Marc Leroy

A Bidirectional Reflectance Model of the Earth's Surface for the Correction of Remote Sensing Data

A surface bidirectional reflectance model has been developed for the correction of surface bidirectional effects in time series of satellite observations, where both sun and viewing angles are varying. The model follows a semiempirical approach and is designed to be applicable to heterogeneous surfaces. It contains only three adjustable parameters describing the surface and can potentially be included in an algorithm of processing and correction of a time series of remote sensing data. The model considers that the observed surface bidirectional reflectance is the sum of two main processes operating at a local scale: (1) a diffuse reflection component taking into account the geometrical structure of opaque reflectors on the surface, and shadowing effects, and (2) a volume scattering contribution by a collection of dispersed facets which simulates the volume scattering properties of canopies and bare soils. Detailed comparisons between the model and in situ observations show satisfactory agreement for most investigated surface types in the visible and near-infrared spectral bands. The model appears therefore as a good candidate to reduce substantially the undesirable fluctuations related to surface bidirectional effects in remotely sensed multitemporal data sets.

Selection and characterization of Saharan and Arabian desert sites for the calibration of optical satellite sensors

Desert areas are good candidates for the assessment of multitemporal, multiband, or multiangular calibration of optical satellite sensors. This article describes a selection procedure of desert sites in North Africa and Saudi Arabia, of size 100 × 100 km2, using a criterion of spatial uniformity in a series of Meteosat-4 visible data. Twenty such sites are selected with a spatial uniformity better than 3% in relative value in a multitemporal series of cloud free images. These sites are among the driest sites in the world. Their meteorological properties are here described in terms of cloud cover with ISCCP data and precipitation using data from a network of meteorological stations. Most of the selected sites are large sand seas, the geomorphology of which can be characterized with Spot data. The temporal stability of the spatially averaged reflectance of each selected site is investigated at seasonal and hourly time scales with multitemporal series of Meteosat-4 data. It is found that the temporal variations, of typical peak-to-peak amplitude 8–15% in relative value, are mostly controlled by directional effects. Once the directional effects are removed, the residual rms variations, representative of random temporal variability, are on the order of 1–2% in relative value. The suitability of use of these selected sites in routine operational calibration procedures is briefly discussed.

A method of biophysical parameter retrieval at global scale by inversion of a vegetation reflectance model

Abstract The objective of the paper is to study a physically based method of retrieval of leaf area index (LAI) and fraction of absorbed photosynthetically active radiation (fAPAR) parameters from global data sets of new generation wide field of view optical satellite sensors, such as POLDER/ADEOS, VGT/SPOT4, MODIS/EOS, MISR/EOS, MERIS/ENVISAT, and so forth. The method uses the bidirectional reflectance distribution function (BRDF) model of Kuusk (1995) , which simultaneously predicts the spectral and directional behavior of reflectances, as a function of LAI, chlorophyll concentration, ratio of leaf size to canopy height, and other optical or structural parameters of the soil and vegetation. The same model is used irrespective of surface type, and no ancillary data is needed. This approach is evaluated with field and airborne data acquired over three different types of surfaces: Sahelian vegetation in the HAPEX-Sahel (1992) experiment, boreal forest in the BOREAS (1994) experiment, and cultivated areas in the Alpilles (1996) experiment. The results show that the LAI is restituted with a fair accuracy [root-mean-square (rms) difference between model results and observations of 0.70], better than that obtained with a semiempirical relation LAI-vegetation index. The daily fAPAR is restituted accurately, with a rms difference between measured and modeled fAPAR of 0.097. In the example of HAPEX, the model reproduces to some extent the temporal evolution of measured LAI and fAPAR. Reflectances reconstructed with the model are in acceptable agreement with observed reflectances, with a rms difference between observed and measured values of 0.017 on average. It is concluded that the retrieval of biophysical parameters from inversion of a BRDF model is promising from the perspective of a quantitative characterization of the terrestrial biosphere from space.

Investigation of directional reflectance in boreal forests with an improved four-scale model and airborne POLDER data

Airborne Polarization and Directional Earth Radiation (POLDER) data acquired during the boreal ecosystem-atmosphere study (BOREAS) and the four-scale model of Chen and Leblanc (1997) are used to investigate radiative transfer in boreal forest. The four-scale model is based on forest canopy architecture at different scales. New aspects are incorporated into the model to improve the physical representation of each canopy, as follows: 1) Elaborate branch architecture is added. 2) Different crown shapes are used for conifer and deciduous forests. 3) Bilayer version of the model is introduced for forest canopies with an important understory. 4) Natural repulsion effect is considered in the tree distribution statistics. Ground measurements from BOREAS sites are used as input parameters by the model to simulate measurements of bidirectional reflectance distribution function (BRDF) from four forest canopies (old black spruce, old aspen, and old and young jack pine) acquired by the POLDER instrument from May-July 1994. The model is able to reproduce with great accuracy the BRDF of the four forests. The importance of the branch architecture and the self-shadowing of the foliage is emphasized.

Sun and view angle corrections on reflectances derived from NOAA/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. >

Bidirectional reflectance distribution function signatures of major biomes observed from space

Land surface bidirectional reflectance distribution function (BRDF) measurements were acquired from November 1996 to June 1997 at global scale and 6km spatial resolution with the POLDER instrument onboard the ADEOS-I satellite. We selected 395 BRDF data sets on areas distributed on the 17 biomes of the IGBP 1-km land cover classification (DISCover data set) at 443, 670, and 865nm, at several periods (November and December 1996, May and June 1997). The selected BRDF data are characterized by a low noise level, a sufficient number of clear days during the month, and a roughly even sampling of directional space. The data show large differences of the directional and spectral signatures of the various land cover classes, both in shape and in magnitude. Except for the desert and ice classes, all signatures present a peak in the backscattering direction, with sometimes an additional strong peak in the specular direction for wetlands. The data permit an assessment of the BRDF temporal evolution due to changes of surface state or Sun elevation, as well as a quantification of the BRDF variability within a land cover class. The maximum error level of the BRDF database is estimated to be of the order of 0.01 and 0.03 (in units of reflectance) in the red and near infrared, respectively. The BRDF database is available to the science community through the Internet. It should be helpful for the prototyping of various science applications, including the test of radiative transfer models and algorithmic schemes of corrections of angular effects on remote sensing data. As an example of application of the database, various semiempirical BRDF models published in the literature are tested and intercompared. Whereas all tested models catch reasonably well the overall shape of the BRDF, some differences appear between the red and the near infrared, between classes, and between models, which the use of the database permits to quantify.

Modeling BRF and Radiation Regime of Boreal and Tropical Forests: I. BRF

Abstract Monitoring of forest evolution and functioning with remote sensing depends on canopy BRF (bidirectional reflectance factor) sensitivity to biophysical parameters and to canopy PAR (photosynthetically active radiation) regime. Here, we study the canopy BRF of a tropical (Sumatra) and three boreal (Canada) forest sites, with the DART (discrete anisotropic radiative transfer) model. The behavior of PAR regime of these forests is analyzed in a companion article. We assessed the BRF sensitivity to some major experimental parameters (scale of analysis, viewing and illumination directions, sky radiation) and compared it with BRF sensitivity to commonly studied biophysical quantities: Leaf area index (LAI) and leaf optical properties. Simulations showed that BRF directional anisotropy is very large for all forests. For example, maximum relative reflectance difference with view zenith angle less than 25° is around 0.5 in the visible, 0.4 in the short wave infrared, and 0.25 in the near-infrared for tropical forest. We showed that this BRF variability associated with experimental conditions can hamper the remote detection of forest LAI and tree cover change such as deforestation of tropical forest. DART BRFs of the boreal sites were favorably compared with ground (PARABOLA) and airborne (POLDER) measured BRFs. This work stressed 1) the potential of the DART model, 2) the importance of accurate field data for validation approaches, and 3) the very strong influence of canopy architecture on forest BRF; for example, depending on forest sites, a LAI increase may imply that nadir near-infrared reflectance increases or decreases.

Surface bidirectional reflectance distribution function observed at global scale by POLDER/ADEOS

The spaceborne POLDER instrument presents the unique capability of sampling the surface Bidirectional Reflectance Distribution Function (BRDF) up to about 60° viewing angle for the full azimuth range of every point on Earth at 6 km resolution, when atmospheric conditions are favorable. This paper presents examples of BRDF signatures acquired at this resolution on several terrestrial biomes (desert, steppe, grassland, boreal forest, savanna, wetland). Well identified directional signatures for all azimuths are obtained and shown to be different for each biome. Specific directional effects in the hot spot and specular directions are well observed in the data.

Normalisation of directional effects in 10-day global syntheses derived from VEGETATION/SPOT:: II. Validation of an operational method on actual data sets

Abstract Since April 1998, the VEGETATION/SPOT-4 sensor has provided global reflectances on a daily basis. Its large field-of-view makes the observations strongly dependent on the Sun-target-sensor geometry. This paper presents the “BiDirectional Compositing” (BDC) method we designed for the VEGETATION operational line to normalise directional effects in 10-day global syntheses. For each spectral band and each pixel, BDC results every 10 days in one nadir view datum derived from the observations acquired at every orbit pass. BDC is based on two main ideas. Firstly, the length of the time window devoted to Bidirectional Reflectance Distribution Functions (BRDFs) retrieval is conceived as a variable in such a way that a constant number of cloud-free data is always available to fit the BRDF model. Secondly, BRDF retrieval is separated from data normalisation and compositing, which operates only on 10-day windows before the date of syntheses in order to keep the reflectances level of the most recent observations. As a reliable BRDF is always available from VEGETATION data, BDC is fully productive at a 10-day step. It allows to use one single method to derive global syntheses, which can be easily adapted for any large field-of-view optical sensor. Using VEGETATION data sets acquired on four regions of the world, we finally compare 10-day syntheses obtained by BDC to the ones derived with two versions of Maximum Value Compositing (MVC) differing by the performance in detection of clouds and aerosols. The seasonal and spatial coherence of reflectances and Normalised Difference Vegetation Index (NDVI) are much larger on BDC than on MVC syntheses, both at a regional and pixel (km 2 ) scale. Compared to the version of MVC that is presently used in the VEGETATION operational line, BDC smoothes 10-day fluctuations of reflectances and NDVI time series by a factor 2.8 on average.

Angular signatures of surface reflectances from airborne POLDER data

Abstract Thanks to its optical design, the airborne POLDER instrument allows a multidirectional measurement of the surface reflectance. During the “La Crau 91” experiment, the radiometer has been flying over a study area of 10 km x 10 km composed of various cultivated fields. The measurements have been registered on a regular grid of resolution 100 m. For each grid point, up to 50 directional measurements provide an adequate description of the surface reflectance directional signature. These signatures are discussed for a few selected targets representative of the area. The measurements show typical features of the reflectance angular signature, including the “hot spot” and specular reflection. The quality of the measurements is shown to be satisfactory. A simple directional model is then applied to the measurements. It provides, for each of the l0 4 grid points, a normalized reflectance corrected for the angular effects, together with two parameters that give a quantitative assessment of the directional signature. The article discusses these parameters and the ability of the model to represent the measurements. Some spatial structure of directional signature is apparent in the study area and is consistent with a land use map of this area. The squared correlation coefficient R2 of the fit between model and observations is larger than 0.8 for 50% of the area and smaller than 0.5 for about 20% of it at 550 nm and 850 nm. The part of the area for which the model is not satisfactory corresponds primarily to pixels with a larger specular reflection signal or with few directional measurements. Angular signatures of reflectances are measured here at the regional scale, a key step for the preparation of the operational processing of spaceborne POLDER measurements at the global scale.