Conversion of sub-surface reflectances to above-surface MERIS reflectance

Factors for converting sub-surface reflectances to above-surface MERIS reflectances have been determined both as analytic functions and average numbers for solar zenith angles in the range 30°–75°, wind speeds up to 10 m s^?1, and the spectral domain 400–700 nm. The conversion factors have been obtained by numerical and statistical computations based on field observations of spectral radiance and irradiance, above and below the surface of the sea. The estimated maximum errors of the different algorithms range from ≤0.1% up to 10%, depending on the chosen method and the types of optical quantities that are available. The errors are smallest for solar zenith angles between 30° and 60° and increase when the solar zenith angle approaches 75°. The influence of the wind on the conversion factors is practically negligible. The algorithms, which have been derived for conditions representative of the Skagerrak and the adjacent seas, are assumed to be valid for both Case 1 and 2 waters.     

MODIS卫星数据地表反照率反演的简化模式

以内蒙西部地区的MODIS遥感图像数据和地表野外同步观测的光谱数据为例,在野外数据量较少且有定标数据的条件下反演地表反照率。使用6S大气1辐射传输模型进行大气校正,并通过MODTRAN4.0模型获取各波段地表入射光通量和窄波段的地表反照率;在窄波段反照率与宽波段反照率之间存在线性关系的前提下,以各波段的入射光通量占总入射通量的比例作为反演参数,实现窄波段到宽波段的反演。反演结果证明此方法简便可行。     

Estimating albedo from limited spectral and angular data

A database of synthetic albedo and directional reflectance values for vegetated surfaces was constructed utilizing mathematical models. This database enables the comparison of albedo with reflectances measured in narrow spectral bands in particular viewing directions for specified vegetation canopy and solar conditions. The analysis reported here is for spectral bands and angular regimes corresponding to the Along-Track Scanning Radiometer (ATSR-2) sensor on ERS-2. In the analysis multiple linear regression is used to calculate the best fit between modelled reflectance and modelled albedo. A primary estimate of albedo is calculated using reflectance data from the nadir direction only. Data from the forward view of the ATSR sensor are then used to provide additional information to correct the nadir estimate. The relationship between the regressed coefficients and the illumination conditions was investigated in order to provide a universal albedo estimation. Preliminary results for representative solar zenith and azimuth angles show an extremely good fit between modelled albedo and that estimated using the modelled ATSR-2 reflectance.     

Arctic tundra albedo and its estimation from spectral hemispheric reflectance

We present the results of a field experiment in which the nearly complete bidirectional reflectance distribution function of Alaskan arctic tundra sites early in the growing season is measured by the PARABOLA instrument. The spectral hemispheric reflectances were computed by angular integration of these measurements for three wavebands: red (650-670nm), near-infrared (810-840nm) and shortwave infrared (1620-1690 nm). Total albedo was then estimated by weighting the spectral hemispheric reflectances by the fraction of total solar irradiance in three broadband spectral regions (300-700, 700-1300 and 1300-4000nm) and representing each spectral region by the narrowband PARABOLA measurements. These calculations resulted in albedo estimates with a mean relative error of 15.7 per cent as compared to pyranometer measured albedo. Since vegetation reflectance varies significantly over each of the three broadband regions, additional reflectance weighting factors were computed from a combination of high spectral resolution canopy reflectance data and corresponding computed spectral solar irradiance. This additional reflectance weighting resulted in a reduction in the mean relative error to 7.5 per cent relative to pyranometer measured albedo. It is noted that the three spectral bands of the PARABOLA instrument data reported here are similar to those of the spectral wavebands planned for future Advanced Very High Resolution Radiometer (AVHRR) sensors on National Oceanic and Atmospheric Administration (NOAA) satellites. Therefore the results and techniques presented here may be useful for future global albedo estimation utilizing AVHRR sensors. The analysis presented here may also be applied to albedo estimation from satellite sensors with higher spectral resolution and more complete spectral coverage, such as the future orbiting MODIS sensor, in which the errors of spectral reflectance weighting will be reduced considerably due to a more complete sampling of the reflected spectrum.     

Remote sensing of leaf water content in the near infrared

A stochastic leaf radiation model was used to predict leaf spectral reflectance as a function of leaf water content for a dicot leaf. Simulated spectral reflectances were analyzed to quantify reflectance differences between different equivalent water thicknesses. Simulated results coupled with consideration of atmoshperic transmission properties and the incident solar spectral irradiance at the earth's surface resulted in the conclusion that the 1.55–1.75 μm region was the best-suited wavelength interval for satellite—platform remote sensing of plant canopy water status in the 0.7–2.5 μm region of the spectrum.     

An atmospheric correction procedure for the ATSR-2 visible and near-infrared land surface data

An atmospheric correction procedure for the solar reflecting channels of the Second Along Track Scanning Radiometer (ATSR-2) is described in which the aerosol optical thickness is retrieved from atmospheric simulations using the top-of-atmosphere (TOA) reflectances. The aerosol optical thickness is obtained when the ratio of the corresponding surface reflectances, in the off-nadir view to the nadir view, for two selected sensor channels, are equal. Directional reflectance models have been used to determine the validity of this approach and to determine suitable channel combinations. Simulations of the procedure for realistic surfaces indicate the retrieval of tau A 550 is better than 0.1 when the aerosol meteorology is known. The accuracy of the retrieval of surface reflectance is significantly affected by deviations from the expected aerosol meteorology. Factors affecting the accuracy of the correction procedure are described.     

Simulated reflectance technique for ATM image enhancement

The thermal band of the Airborne Thematic Mapper (ATM) has the same spatial resolution as that of the reflective spectral bands and therefore can be used together with other bands without degrading the resolution as happens with Landsat TM. Here we show that the total solar irradiance on the land surface can be simulated by a properly weighted summation of the thermal band and all the reflective spectral bands. The simulated reflectance as well as emittance can then be derived from the relation between irradiance, thermal emittance, spectral reflectance and albedo based on a simplified energy conservation model of solar radiation incident on a land surface. Compared with other methods for spectral enhancement and topography suppression, such as ratio and logarithmic residual, this method has the advantage of conserving surface albedo information and thus produces a reasonable simulation of reflectance.     

Characterization of the Reflectance Anisotropy of Three Boreal Forest Canopies in Spring–Summer

As a part of the Boreal Ecosystem-Atmosphere Study (BOREAS), measurements of the spectral reflectance anisotropy of three boreal forest canopies were studied for cloudless sky conditions at the phenological growth stages which were at or near maximum leaf area index at each site. The three sites were relatively homogeneous mature stands of black spruce, jack pine, and aspen located in the southern boreal zone of central Saskatchewan. Measurements of the spectal bidirectional reflectance factors with a 15° instrument field of view in three spectral bands centered at 662 nm, 826 nm, and 1658 nm were made with the PARABOLA instrument over a range of solar zenith angles typically varying from 35° (near solar noon) to 70°. The measured reflectance factors showed large anisotropy at all three sites and for all three wavelengths, with prominant backscatter peak reflectances, and strong retro solar view angle (hot spot) maximum reflectances in the visible (662 nm) and shortwave infrared (1658 nm) for the jack pine and black spruce sites, with a less pronounced hot spot at the aspen site. Pronounced effects of canopy and understory shadowing in the visible, as a function of solar zenith angle (SZA), were observed for the black spruce and jack pine sites, with resultant large linear increases in computed normalized difference and simple ratio vegetation indices as SZA increased for near-nadir view angles. Hemispheric spectral reflectances or spectral albedos were computed from angular integration of PARABOLA measured bidirectional reflectances. Visible (662 nm) hemispheric reflectances for the jack pine and black spruce canopies showed very little variation with solar zenith angle, while near-infrared hemispheric reflectances increased strongly with increasing SZA. Estimates were made of the total shortwave albedo for the aspen and jack pine sites from irradiance and reflectance weighting of the spectral hemispheric reflectances in the three measured wavelengths. Comparison of estimated to pyranometer measured total albedo showed all estimates to be biased high, but only by about 0.007–0.018, depending on which of two sets of pyranometer measured albedos were utilized for the comparison. The measured bidirectional reflectance factor (BRF) data sets reported in this study coupled with ancillary data of biophysical parameters collected at the same sites by BOREAS researchers provide a unique data set for the development and characterization of canopy bidirectional reflectance modeling and for the interpretation of remotely sensed data for boreal forest canopies.     

Initial assessment of ATSR-2 data structure for land surface applications

Principal component analysis was used to assess the structure of 1 km spatial resolution data from the second Along Track Scanning Radiometer (ATSR-2). Results indicate that the nadir spectral data have a structure similar to Landsat Thematic Mapper data and could be reduced to two eigenvectors with a loss of under 10 per cent of the total variance. The dual-view spectral data could be reduced to three eigenvectors with a loss of under 12 per cent of the total variance. The thermal data may provide additional information to that contained in the spectral data at some sites. The physical basis of the data structure is interpreted. Thus, ATSR-2 data are of considerable interest for estimating land surface characteristics at a regional scale.     

Estimation of multiple reflection and lowest order adjacency effects on remotely-sensed data

Abstract

The aims of this paper are (i) to investigate the lowest-order adjacency effect and the effect of multiple reflections between the ground and the atmosphere on the Advanced Very High Resolution Radiometer (AVHRR) channel 1 and channel 2 reflectances and on the normalized difference vegetation index (NDVI) and (ii) to make a comparison between global irradiances calculated from two empirical relations, one theoretical and the other experimental. Derivation of the theoretical empirical relation for global solar irradiance is in accordance with Iqbal (1983) whereas the experimental empirical relation is taken from 'Singh et al. (1985).The effect of multiple reflection on each channel reflectance is about 1·2 per cent so that the NDVI is not affected by including a multiple reflection term in the atmospheric correction algorithm. The lowest-order adjacency effect in channel 1 is about 7 per cent and in channel 2 is about 5 per cent and the NDVI changes by about 0·5 per cent which is negligible. These results show that the multiplereflection and lowest-order adjacency effects can be ignored if one is interested in the NDVI. But such effects could be important if one is using reflectances, say, for scene classification. The variations of global solar irradiance with the solar zenith angle for both empirical relations are presented. These relations vary in a remarkably similar manner. However, the experimental relation underestimates global irradiance compared to the value obtained from the theoretical empirical relation. As a result of this, channel 1 and channel 2 reflectances differ by about 16 and 10 per cent, respectively. But the effect on the NDVI is only about 2·4 per cent.     

The Mutsu Bay Experiment: Validation of ATSR-1 and ATSR-2 sea surface temperature

A comparison is presented between sea surface temperature measurements from the Along Track Scanning Radiometer (ATSR) space instruments and coincident in situ measurements made using the high-accuracy Scanning Infrared Sea Surface Temperature Radiometer (SISTeR). The in situ measurements were obtained as part of the Mutsu Bay Experiment (MUBEX) conducted in northern Japan during July and August 1996. Differences between the satellite and in situ measurements are lower than 0.2 deg K for ATSR-2 and 0.5 deg K for ATSR-1. This indicates that both ATSR-1 (after 5 years of continuous operation) and ATSR-2 are operating well within their accuracy design specification.     

An ATSR-2 Mosaic Image of Australia

A cloud free mosaic of Australia has been assembled using images from the Along Track Scanning Radiometer (ATSR-2) onboard the European Remote Sensing satellite (ERS-2). The mosaic is composed of daytime ATSR-2 images selected from the months of October and November 1995. The Australian mosaic is presented as a Normalised Difference Vegetation Index (NDVI ) image and a false colour image with the nadir 0.67, 0.87 and 1.6 w m channels being assigned to blue, green and red respectively. The individual ATSR-2 scenes show a strong systematic increase in brightness from east to west. This variation is identified as an illumination and viewing geometry effect and is a good example of how the surface Bidirectional Reflectance Distribution Function (BRDF) can effect satellite sensor imagery. The mosaic is intended to be the first in a series to be generated at bi-monthly intervals. The mosaic is being made available via the CSIRO Atmospheric Research web site to the general scientific community as a demonstration of ATSR-2 capabilities. The data includes all ATSR-2 spectral channels and views, and provides the basis for the derivation of various geophysical data products on a continental scale.     

一种面向低轨卫星网络的高效无证书身份认证方案

针对现有低轨卫星网络认证方案采用集中认证方式存在认证时延大和采用复杂的双线性映射存在计算开销大的问题。引入无证书认证模型,在Gayathri方案的基础上;设计了一种高效无证书认证方案。该方案将用户的公钥和真实身份统一起来,使得认证过程中不需要第三方参与,降低了认证时延;通过椭圆曲线上少量点乘和点加运算构建认证消息,避免使用双线性映射,降低了计算开销;并在随机预言模型下,基于椭圆曲线离散数学对问题假设,对其安全性进行了证明。最后,通过实验仿真,与现有低轨卫星身份认证方案相比,所提方案的认证时延、计算开销和通信开销较低。     

La Crau: A European test site for remote sensing validation

Well characterized reference sites are of major importance for the calibration of remote sensors and the validation of retrieval algorithms. A 0.4 km square zone of 'La Crau', an arid area in the South of France, is well established for the calibration of SPOT HRV sensors, and has been regularly used for a variety of remote sensing campaigns. This study reports an investigation of the potential use of a more extended area of La Crau as a validation site for lower spatial resolution satellite sensors. Sample Bidirectional Reflectance Distribution Function (BRDF) measurements were acquired over a large part of the area of La Crau during a 3-week period in July 1995. The data were analysed to provide a detailed spectral characterization of the surface of La Crau and its temporal, directional and spatial variability. The results were used to simulate the three optical channels and the 1.6 mu m channel of the ATSR-2 sensor and compared with ATSR-2 image data from the site.     

Mapping sediment-laden sea ice in the Arctic using AVHRR remote-sensing data: Atmospheric correction and determination of reflectances as a function of ice type and sediment load

Exploiting the fact that the spectral characteristics of light backscattered from sediment-laden ice differ substantially from those of clean ice and that sediment tends to accumulate at the ice surface during the first melt season, remote-sensing techniques provide a valuable tool for mapping the extent of particle-laden ice in the Arctic basin and assessing its particulate loading. This study considers two fundamental problems that still need to be addressed in order to make full use of satellite observations for this type of assessment: (i) the effects of the atmosphere on surface reflectances derived from radiances measured by the satellite sensor need to be quantified and ultimately corrected for, and (ii) the spectral reflectance of the ice surface as a function of particle loading and sub-pixel distribution needs to be determined in order to derive quantitative estimates from the at-sensor satellite signal. Here, spectral albedos have been computed for different ice surfaces of variable sediment load with a radiative transfer model for sea ice coupled with an optical model for particulates included in sea ice. In a second step, the role of the atmosphere in modulating the surface reflectance signal is assessed with the aid of an atmospheric radiative transfer model applied to a “standard” Arctic atmosphere and surface boundary conditions as prescribed by the sea ice radiative transfer model. A series of sensitivity studies helps assess differences between top-of-the-atmosphere and true surface reflectance and has been utilized to derive a look-up table for atmospheric correction of Advanced Very High Resolution Radiometer (AVHRR) data over sediment-laden sea ice surfaces. In particular, the effects of solar elevation, viewing geometry, and atmospheric properties are considered. The atmospheric corrections are necessary for certain geometries and surface types. Large discrepancies between raw and corrected data are particularly evident in the derived coverage of clean ice and ice with small sediment loading.     

更安全的匿名三因子多服务器身份认证协议研究

多服务器架构下的身份认证协议是远程认证的关键,但许多现有方案都存在潜在的攻击,未实现三因子安全性,忽略了匿名性。因此,需要指出其中的错误,并提出一个匿名的三因子方案。通过攻击者模型,攻击了温翔等人的方案,检验了新方案;使用椭圆曲线密码,保障认证阶段的核心安全性;使用模糊提取器与验证器,保护生物特征与口令;经与同类协议比较,分析了新协议的优势。分析表明,温翔等人的方案不能抵抗服务器仿冒用户,不具匿名性等。而新协议能有效防范智能卡丢失攻击、仿冒攻击等更多样的攻击,实现了匿名性、前向安全性等更全面的功能,计算效率也比前人提高了约14.8%。因此,可以应用于对安全性、可靠性要求较高的多服务器认证网络。     

A multi-angle spectrometer for automatic measurement of plant canopy reflectance spectra

The paper describes the design and operation of a multi-angle spectrometer (MAS) for automatic measurement of near-field spectral reflectances of plant canopies at hourly intervals. A novel feature of the instrument is a rotating periscope connected to a spectrometer via a fiber optic cable. Canopy reflectances are calculated for multiple view azimuths, at a single zenith angle from measurements of spectrometer dark current, incoming solar irradiance and reflected radiances. Spectral measurements are made between 300 and 1150 nm wavelength at a band-to-band spacing of 3 nm, and a bandwidth (full-width, half maximum) of 10 nm. Preliminary data analysis showed that the canopy reflectance model of Kuusk [Kuusk, A. (1995). A fast, invertible canopy reflectance model. Remote Sensing of Environment 51, 342-350] reproduced the observed large differences in visible and near-infrared (NIR) reflectances, but the model was unable to predict quantitatively the observed variations in the measured reflectance spectra with azimuth, particularly in the NIR. Discrepancies between model and measurements are likely due to the inhomogeneous nature of the forest canopy in contrast to the assumption of a uniformly absorbing turbid medium in the model. Measurements using the MAS can be used to investigate directional dependences of reflectance indices and for testing BRDF models used to separate geometrical and plant physiological contributions to the reflectance signals. The MAS provides continuous sampling of reflectance indices which can be compared with canopy properties such as chlorophyll content and photosynthetic capacity.     

In orbit calibration of the ATSR-1 1-6/ mi channel using high resolution data from the JERS-1 (Fuyo-l) Optical Sensor

The 1-6 ^m channel of the Along Track Scanning Radiometer-1 (ATSR-1) does not benefit from a pre-launch or onboard digital count to spectral radiance calibration mechanism. This Letter outlines the results of a post-launch calibration procedure, accomplished by comparing near-coincident data from the uncalibrated ATSR-1 instrument onboard ERS-1 and the (calibrated) OPS instrument onboard JERS-1. These results show that the ATSR-1 1-6(im channel can be considered linear in response to incoming radiance and equations are produced that allow data from this channel to be calibrated in units of spectral radiance and/ or spectral reflectance.     

Validation of MISR land surface broadband albedo

Land surface broadband albedo is a critical variable for many scientific applications. Due to the scarcity of spectral albedo measurements of the Earth's surface environments, it is useful to construct broadband albedo from spectral albedo data obtained by multi‐angle satellite observations. The Multi‐angle Imaging SpectroRadiometer (MISR) onboard NASA's Earth Observing System (EOS) Terra satellite provides land surface albedo products from multi‐angular observations; however, the products have not been comprehensively validated. We convert MISR spectral albedos to total shortwave albedos and validate them using ground measurements at different validation sites. For most surface types, a published narrowband to broadband conversion formula was used, but a new conversion formula for snow and ice covered sites is developed in this study where the spectral range of the instrument is different. Several comparisons are made: (1) between MISR directional‐hemispherical reflectance (DHR) or albedo and MODIS (Moderate Resolution Imaging Spectroradiometer) DHR; and (2) between MISR spectral DHR and bi‐hemispherical reflectance (BHR). The results show that: (1) both the value and the temporal trends of the MISR shortwave albedo and the ground measured shortwave albedo are in good agreement, with the exception of the snow and ice sites; (2) the MISR DHR conforms well to MODIS DHR; and (3) the values of MISR DHR and BHR are nearly identical.