Pierre-Yves Deschamps

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.

Spectral reflectance of sea foam in the visible and near‐infrared: In situ measurements and remote sensing implications

The spectral reflectance of sea foam was measured at the Scripps Institution of Oceanography Pier, La Jolla, California, by viewing the sea surface radiometrically in a region of breaking waves. Foam reflectance was found to decrease substantially with wavelength in the near-infrared, contrary to the findings of previous studies, theoretical as well as experimental. Values in the visible (0.44 μm) were reduced by typically 40% at 0.85 μm, 50% at 1.02 μm, and 85% at 1.65 μm. The spectral effect was explained by the nature of the foam, which is composed of large bubbles of air separated by a thin layer of water (foam stricto sensu) and of bubbles of air injected in the underlayer. The presence of bubbles in the underlayer enhances water absorption and thus reduces reflectance in the near-infrared. For ocean color remote sensing, affected by the presence of foam and aerosols, the consequences of neglecting the spectral dependence of foam are dramatic. With only a small amount of foam, in the presence of aerosols or not and thus irrespective of aerosol type, the errors in the retrieved water reflectance at 0.44 μm are above 0.01, which does not meet the accuracy goal of 0.001 for biological applications. Since under normal conditions the effect of foam may have the same magnitude as the effect of aerosols, atmospheric corrections will be inaccurate (and useless) in many cases, even taking into account the spectral dependence of the foam reflectance. Space observations potentially contaminated by an effective foam reflectance (product of reflectance and fractional coverage) above 0.001, i.e., corresponding to wind speeds above 8 m s−1, should be eliminated systematically. Utilization of near-infrared wavelengths above 0.9 μm for atmospheric corrections of ocean color, possible with the moderate-resolution imaging spectrometer (MODIS), would aggravate the problem. The measurements also indicated that foam significantly affects the retrieval of aerosol turbidity at 0.85 and 1.02 μm for wind speeds above 10 m s−1 but impacts minimally turbidity estimates at 1.65 μm. Over the oceans the spectral range above 1 μm is definitely recommended for remote sensing of tropospheric aerosol load and type from space.

Influence of the background contribution upon space measurements of ground reflectance

The influence of Thebackground contamination on the apparent reflectance of a target as viewed from space has been studied as a function of the characteristics of atmospheric aerosols and the simple geometry of the target. For relatively common aerosol characteristics, the main features of the environment effect may be accounted for by simple correction terms, which depend only upon the optical thicknesses of aerosols and the molecular scattering of the atmosphere.

Results of POLDER in-flight calibration

POLDER is a CNES instrument on board NASDAs ADEOS polar orbiting satellite, which was successfully launched in August 1996. On October 30, 1996, POLDER entered its nominal acquisition phase and worked perfectly until ADEOSs early end of service on June 30, 1997. POLDER is a multispectral imaging radiometer/polarimeter designed to collect global and repetitive observations of the solar radiation reflected by the Earth/atmosphere system, with a wide field of view (2400 km) and a moderate geometric resolution (6 km). The instrument concept is based on telecentric optics, on a rotating wheel carrying 15 spectral filters and polarizers, and on a bidimensional charge coupled device (CCD) detector array. In addition to the classical measurement and mapping characteristics of a narrow-band imaging radiometer, POLDER has a unique ability to measure polarized reflectances using three polarizers (for three of its eight spectral bands, 443 to 910 nm) and to observe target reflectances from 13 different viewing directions during a single satellite pass. One of POLDERs original features is that its in-flight radiometric calibration does not rely on any on-board device. Many calibration methods using well-characterized calibration targets have been developed to achieve a very high calibration accuracy. This paper presents the various methods implemented in the in-flight calibration plan and the results obtained during the instrument calibration phase: absolute calibration over molecular scattering, interband calibration over sunglint and clouds, multiangular calibration over deserts and clouds, intercalibration with Ocean Color and Temperature Scanner (OCTS), and water vapor channels calibration over sunglint using meteorological analysis. A brief description of the algorithm and of the performances of each method is given.

Determination from space of atmospheric total water vapor amounts by differential absorption near 940 nm : theory and airborne verification

Abstract A new technique is proposed to estimate atmospheric total water vapor amounts from space. The technique consists of viewing the Earths surface in two spectral channels, one narrow, the other wide, centered on the same wavelength at the water vapor absorption maximum near 940 nm. With these characteristics, the ratio of the solar radiance measured in the two channels is independent of the surface reflectance and yields a direct estimate of the water vapor amount integrated along the optical path. To test the technique, we designed and built a two-channel radiometer based on the above concept. Airborne experiments carried out with the new device demonstrate the techniques feasibility under clear sky conditions over both sea and land. Over the ocean and in the presence of thick aerosol layers, however, total water vapor amounts may be underestimated by as much as 20%. Compared to satellite microwave techniques, which are applicable under most weather conditions, the proposed technique has the adva...

Modeling of the atmospheric effects and its application to the remote sensing of ocean color

The effect of atmospheric scattering on ocean color measurements from space is considered. It is shown that modeling of the atmospheric effects can be improved by taking into account not only the direct but also the diffuse component of atmospheric transmittance and by a more precise formulation of the interaction between molecular and aerosol scattering in the calculation of atmospheric reflectance. This method, necessitating two near-infrared channels, should be used in future ocean color experiments to better correct for variable aerosol reflectance. The relative accuracy of the aerosol reflectance correction would then be to within 5%, as opposed to the more than 10% obtained with previous modelings.

Influence of the atmosphere on space measurements of directional properties

To compare measurements performed in different geometrical conditions, one must take into account the angular anisotropy of the reflection properties of natural surfaces. As use of the exact boundary conditions in the radiative transfer codes seems prohibitive, a simple but accurate formulation of the problem has been sought. In this paper, two average angular reflectances are defined from which the reflected radiance may be deduced for any distribution of the downward radiance. Calculations made for different atmospheric models show that the solar directionality is partly preserved in the downward radiation field, so that the average reflectances can be written as a linear combination of actual reflectance and spherical albedo of the surface. Finally, the feasibility of detecting directional properties from space measurements is discussed.

Analysis of the POLDER (POLarization and directionality of earth's reflectances) airborne instrument observations over land surfaces

Abstract The POLDER (POLarization and Directionality of the Earths Reflectances) instrument provides polarized reflectance measurements that can be used to distinguish atmospheric and surface contributions to reflectance. Polarized reflectance measurements can then be used for an accurate aerosol estimation over land. The POLDER instrument was flown for the first time on 17 June 1990 over the “La Crau” site, in southern France and the results are present in this article. The POLDER instrument is scheduled for launch in 1995 on the Japanese ADEOS (ADvanced Earth Observing System) platform. Surface based atmospheric optic measurements (spectral optical thickness, sky radiance) are used to estimate the aerosol refractive index and size distribution. The corresponding aerosol model is then used in a radiative transfer model to simulate POLDER polarized measurements. The correlation between the observations and the simulations is good for the 550 nm and 650 nm wavelengths, but the simulation is biased for the 850 nm wavelength. These results indicate that, compared with the atmospheric contribution to the polarized reflectance, the surface contribution can be neglected at shorter wavelengths, but not so in near infrared wavelengths. The POLDER instrument allows multidirectional reflectance measurements. A surface target bidirectional reflectance, therefore, can be sampled at various viewing angles. In this article, we investigate the angular variations of the surface reflectance of various surface covers. The main observed variations are: i) a limb brightening; ii) a larger reflectance in the backscattering direction; iii) a local maximum in the forward direction for the shorter wavelengths, indicating specular reflection by the leaves. A very simple empirical model is proposed to quantify the main reflectance angular variations.

Reduction of skylight reflection effects in the above-water measurement of diffuse marine reflectance

Reflected skylight in above-water measurements of diffuse marine reflectance can be reduced substantially by viewing the surface through an analyzer transmitting the vertically polarized component of incident radiance. For maximum reduction of effects, radiometric measurements should be made at a viewing zenith angle of approximately 45 degrees (near the Brewster angle) and a relative azimuth angle between solar and viewing directions greater than 90 degrees (backscattering), preferably 135 degrees. In this case the residual reflected skylight in the polarized signal exhibits minimum sensitivity to the sea state and can be corrected to within a few 10(-4) in reflectance units. For most oceanic waters the resulting relative error on the diffuse marine reflectance in the blue and green is less than 1%. Since the water body polarizes incident skylight, the measured polarized reflectance differs from the total reflectance. The difference, however, is small for the considered geometry. Measurements made at the Scripps Institution of Oceanography pier in La Jolla, Calif., with a specifically designed scanning polarization radiometer, confirm the theoretical findings and demonstrate the usefulness of polarization radiometry for measuring diffuse marine reflectance.

Atmospheric correction in presence of sun glint: application to MERIS

The sun glint is a major issue for the observation of ocean color from space. For sensors without a tilting capacity, the observations at sub-tropical latitudes are contaminated by the bright pattern of the specular reflexion of the sun by the wavy sea surface. Common atmospheric correction algorithms are not designed to work in these observation conditions, reducing the spatial coverage at such latitudes by nearly a half. We describe an original atmospheric correction algorithm, named POLYMER, designed to recover ocean color parameters in the whole sun glint pattern. It has been applied to MERIS data, and validated against in-situ data from SIMBADA. The increase of useful coverage of MERIS measurements for ocean color is major, and the accuracy of the retrieved parameters is not significantly reduced in the presence of high sunglint, while, outside the sunglint area, it remains about the same as by using the standard algorithm.