a spectral model for the MERIS instrument. Preliminary results of the spectral variation with time are reported. Except camera 4, the instrument is quite stable with time. Camera 4 needs further investigations to better understand its behaviour. Except for the use of the MERIS oxygen band, the spectral characterization of the other MERIS bands is achieved within the nominal accuracy (1 nm).
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Simulation of MERIS measurements above selected ocean waters
International Journal of Remote Sensing, 1999Co-Authors: Jurgen Fischer, Frank FellAbstract:We report on a number of radiative transfer calculations that have been performed in order to predict Medium Resolution Imaging Spectrometer (MERIS) measurements for the anticipated spectral bands and observation geometries for selected open ocean and coastal waters. The simulations show that, above coastal waters, significant levels of the water-leaving radiance must be expected in several of the MERIS channels dedicated to the atmospheric correction. These channels should be included in the algorithms to retrieve concentrations of water constituents. In addition, sunglint may affect large parts of MERIS satellite images, especially in the lower latitudes between ca 20 N and 20 S. Radiative transfer calculations similar to those presented here are used to derive algorithms for the assessment of constituent concentrations in Case II waters from ocean colour measurements.
Nadine Gobron - One of the best experts on this subject based on the ideXlab platform.
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MERIS Land Algorithm: Preliminary Validation Results
2020Co-Authors: Nadine Gobron, Malcolm Taberner, B. Pinty, F. Mélin, M. M. Verstraete, Jean-luc WidlowskiAbstract:This paper presents a first and preliminary evaluation of the performance of the algorithm implemented in the Medium Resolution Imaging Spectrometer (MERIS) ground segment for assessing the status of land surfaces. First, we propose an updated version of the MERIS algorithm itself, which improves the accuracy of the product. Second, we analyze the first results by inter-comparing the MERIS Global Vegetation Index (MGVI) to similar products derived from the Sea- viewing Wide Field-of-view Sensor (SeaWiFS) that are generated at the European Commission Joint Research Center (EC-JRC). The first evaluation between MERIS and SeaWiFS derived products is made using data acquired on August 3, 2002 by both instruments. The results show acceptable agreement and the differences are well understood by radiation transfer models. Finally, we propose some recommendations to improve the current MGVI-land algorithm.
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Validation of the Operational MERIS FAPAR (Invited Paper)
2020Co-Authors: Nadine Gobron, Bernard Pinty, Ophelie Aussedat, Thomas Lavergne, Frederic Melin, Monica Robustelli, Malcolm TabernerAbstract:This paper discusses the validation of the operational Medium Resolution Imaging Spectrometer (MERIS) land product which corresponds to the Fraction of Absorbed Photosynthetically Active Radiation (FAPAR). This biophysical variable acts as an indicator of the presence and state of the vegetation and it is currently estimated from MERIS data at both reduced and full resolution using a physically-based approach. The quality of the MERIS FAPAR products, derived from the MERIS Global Vegetation Index (MGVI) algorithm, capitalizes on the availability of MERIS data since June 2002. The validation protocol to assess the accuracy of FAPAR product includes 1) the estimates of theoretical uncertainties (versus the algorithm formulae and instrument calibration performance), 2) the assessment of the performance for detecting various events (verisimilitude) against the time-series of well-known land surfaces and 3) the direct comparisons of the FAPAR MERIS values to similar products generated by other independent sensors, like the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) and the MODerate Resolution Imaging Spectro- radiometer (MODIS), and against ground-estimates of FAPAR performed over various vegetation types. optimization and is applied here for the evaluation of the theoretical uncertainties. The design of the FAPAR algorithm, named MERIS Global Vegetation Index (MGVI), is based on a two steps procedure where the spectral reflectances measured in the red and near-infrared bands are, first rectified in order to ensure their 'decontamination' from atmospheric and angular effects and, second combined together in a mathematical formula to generate the FAPAR value. The reflectance in the blue band is employed to decontaminate the red and the near- infrared bands from atmospheric effects because of its sensitivity to the aerosol optical thickness. The decontamination from directional effects is achieved using a parametric angular model (6). Similar FAPAR algorithms have been also developed for additional optical sensors (7). The first section summarizes the method for assessing the theoretical FAPAR accuracy using the derivative of the algorithm formulae versus the instrumental calibration quality and presents few results. Examples of FAPAR time-series are presented in the second part to verify the performance of the operational MERIS FAPAR products with respect to the expected seasonal cycle over two well-known land surfaces. The third section summarizes the results of comparisons between MERIS daily FAPAR values and similar products generated by other independent sensors like SeaWiFS and MODIS over validation sites. This last step is performed from a previous analysis of the problem which ended up in the grouping of available field information into broad categories representing different radiative transfer regimes (8)(9). This greatly helps the interpretation of the results since it recognizes the various level of difficulties and sources of uncertainties associated with the radiative sampling of different types of vegetation canopies.
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POTENTIAL OF MERIS fAPAR FOR DROUGHT DETECTION
2020Co-Authors: S. Rossi, Monica Robustelli, C. Weissteiner, Giovanni Laguardia, B. Kurnik, M. Niemeyer, Nadine GobronAbstract:This work evaluates the potential for drought detection of the Fraction of Absorbed Photosynthetically Active Radiation (fAPAR) estimations operationally produced by the European Space Agency (ESA) as the Medium Resolution Imaging Spectroradiometer (MERIS) Level 2 Land Product. MERIS fAPAR daily data were used to produce 10-day time composites. The selected study area was the Iberia Peninsula. Seasonal anomalies were calculated for the fAPAR dataset as well as for those of other drought indicators from different sources, such as the Standardized Precipitation Index, a widely used drought index, or the Normalized Difference Vegetation Index (NDVI). The resulting time series of anomaly maps of different drought indicators were then compared to assess the correlation among them and their ability to detect recent drought events in the area. Results show that MERIS fAPAR has a capacity to detect droughts and offers an enhanced performance compared to NDVI.
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Exploiting ten years of MERIS data over land surfaces
2012 IEEE International Geoscience and Remote Sensing Symposium, 2012Co-Authors: Nadine Gobron, Olivier Arino, Jadunandan Dash, Lorena Hojas Gascon, Janpeter MullerAbstract:Envisat's Medium Resolution Imaging Spectrometer (MERIS) acquired multi-spectral imagery of the Earth in the optical domain over terrestrial surfaces for a decade at global scale. For the last ten years, scientists have used multi-spectral data or terrestrial geophysical products for characterizing the state of the global system and its variability. Our paper shows highlights of several achievements of the use of MERIS data over terrestrial surfaces but specifically focuses on regional to global scale applications. We first summarize daily operational biophysical parameters and present examples of their uses for the monitoring of land surface states and changes, especially related to ECVs. In addition, specific projects for deriving a series of land cover maps will be presented and we conclude on the MERIS data exploitation and highlights future applications.
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uncertainty estimates for the fapar operational products derived from MERIS impact of top of atmosphere radiance uncertainties and validation with field data
Remote Sensing of Environment, 2008Co-Authors: Nadine Gobron, Bernard Pinty, Ophelie Aussedat, Thomas Lavergne, Frederic Melin, Monica Robustelli, Malcolm Taberner, Olga Faber, Paul SnoeijAbstract:Abstract This paper discusses the accuracy of the operational Medium Resolution Imaging Spectrometer (MERIS) Level 2 land product which corresponds to the Fraction of Absorbed Photosynthetically Active Radiation (FAPAR). The FAPAR value is estimated from daily MERIS spectral measurements acquired at the top-of-atmosphere, using a physically based approach. The products are operationally available at the reduced spatial resolution, i.e. 1.2 km, and can be computed at the full spatial resolution, i.e. at 300 m, from the top-of-atmosphere MERIS data by using the same algorithm. The quality assessment of the MERIS FAPAR products capitalizes on the availability of five years of data acquired globally. The actual validation exercise is performed in two steps including, first, an analysis of the accuracy of the FAPAR algorithm itself with respect to the spectral measurements uncertainties and, second, with a direct comparison of the FAPAR time series against ground-based estimations as well as similar FAPAR products derived from other optical sensor data. The results indicate that the impact of top-of-atmosphere radiance uncertainties on the operational MERIS FAPAR products accuracy is expected to be at about 5–10% and the agreement with the ground-based estimates over different canopy types is achieved within ± 0.1.
Ralf Bennartz - One of the best experts on this subject based on the ideXlab platform.
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1d var retrieval of daytime total columnar water vapour from MERIS measurements
Atmospheric Measurement Techniques, 2011Co-Authors: R Lindstrot, Ralf Bennartz, Rene Preusker, Hannes Diedrich, Lionel Doppler, Jurgen FischerAbstract:Abstract. A new scheme for the retrieval of total columnar water vapour from measurements of MERIS (Medium Resolution Imaging Spectrometer) on ENVISAT (ENVIronmental SATellite) is presented. The algorithm is based on a fast forward model of the water vapour transmittance around 900nm, including a correction for atmospheric scattering and the influence of the temperature- and pressure-profile on the water vapour absorption lines. It provides the water vapour column amount for cloud-free scenes above land and ocean at a spatial resolution of 0.25 km × 0.3 km and 1 km × 1.2 km, depending on whether applied to the "full resolution" or the operational "reduced resolution" mode of MERIS. Uncertainties are provided on a pixel-by-pixel basis, taking into account all relevant sources of error. An extensive validation against various sources of ground-based reference data reveals a high accuracy of MERIS water vapour above land (root mean square deviations between 1.4 mm and 3.7 mm), apart from a wet bias of MERIS between 5 and 10% that is found in all comparison studies. This wet bias might be caused by spectroscopic uncertainties, such as the description of the water vapour continuum. Above ocean the accuracy is reduced, due to the uncertainty introduced by the unknown atmospheric scattering. Consequently, an increased root mean square deviation of ≥5 mm was found by comparing MERIS total columnar water vapour above ocean against SSM/I and ENVISAT MWR data. An increased wet bias of 2–3 mm is found over ocean, potentially due to a not properly working atmospheric correction scheme.
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assessment of the potential of MERIS near infrared water vapour products to correct asar interferometric measurements
International Journal of Remote Sensing, 2006Co-Authors: Zhenhong Li, Janpeter Muller, P A Cross, P Albert, J Fischer, Ralf BennartzAbstract:Atmospheric water vapour is a major limitation for high precision Interferometric Synthetic Aperture Radar (InSAR) applications due to its significant impact on microwave signals. We propose a statistical criterion to test whether an independent water vapour product can reduce water vapour effects on InSAR interferograms, and assess the potential of the Medium Resolution Imaging Spectrometer (MERIS) near‐infrared water vapour products for correcting Advanced SAR (ASAR) data. Spatio‐temporal comparisons show c. 1.1 mm agreement between MERIS and GPS/radiosonde water vapour products in terms of standard deviations. One major limitation with the use of MERIS water vapour products is the frequency of cloud free conditions. Our analysis indicates that in spite of the low global cloud free conditions (∼25%), the frequency can be much higher for certain areas such as Eastern Tibet (∼38%) and Southern California (∼48%). This suggests that MERIS water vapour products show potential for correcting ASAR interferomet...
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Assessment of the potential of MERIS near-infrared water vapour products to correct ASAR interferometric measurements
INT J REMOTE SENS, 2006Co-Authors: Ralf BennartzAbstract:Atmospheric water vapour is a major limitation for high precision Interferometric Synthetic Aperture Radar (InSAR) applications due to its significant impact on microwave signals. We propose a statistical criterion to test whether an independent water vapour product can reduce water vapour effects on InSAR interferograms, and assess the potential of the Medium Resolution Imaging Spectrometer (MERIS) near-infrared water vapour products for correcting Advanced SAR ( ASAR) data. Spatio-temporal comparisons show c. 1.1mm agreement between MERIS and GPS/ radiosonde water vapour products in terms of standard deviations. One major limitation with the use of MERIS water vapour products is the frequency of cloud free conditions. Our analysis indicates that in spite of the low global cloud free conditions (similar to 25%), the frequency can be much higher for certain areas such as Eastern Tibet (similar to 38%) and Southern California (similar to 48%). This suggests that MERIS water vapour products show potential for correcting ASAR interferometric measurements in certain regions.