Bidirectional NDVI and atmospherically resistant BRDF inversion for vegetation canopy

The normalized difference vegetation index (NDVI) has been widely applied in optical remote sensing. However, it has been demonstrated that NDVI is still partially affected by atmospheric path scattering and bidirectional (illumination and viewing geometry) effects. In this paper we present the benefit of using a bidirectional NDVI, and we discuss the problems in using the maximum NDVI composite method. Based on the assumption that a clear day has a larger NDVI value and a cloudy day has a smaller NDVI value (smaller reflectance in the near-infrared band and larger reflectance in red band due to atmospheric path scattering), the ratio of squared observed NDVI values and calculated NDVI values is used as a weight in our inversion method. The calculated NDVI values are derived from previously inverted bidirectional reflectance distribution functions (BRDFs). The inversion process will loop until all weights converge. Our research on the early Terra/MODIS data using a semiempirical kernel-driven BRDF model (the RossThick-LiTransit model) shows that this new method can improve inversion results whenever some cloudy pixels are not filtered out. As cloud detection and subpixel cloudiness are always a problem, this technique should still be very useful in improving the quality of BRDF inversion.     

Designing optimal spectral indexes for remote sensing applications

Satellite remote sensing data constitute a significant potential source of information on our environment, provided they can be adequately interpreted. Vegetation indexes, a subset of the class of spectral indexes, represent one of the most commonly used approaches to analyze data in the optical domain. An optimal spectral index is very sensitive to the desired information (e.g. the amount of vegetation), and as insensitive as possible to perturbing factors (such as soil color changes or atmospheric effects). Since both the desired signal and the perturbing factors vary spectrally, and since the instruments themselves only provide data for particular spectral bands, optimal indexes should be designed for specific applications and particular instruments. This paper describes a rational approach to the design of an optimal index to estimate vegetation properties on the basis of the red and near-infrared reflectances of the AVHRR instrument, taking into account the perturbing effects of soil brightness changes, atmospheric absorption and scattering. The rationale behind the Global Environment Monitoring index (GEMI) is explained, and this index is proposed as an alternative to the Normalized Difference Vegetation Index (NDVI) for global applications. The techniques described here are generally applicable to any multispectral sensor and application     

Scattering of Solar and Atmospheric Background Radiation from a Target

Light scattering property of environment is very important in theoretical study and application of the remote sensing. What's more, it is valuable for infrared radiation, imaging, and the detection of target and tracking. In this paper, solar and atmospheric background radiation, and their scattering property from target are discussed. BRDF (Biodirectional Reflectance Distribution Function) is a very important quantity that shows the radiation and reflection feature of target. According to electromagnetic radiant and scattering theories, the relationship between laser radar scattering cross section (LRCS) and BRDF is introduced. LOWTRAN model is an effective method of calculating the spectral distribution of solar and atmospheric radiation. Here it is applied to compute solar and atmospheric background radiation scattered from a target. The relative equations are deduced. Thus, the spatial and spectral distribution of scattering light is given. As a special example, for the Lambert's surface, the equations are simplified. As a result, the spatial and spectral distributions scattering radiation of solar and atmospheric background from a rough painted surface are present. The scattering of solar radiation plays a primary role in MIR region, but scattering of atmospheric background radiation is higher in LIR region. At the same time, there is obviously specular reflectance for solar radiation due to coherent scattering from rough surface.     

Sun and view angle corrections on reflectances derived fromNOAA/AVHRR data

A new method is proposed for the reduction of the noise-like fluctuations associated with variations of Sun-target-sensor geometry in multitemporal AVHRR data in the visible and near-infrared bands. Its principle is to adjust, over a monthly period, a three-parameter model of surface bidirectional reflectance on a time series of cloud-free atmospherically corrected AVHRR data. One parameter of the model represents the surface reflectance corrected for angular effects. Time profiles of corrected reflectances are obtained by making the monthly period slide over the annual vegetation cycle. This procedure is applied to an annual cycle of AVHRR data on seven test sites in France representative of bare soils, agricultural crops, and forested thematic areas. The method is evaluated according to two criteria, which are the ability of the bidirectional reflectance model to reduce the amount of noise-like temporal fluctuations of AVHRR data, and the stability of the retrieved parameters when random noise is artificially added to the original AVHRR data set. The perturbations induced by the coupling between diffuse sky radiance effects and the non-Lambertian behavior of ground reflectance are also discussed. The method is proved to give satisfactory results, and can potentially be used to compare satellite-derived reflectances obtained not only at different times, but also at different places and with different sensors     

Modeling of atmospheric effects on the angular distribution of abackscattering peak

Atmospheric radiative transfer calculations with extremely high angular resolution of the radiance distribution are used to analyze the effects of atmospheric multiple scattering and absorption on the angular distribution of a narrow retroreflection peak such as the canopy hot spot, or Heiligenschein. Using a realistic aerosol-loaded atmospheric model, the results demonstrate that the angular width of the model hot spot (for half widths between 1° and 4° and various types of vegetative canopies) is, to within about 10%, invariant to atmospheric perturbations for total optical depths of the atmosphere up to 1.0 at 0.55 μm and up to 0.9 at 0.86 μm. This result is a consequence of the angular filter effect of the surface bidirectional reflectance distribution function and the comparatively broad angular signature of atmospheric backscattering. However, the contrast ratio of the backscattering peak is strongly influenced by atmospheric extinction. As a consequence for satellite remote sensing, the results indicate that the canopy hot spot can be classified as an angular reflectance signature with an angular width that remains invariant to atmospheric scattering and absorption     

Effect of orbital drift and sensor changes on the time series of AVHRR vegetation index data

This paper assesses the effect of changes in solar zenith angle (SZA) and sensor changes on reflectances in channel 1, channel 2, and normalized difference vegetation index (NDVI) from the advanced very high resolution radiometer (AVHRR) Pathfinder land data set for the period July 1981 through September 1994. First, the effect of changes in SZA on channel reflectances and NDVI is derived from equations of radiative transfer in vegetation media. Starting from first principles, it is rigorously shown that the NDVI of a vegetated surface is a function of the maximum positive eigenvalue of the radiative transfer equation within the framework of the theory used and its assumptions. A sensitivity analysis of this relation indicates that NDVI is minimally sensitive to SZA changes, and this sensitivity decreases as leaf area increases. Second, statistical methods are used to analyze the relationship between SZA and channel reflectances or NDVI. It is shown that the use of ordinary least squares can generate spurious regressions because of the nonstationary property of time series. To avoid such a confusion, the authors use the notion of cointegration to analyze the relation between SZA and AVHRR data. Results are consistent with the conclusion of theoretical analysis from equations of radiative transfer. NDVI is not related to SZA in a statistically significant manner except for biomes with relatively low leaf area. From the theoretical and empirical analysis, they conclude that the NDVI data generated from the AVHRR Pathfinder land data set are not contaminated by trends introduced from changes in solar zenith angle due to orbital decay and changes in satellites (NOAA-7, 9, 11). As such, the NDVI data can be used to analyze interannual variability of global vegetation activity.     

Non-Lambertian Effects on Remote Sensing of Surface Reflectance and Vegetation Index

This paper discusses the effects of non-Lambertian reflection from a homogeneous surface on remote sensing of the surface reflectance and vegetation index from a satellite. Remote measurement of the surface characteristics is perturbed by atmospheric scattering of sun light. This scattering tends to smooth the angular dependence of non-Lambertian surface reflectances, an effect that is not present in the case of Lambertian surfaces. This effect is calculated to test the validity of a Lambertian assumption used in remote sensing. For the three types of vegetations considered in this study, the assumption of Lambertian surface can be used satisfactorily in the derivation of surface reflectance from remotely measured radiance for a view angle outside the backscattering region. Within the backscattering region, however, the use of the assumption can result in a considerable error in the derived surface reflectance. Accuracy also deteriorates with increasing solar zenith angle. The angular distribution of the surface reflectance derived from remote measurements is smoother than that at the surface. The effect of surface non-Lambertianity on remote sensing of vegetation index is very weak. Since the effect is similar in the visible and near infrared part of the solar spectrum for the vegetations treated in this study, it is canceled in deriving the vegetation index. The effect of the diffuse skylight on surface reflectance measurements at ground level is also discussed.     

中巴地球资源02星CCD图像交叉定标与大气校正研究

利用LANDSAT-5的TM对CBERS-2的CCD 1-4波段进行交叉定标,得到了CCD 1-4波段的参考辐射定标系数,并对这组定标系数进行了验证.利用6S模型和同步气象资料,对CBERS-2的CCD数据,进行了大气校正和反射率反演.最后对CCD 1-4波段大气校正前后的反射率和归一化植被指数(NDVI)的变化进行了对比研究.发现大气校正后的CCD3和CCD4波段的地面反射率明显升高,大气校正后的CCD图像呈现出高植被覆盖区NDVI增大,低植被覆盖区NDVI减小的趋势.     

Estimation of the ratio of sensor degradation between NOAA AVHRRchannels 1 and 2 from monthly NDVI composites

Vegetation monitoring, based on the normalized difference vegetation index (NDVI) calculated from the Advanced Very High Resolution Radiometer (AVHRR) channels 1 and 2 data, requires continuous updates of calibration coefficients to correct for the sensor degradation in these channels. A method was developed to estimate calibration coefficients with monthly composited NDVI data from desert targets without recourse to the component channels 1 and 2 data. The method was tested on NDVI data from the AVHRR onboard the NOAA-7, -9, and -11 satellites for the period from 1982 until 1993. The results of the method outlined in this paper correlated high r, between 0.94 and 0.95, with the results from other studies that estimated sensor degradation for the individual AVHRR bands     

基于短波红外波段的Ⅱ类水体MODIS影像大气校正算法

提出了一种基于短波红外(SWIR,short wave infrared)波段离水反射率为0的Ⅱ类水体大气校正算法.采用MODIS的1.240μm和1.640μm两个SWIR波段的反射率计算出了可见光及近红外(NIR, near infrared)波段气溶胶散射反射率,进而反演得到了这些波段的离水反射率.应用该算法对中国东部近海及湖泊的Ⅱ类水体进行了大气校正,并与实测数据和常用的大气校正算法进行了比较分析,结果表明该算法能够有效地去除大气的影响.     

Aerosol retrieval over land from AVHRR data-application foratmospheric correction

Correction of Advanced Very High Resolution Radiometer (AVHRR) imagery for the aerosol effect requires retrieval of the aerosol loading from the images. Two retrieval algorithms that were previously developed for Landsat are modified for the AVHRR. The methods determine the aerosol optical thickness over land surfaces from AVHRR band one data independently of ancillary information. The first method retrieves aerosols based on the atmospheric effect on the path radiance. This method requires the surface reflectance to be 0.02±0.01, which is found over forests in the red channel. Two techniques are used to screen an AVHRR scene for pixels that have this low reflectance. The qualifying requirements for these techniques are discussed, and the method is demonstrated to retrieve aerosol optical thicknesses to ~±0.1. The second method uses the change in contrast for several scenes to determine the change in the optical thickness between the scenes. A reference scene allows absolute determination. The method has an rms error of ~0.1     

Influence of Aerosol Scattering on Atmospheric Blurring of Surface Features

We investigate the influence of the shape of atmospheric scattering phase functions on blurring of surface detail in images acquired from space. The effects are characterized by computing atmospheric transfer functions and by solving the multidimensional equation of radiative transfer using a Fourier transform method. We predict that increased forward scattering enhances the detectability of atmospheric blurring near reflectance boundaries. Results for off-nadir viewing are also presented.     

Atmospheric and angular effects on NDVI temporal profiles derivedfrom ADEOS-POLDER data over India

The variations of NDVI for crops, semi-evergreen forest, dry deciduous forest, and sand have been analyzed as a function of date of acquisition at 670 and 865 am using ADEOS-Polarization and Directionality of Earth Resources (POLDER) data acquired over India. After correcting the data for atmospheric effects, a semi-empirical bidirectional reflectance distribution function (BRDF) model has been fitted to the data to extract angularly normalized target reflectances. It is shown that atmospheric corrections and angular normalization are important in the quantitative analysis of NDVI and its temporal variations     

Remote sensing of solar radiation absorbed and reflected byvegetated land surfaces

The problem of remotely sensing the amount of solar radiation absorbed and reflected by vegetated land surfaces was investigated with the aid of one- and three-dimensional radiative transfer models. Desert vegetation was modeled as clumps of leaves randomly distributed on a bright dry soil with a ground cover of generally less than 100%. Surface albedo (ALB), fraction of photosynthetically active radiation absorbed by the canopy (FAPAR), fractions of solar radiation absorbed by the canopy (FASOLAR) and soil (FASOIL), and normalized difference vegetation index (NDVI) were calculated for various illumination conditions. The magnitude of errors involved in the estimation of surface albedo from broadband monodirectional measurements was assessed. The nature of the relationships between NDVI vs. FASOLAR, FAPAR, FASOIL, and ALB and their sensitivity to all problem parameters were investigated in order to develop simple predictive models. The relationship between NDVI measured above the atmosphere and that sensed above the canopy at the ground surface was studied to characterize atmospheric effects     

Impact of variable atmospheric water vapor content on AVHRR datacorrections over land

This paper explores the impact of the integrated water vapor content (IWV) in the atmospheric column on the corrections of optical satellite data over land. First, simulation runs were used to quantify the trends in red and near infrared parts of the electromagnetic spectrum. Second, advanced very high resolution radiometer (AVHRR) measurements obtained over Canada during the 1996 growing season, together with reanalyzed IWV content data, were employed to determine the actual impact of constant IWV values. Third, various options in characterizing IWV for atmospheric corrections of AVHRR composites were examined. It was found that (1) as expected, IWV affects near-infrared radiation substantially more than red, although the latter is also altered; (2) that additional, subtle interactions take place between IWV, radiance levels, and viewing geometry that influence the retrieved surface reflectance; (3) that spatial and temporal variation in IWV caused changes in the normalized difference vegetation index up to 7.5% in relative terms during the peak green period; and (4) that IWV varies so substantially that pixel and date-specific values need to be used for the atmospheric correction of AVHRR data. At present, subdaily gridded IWV data sets from atmospheric data reanalysis projects are the only candidate source for such purpose     

Atmospheric correction of hyperspectral data over dark surfaces via simulated annealing

A method [atmospheric correction via simulated annealing (ACSA)] is proposed that enhances the atmospheric correction of hyperspectral images over dark surfaces. It is based on the minimization of a smoothness criterion to avoid the assumption of linear variations of the reflectance within gas absorption bands. We first show that this commonly used approach generally fails over dark surfaces when the signal to noise ratio strongly declines. In this case, important residual features highly correlated with the shape of gas absorption bands are observed in the estimated surface reflectance. We add a geometrical constraint to deal with this correlation. A simulated annealing approach is used to solve this constrained optimization problem. The parameters involved in the implementation of the algorithm (initial temperature, number of iterations, cooling schedule, and correlation threshold) are automatically determined by using a standard simulated annealing theory, reflectance databases, and sensor characteristics. Applied to a HyMap image with available ground truths, we verify that ACSA adequately recovers ground reflectance over clear land surfaces, and that the added spectral shape constraint does not introduce any spurious feature in the spectrum. The analysis of an AVIRIS image of Central Switzerland clearly shows the ability of the method to perform enhanced water vapor estimations over dark surfaces. Over a lake (reflectance equal to 0.02, low signal to noise ratio equal to about 6), ACSA retrieves unbiased water vapor amounts (2.86 cm/spl plusmn/0.36 cm) in agreement with in situ measurements (2.97 cm/spl plusmn/0.30 cm). This corresponds to a reduction of the standard deviation by a factor 3 in comparison with standard unconstrained procedures (1.95 cm/spl plusmn/1.08 cm). Similar results are obtained using a Hyperion image of DoE ARM SGP test site containing a very dark area of the land surface.     

Analysis of interannual changes in northern vegetation activityobserved in AVHRR data from 1981 to 1994

This paper reports on the analysis of Pathfinder AVHRR land (PAL) data set that spans the period July 1981 to September 1994. The time series of normalized difference vegetation index (NDVI) data for land areas north of 45° N assembled by correcting the PAL data with spectral methods confirms the northerly greening trend and extension of the photosynthetically active growing season. Analysis of the channel reflectance data indicates that the interannual changes in red and near-infrared reflectances are similar to seasonal changes in the spring time period when green leaf area increases and photosynthetic activity ramps up. Model calculations and theoretical analysis of the sensitivity of NDVI to background reflectance variations confirm the hypothesis that warming driven reductions in snow cover extent and earlier onset of greening are responsible for the observed changes in spectral reflectances over vegetated land areas     

Improved semi-arid community type differentiation with the NOAAAVHRR via exploitation of the directional signal

Mapping semi-arid vegetation types at the community level is extremely difficult for optical sensors with large ground footprints such as the National Oceanic and Atmospheric Administration (NOAA) Advanced Very High Resolution Radiometer (AVHRR). Attempts to use solar wavelength AVHRR data in community type differentiation have often resulted in unacceptable classification errors which are usually attributed to noise from topographic and soil background variations, inaccurate reflectance retrieval and poor registration. One source of variation which is rarely accounted for adequately is the directional signal resulting from the combined effects of the surface bidirectional reflectance distribution function (BRDF) and the variation of viewing and illumination geometry as a function of scan angle, season, latitude and orbital overpass time. In this study, a linear semiempirical kernel-driven BRDF model is used to examine the utility:of the directional signal in community and cover type differentiation over discontinuous but statistically homogeneous semi-arid canopies in Inner Mongolia Autonomous Region, China, and New Mexico, USA. This research shows that the directional signal resulting from the physical structure of the canopy-soil complex can be retrieved to provide information which is highly complementary to that obtained in the spectral domain     

Assessment of regional biomass-soil relationships using vegetationindexes

The development of photosynthetic active biomass in different ecological conditions, as indicated by normalized difference vegetation indices (NDVIs) is compared by performing a stratified sampling (based on soil associations) on data acquired over Indiana. Data from the NOAA-10 Advanced Very High Resolution Radiometer (AVHRR) were collected for the 1987 and 1988 growing seasons. An NDVI transformation was performed using the two optical bands of the sensor (0.58-0.68 μm and 0.72-1.10 μm). The NDVI is related to the amount of active photosynthetic biomass present on the ground. Statistical analysis of results indicate that land-cover types (forest, forest/pasture, and crops), soil texture, and soil water-holding capacity have an important effect on vegetation biomass changes as measured by AVHRR data     

Solar zenith angle effects on forest canopy hemisphericalreflectances calculated with a geometric-optical bidirectionalreflectance model

The bidirectional reflectance distribution function (BRDF) provided by the Li-Strahler geometric-optical forest canopy model has been integrated to provide spectral instantaneous hemispherical reflectances of sparsely vegetated surfaces. Further integration over the Sun's zenith angles can yield daily or longer interval hemispherical reflectances as well. A variety of simulated canopies were modeled with varying solar angles. In all cases, as the geometric-optical model introduced increased shadowing of the surface with increased solar zenith angle, the direct-beam hemispherical surface reflectance gradually decreased. The hemispherical reflectance values are direct beam calculations and do not directly include canopy multiple scattering and leaf specularity or consider the impact of diffuse irradiance. These limitations are acceptable for sparse canopies, in which 3D shadowing effects are large. However, radiative transfer calculations have shown that these phenomena must be incorporated before truly realistic modeling of hemispherical surface reflectances can be achieved for dense canopies