MERIS atmospheric correction over coastal waters: validation of the MERIS aerosol models using AERONET

Standard aerosol models (SAMs) are used for the Medium-Resolution Imaging Spectrometer (MERIS) level-2 processing over water, first to remotely sense the aerosols in the near-infrared and secondly to perform the atmospheric correction for ocean colour analysis. However, are these SAMs still suitable over coastal areas? The present work was intended to answer that question through the use of the Aerosol Robotic Network (AERONET) by selecting CIMEL radiometers operating over the sea surface or near the coastline. The current official MERIS algorithm overestimates aerosol optical thickness (AOT) over coastal waters at 865 nm. This can be related either to incorrect assumptions of the underlying surface assumption or to the assumptions of the aerosol properties (e.g. phase function). This study looks at the importance of aerosol modelling and confirms that the improved aerosol models must be used in the retrieval chain. Extinction measurements were first used to derive the aerosol optical thicknesses (AOTs). The spectral dependency of the AOTs between 670 nm and 865 nm allowed the selection of a standard aerosol model. The ability of the standard aerosol models to retrieve the AOTs at 440 nm was then analysed as a key element in the extrapolation of the aerosol path radiance from the near-infrared to the blue spectral range. The two outputs of this analysis are systematic biases in this retrieval process and accordingly they are an estimation of the dispersion. The first output can be defined as a corrective factor in the aerosol path radiance at 440 nm and the second output can be used for error analysis. A radiative transfer code was used to simulate the sky radiance in the principal plane of acquisition. Comparisons at 870 nm illustrated the ability of the standard aerosol models to retrieve the aerosol path radiances with a direct impact on the AOT retrieval from satellite observations at 865 nm.     

The sensitivity of MERIS atmospheric correction over water to aerosol climatology

Performing a classical atmospheric correction over water requires a well-defined climatology representative of the aerosols encountered in the remote areas of oceans. Different climatologies built up at the global scale are candidates to be implemented, as an auxiliary data file (ADF) including look-up tables (LUTs) with radiative properties of the aerosols, in a traditional atmospheric correction algorithm. In addition to these, two regional climatologies were developed in the 2-Seas region, comprising both the Eastern English Channel and the North Sea, and at the Acqua Alta Oceanographic Tower (AAOT) in the Adriatic Sea. By using the optical data processor of the European Space Agency (ODESA), Medium Resolution Imaging Spectrometer (MERIS) level-1 (L1) data extracted from the MERis MAtchup In-situ Database (MERMAID) were processed to obtain the level-2 (L2) products over water. For a given climatology, a full processing chain was developed to generate the MERIS aerosol LUTs suitable to ODESA. The final step consisted of an analysis of the L2 products, for both the aerosols and marine reflectance, in the framework of the evaluation of the performance of each climatology in atmospheric correction over oceans. Finally, we recommend using the regional aerosol climatologies available from the AErosol RObotic NETwork (AERONET) database, at least for the retrieval of the L2 aerosol product. In regard to marine reflectance, this remains more challenging and needs a more extensive analysis.     

Assimilation of leaf area index derived from ASAR and MERIS data into CERES-Wheat model to map wheat yield

This study presents a method to assimilate leaf area index retrieved from ENVISAT ASAR and MERIS data into CERES-Wheat crop growth model with the objective to improve the accuracy of the wheat yield predictions at catchment scale. The assimilation method consists in re-initialising the model with optimal input parameters allowing a better temporal agreement between the LAI simulated by the model and the LAI estimated by remote sensing data. A variational assimilation algorithm has been applied to minimise the difference between simulated and remotely-sensed LAI and to determine the optimal set of input parameters. After the re-initialisation, the wheat yield maps have been obtained and their accuracy evaluated.The method has been applied over Matera site located in Southern Italy and validated by using the dataset of an experimental campaign carried out during the 2004 wheat growing season.Results indicate that, LAI maps retrieved from MERIS and ASAR data can be effectively assimilated into CERES-Wheat model thus leading to accuracies of the yield maps ranging from 360 kg/ha to 420 kg/ha.     

A comparison between two radiative transfer models for atmospheric correction over a wide range of wavelengths

Correction of atmospheric effects caused by molecular and particulate scattering and absorption from hyperspectral measurements is desirable whenever comparisons are to be made with data acquired under different atmospheric or viewing conditions. Absolute atmospheric correction of optical remotely sensed data relies on Radiative Transfer Codes (RTCs) capable of simulating interactions between the radiation and the medium. Several RTCs are available with different features, but the two most widely used nowadays are the Second Simulation of Satellite Signal in the Solar Spectrum (6S) and the MODerate-Resolution TRANsmittance Code (MODTRAN). In this paper, we perform a comparison between these two models and report the results obtained, specifically investigating the differences that emerged during the various test stages.     

利用MERIS和AATSR资料估算黄土高原塬区植被含水量时空变化

通过利用2005年黄土高原塬区夏季地表过程野外观测试验期间收集的地面观测的植被含水量、中分辨率影像光谱仪(Medium Resolution Imaging Spectrometer,MERIS)和高级沿轨迹扫描辐射计(Advanced Along-Track Scanning Radiometer,AATSR)卫星遥感资料,分别对归一化差值植被指数(Normalized Different Vegetation Index)和归一化差值水分指数(NormalizedDifferent Water Index)与植被含水量(Vegetation water content)变化关系进行了分析比较,得到了两种不同的植被指数在作物生长背景影响下的异同。并分别利用MERIS的观测资料计算了NDVI,利用AATSR观测资料计算了NDWI,通过分析得出:随着作物的生长或生物量的增加,归一化差值植被指数变化趋于饱和,而归一化差值水分指数仍然继续增加。进一步通过同步地面野外观测植被含水量与卫星遥感观测资料的对比,建立了归一化差值植被指数、归一化差值水分指数和实际野外测量植被含水量的关系,并且得到由两种植被指数反演植被含水量的方法和地面观测之间的误差分别为1.030 64 kg·m-2和0.940 45 kg·m-2。最后通过分析后总结出,利用归一化差值水分指数来反演黄土高原塬区夏季玉米冠层的含水量优于利用归一化差值植被指数。     

Validation of atmospheric scattering functions used in atmospheric correction over the ocean

A new approach has been developed to validate atmospheric correction (AC) over the ocean. The latter has been applied to the ground-segment data from the Medium Resolution Imaging Spectrometer (MERIS) on board the Environmental Satellite (Envisat) platform. An atmospheric validation database has been built up with a ground-based instrument, i.e. the Cimel radiometer from the Aerosol Robotic Network (AERONET). The aim of this work is to assess the atmospheric scattering functions needed to perform AC of remotely sensed data. The inputs to this new methodology were provided by AERONET, after inversion of radiometric measurements (i.e. solar direct extinctions and sky diffuse radiances) to get the inherent optical properties (IOPs) of the aerosols. The successive orders (SOs) of scattering code have been used as the radiative transfer tool in this study. This new concept for the validation of AC has been illustrated with the MERIS level-2 data extracted from the Meris Matchup In-situ Database (MERMAID) over the Acqua Alta Oceanographic Tower (AAOT, Venice – Italy). Results indicate first, an overestimate of the MERIS aerosol optical thickness (AOT) at 865 nm, and second, a marine reflectance affected by a negative bias of about 13% at 412.5 nm. This yields to an overestimate of the MERIS algal-1 pigment index, which may exceed 50%, over AAOT. The same trend is also observed in the determination of the algal-2 pigment index.     

Lidar and spectroradiometer measurements of atmospheric aerosol optical characteristics over an urban area in Sofia,Bulgaria

A series of campaigns involving a systematic investigation of the atmosphere over an urban area of Sofia city were carried out. A European Aerosol Research Lidar Network (EARLINET) scanning aerosol lidar, a spectroradiometer, a standard sun photometer and a ground meteorological station were used in the observations. Multiple aerosol layers of variable thickness (200–600 m) were observed systematically in the planetary boundary layer (PBL) over the study area and the experimental data were compared with theoretical data. A study of the optical characteristics of the atmospheric aerosol, including the extinction coefficient, aerosol optical depth (AOD) and Angstrom parameters α and β, was performed and their variations followed during the convective boundary layer (CBL) formation. Values of the AOD obtained using the different instruments during simultaneous measurements were compared. Preliminary results show that the AOD values recorded by the sun photometer and those calculated on the basis of the spectroradiometer data are higher than those retrieved from the lidar data. Determination of the atmospheric optical depth and extinction coefficient using a ground-based spectral instrument is a relatively simple and inexpensive method of monitoring the total aerosol content in the atmosphere as well as the air quality over the region.     

Comparison of vegetation phenological metrics extracted from GIMMS NDVIg and MERIS MTCI data sets over China

Evaluating vegetation phenology is crucial for a better understanding of the effects of climate change on the terrestrial ecosystem. The scientific community has used various vegetation index data sets from different sensors to quantify vegetation phenology from regional to global scales. The normalized difference vegetation index (NDVI) related to photosynthetic activities is the most widely used index. Recently, a number of published articles have used the Medium Resolution Imaging Spectrometer (MERIS) terrestrial chlorophyll index (MTCI) to measure vegetation phenology. MTCI can closely represent the red-edge position (REP). Unlike NDVI, MTCI is more sensitive to high values of chlorophyll content. However, the consistency of vegetation phenological metrics derived from MTCI and NDVI needs to be further explored. This study compared two phenological metrics, i.e. onset of greenness (OG) and end of senescence (ES), extracted from MERIS MTCI data and Advanced Very High Resolution Radiometer (AVHRR) Global Inventory Modeling and Mapping Studies (GIMMS) first generation NDVI (NDVIg) data, which has the longest time records, at nine regions in China from 2003 to 2006. The results showed that the differences of OG and ES vary between different vegetation types, regions, and years, although both NDVI and MTCI time series capture the growth patterns well for most vegetation types. Compared to ES, the OG estimates are more consistent. NDVI yields in general later ES estimates than MTCI.     

Locating congested segments over the Internet by clustering the delay performance of multiple paths

Delay variation-based detection and location of congestion in a large network is considered. Since the Internet is still highly prone to performance deterioration due to transient large delays, locating a part of the network (segments) responsible is vital to ensure that Internet Service Providers can mitigate or prevent such performance deterioration. In the proposed method, the end-to-end packet delays from multiple origins to multiple destinations are actively and continuously measured. By analyzing those data on delay variation along each monitored path, congestion is detected by finding a delay performance deterioration worse than a predefined criteria and a congested segment responsible could be inferred by finding a set of paths among which delay variations are strongly correlated. This is a network tomographic approach based on a clustering technique that effectively tackles the correlation among packet delay variation along individual paths. The proposed method was evaluated through a real-world long-term experiment on the Japan’s commercial Internet, and was shown to have considerable potential to promptly locate congested segments through various analyses on the experimental results.     

ENVISAT-1 ASAR成像系统的设计和实现

针对ENVISAT-1卫星,研究了ASAR数据成像关键算法,给出一种提高系统多普勒估计稳定性的渐进策略。文章详细论述了系统的设计和实现方法,并给出了成像系统的实际处理结果。     

Potential of ASAR/ENVISAT for the characterization of soil surface parameters over bare agricultural fields

The objective of this investigation is to analyze the sensitivity of ASAR (Advanced Synthetic Aperture Radar) data to soil surface parameters (surface roughness and soil moisture) over bare fields, at various polarizations (HH, HV, and VV) and incidence angles (20°-43°). The relationships between backscattering coefficients and soil parameters were examined by means of 16 ASAR images and several field campaigns. We have found that HH and HV polarizations are more sensitive than VV polarization to surface roughness. The results also show that the radar signal is more sensitive to surface roughness at high incidence angle (43°). However, the dynamics of the radar signal as a function of soil roughness are weak for root mean square (rms) surface heights between 0.5 cm and 3.56 cm (only 3 dB for HH polarization and 43° incidence angle). The estimation of soil moisture is optimal at low and medium incidence angles (20°-37°). The backscattering coefficient is more sensitive to volumetric soil moisture in HH polarization than in HV polarization. In fact, the results show that the depolarization ratio σ_HH⁰/σ_HV⁰ is weakly dependent on the roughness condition, whatever the radar incidence. On the other hand, we observe a linear relationship between the ratio σ_HH⁰/σ_HV⁰ and the soil moisture. The backscattering coefficient ratio between a low and a high incidence angle decreases with the rms surface height, and minimizes the effect of the soil moisture.     

The atmospheric correction of water colour and the quantitative retrieval of suspended particulate matter in Case II waters: Application to MERIS

The remote sensing of turbid waters (Case II) using the Medium Resolution Imaging Spectrometer (MERIS) requires new approaches for atmospheric correction of the data. Unlike the open ocean (Case I waters) there are no wavelengths where the water-leaving radiance is zero. A coupled hydrological atmospheric model is described here. The model solves the water-leaving radiance and atmospheric path radiance in the near-infrared (NIR) over Case II turbid waters. The theoretical basis of this model is described, together with its place in the proposed MERIS processing architecture. Flagging procedures are presented that allow seamless correction of both Case I waters, using conventional models, and Case II waters using the proposed model. Preliminary validation of the model over turbid waters in the Humber estuary, UK is presented using Compact Airborne Spectrographic Imager (CASI) imagery to simulate the MERIS satellite sensor. The results presented show that the atmospheric correction scheme has superior performance over the standard single scattering approach, which assumes that water-leaving radiance in the NIR is zero. Despite problems of validating data in such highly dynamic tidal waters, the results show that retrievals of sediments within 50% are possible from algorithms derived from the theoretical models.     

A generalized relay identification method for time delay and non-minimum phase processes

A generalized relay identification method is proposed in this paper for time delay and non-minimum phase processes. A distinct merit of the proposed method is that the process gain can be uniformly derived from a single run of the relay test, independent of the relay type being unbiased or biased. An analytical relay response expression is derived for assessing the process response under an unbiased or biased relay test. A frequency response algorithm is also derived for estimating the multiple frequency response points of the process. By categorizing cases of the model parameters to be identified, the corresponding identification algorithms are developed using fitting conditions established for the process response at the oscillation frequency. Based on the improved estimation of multiple frequency response points, a recursive least-squares identification algorithm is then proposed for obtaining further enhanced fitting accuracy over a user specified frequency range, e.g., the low frequency range often concerned in practical controller tuning. Illustrative examples from the recent literature are given to demonstrate the effectiveness and merits of the proposed algorithms.     

Polarimetric investigation of the atmospheric aerosols over the Pacific ocean

Abstract

For a feasibility study setting up the maritime aerosol model, measurements of the radiance and degree of polarization of the skylight radiation were carried out over the Pacific ocean during the cruise of the Hakuho Maru KH-88-2. The wavelengths 0.4,0.6 and 0.9 μm were considered using the portable spectrora-diometer (resolution about 30nm). The derived visibility is in good agreement with that reported from ships. Furthermore the optical thickness and the mixing ratio of aerosol constituents are shown to be derived from the measurements of maximum degree of polarization for moderate solar elevations, provided that the maritime aerosol is a mixture of oceanic and water-soluble aerosols.     

Reducing power dissipation, delay, and area in logic circuits by narrowing transistors

An important aspect of fast CMOS logic-circuit design is transistor sizing. Designers routinely set transistor channel lengths at the minimal values the process permits (unless there is a need to introduce delay). Specification of channel widths, however, requires careful consideration and is based mainly on the capacitive load the circuit must drive and on considerations of energy dissipation and chip area. Experienced designers use heuristic techniques to help with this aspect of circuit design. Sutherland and his associates have developed a powerful systematic method called logical effort. We present a method, based on an analysis at the logic level, enables designers to identify transistors in certain logic elements whose channels can be narrowed to a minimum without incurring penalties. The logical effort method, or some other procedure, can then follow this preliminary step.     

A comparison of two methods for reducing take-off delay at London Heathrow airport

This paper describes recent research into the departure process at London Heathrow, with the primary motivation of reducing the amount of fuel used, improving both the economic and environmental costs. Two proposals are considered here. The first proposal considers the practicality and potential benefits of aiding the controller in improving the take-off sequencing. The second proposal aims to absorb some of the inevitable delay for aircraft at the stands, before the engines are started, but also involves a take-off sequencing aspect. Models for the two take-off sequencing problems are presented in this paper, the second of which includes an additional pushback time (or TSAT) allocation sub-problem which has to be solved subsequently. These models have distinctive differences from the models for the take-off and arrival sequencing problems which are usually considered in the literature, since they take into account necessary constraints imposed due to the control problem (whether a sequence can actually be achieved, and how) in each case. As discussed in this paper, the control problem cannot be ignored by the controllers at Heathrow, and so it cannot be ignored by any realistic system to aid them in their tasks. Comparative take-off sequencing results are presented for the two systems, and the potential benefits from providing decision support to the runway controllers or improved TSAT allocation at the stands are considered. The paper ends with some overall conclusions from the research, showing the large potential benefits of these systems. The TSAT allocation system which is discussed in this paper has been developed for implementation at London Heathrow as one element of a major project which focuses upon collaborative decision making.     

Estimating the uncertainty in the estimated mean area under the ROC curve of a classifier

This article considers the problem of binary classification and its assessment in a distribution-free approach. We estimate the area under the ROC curve (a more general performance metric than the error rate) of a classifier using a bootstrap-based estimator. We then use the method of the influence function to estimate the uncertainty of that estimate from the very same bootstrap samples. Monte Carlo trials show that small-sample estimates can be obtained with little bias.     

Finding the optimal tradeoff between area and delay in multiple constant multiplications

Over the years many efficient algorithms for the multiplierless design of multiple constant multiplications (MCMs) have been introduced. These algorithms primarily focus on finding the fewest number of addition/subtraction operations that generate the MCM. Although the complexity of an MCM design is decreased by reducing the number of operations, their solutions may not lead to an MCM design with optimal area at gate-level since they do not consider the implementation costs of the operations in hardware. This article introduces two approximate algorithms that aim to optimize the area of the MCM operation by taking into account the gate-level implementation of each addition and subtraction operation which realizes a constant multiplication. To find the optimal tradeoff between area and delay, the proposed algorithms are further extended to find an MCM design with optimal area under a delay constraint. Experimental results clearly indicate that the solutions of the proposed algorithms lead to significantly better MCM designs at gate-level when compared to those obtained by the solutions of algorithms designed for the optimization of the number of operations.     

A multiple scattering algorithm for atmospheric correction of remotely sensed ocean colour (MERIS instrument): Principle and implementation for atmospheres carrying various aerosols including absorbing ones

A multiple scattering algorithm for atmospheric correction of satellite ocean colour observations is described. This algorithm, precisely designed for the MERIS instrument, globally assesses the combined contributions of aerosols and molecules to the multiple scattering regime. The approach was introduced in a previous work, where it was shown that, for a given aerosol, multiple scattering effects can be assessed through the relationship between the aerosol optical thickness and the relative increase in the path radiance that results from the progressive introduction of this aerosol within an aerosol-free atmosphere. Based on considerations about the accuracy to which the water-leaving radiances should be retrieved, the need to account for multiple scattering is argued. The principle of the algorithm is then presented, and tests and sensitivity studies (especially as regards aerosol type and vertical distribution) are performed to assess its performance in terms of errors on the retrieved water-leaving reflectances and pigment concentrations. The algorithm is able to perform the correction for atmospheres carrying several aerosol types, including absorbing ones, through their identification in the near-infrared, and through the detection of their absorption by means of appropriate assumptions on the marine signals at 510 and 705nm.