Richard Santer

Adjacency effects on water surfaces: primary scattering approximation and sensitivity study.

The making of atmospheric corrections is a critical task in the interpretation of ocean color imagery. In coastal areas, a fraction of the light reflected by the land reaches a sensor. Modeling the reduction of image contrast when the atmospheric turbidity increases, the so-called adjacency effect, requires large amounts of computing time. To model this effect we developed a simple approach based on the primary scattering approximation for both nadir and off-nadir views. A sensitivity study indicates that the decisive criterion for measurement accuracy for aerosols is their vertical distribution. As this distribution cannot generally be determined from space, it is not possible to include a suitable correction of the adjacency effects on satellite imagery. Conversely, we propose a simple correction for molecular scattering based on the isotropic approximation. We also address the problem of reduction of the coupling between the Fresnel reflection and the atmosphere for observations of coastal water. We study the influence of the adjacency effects on determination of the abundance of chlorophyll in water by combining use of the red and the infrared bands for aerosol remote sensing and the blue/green-ratio technique for retrieval of these data.

Radiative transfer model for the computation of radiance and polarization in an ocean–atmosphere system: polarization properties of suspended matter for remote sensing

A radiative transfer code termed OSOA for the ocean-atmosphere system that is able to predict the total and the polarized signals has been developed. The successive-orders-of-scattering method is used. The air-water interface is modeled as a planar mirror. Four components grouped by their optical properties, pure seawater, phytoplankton, nonchlorophyllose matter, and yellow substances, are included in the water column. Models are validated through comparisons with standard models. The numerical accuracy of the method is better than 2%; high computational efficiency is maintained. The model is used to study the influence of polarization on the detection of suspended matter. Polarizing properties of hydrosols are discussed: phytoplankton cells exhibit weak polarization and small inorganic particles, which are strong backscatterers, contribute appreciably to the polarized signal. Therefore the use of the polarized signal to extract the sediment signature promises good results. Also, polarized radiance could improve characterization of aerosols when open ocean waters are treated.

Atmospheric correction over land for MERIS

A three-stage atmospheric correction is proposed for the Medium Resolution Imaging Spectrometer (MERIS) from a validated formulation of the signal. We correct first for the gaseous transmittance. Assuming the ozone correction is well defined, we illustrate the need to include a correction for water vapour continuum which covers most of the MERIS bands. The water vapour transmittance can be computed from the water vapour content obtained from a twoband ratio at 900nm and 890nm. We demonstrate that a direct association between the transmittance in a given band and the two band ratio is more accurate due to the removal of the coupling between absorption and scattering. Secondly, the Rayleigh correction depends on the barometric pressure determined here from a two band ratio method with the oxygen A band. Good accuracy is obtained for the pressure when accounting for the coupling between scattering and gas absorption, which mostly depends on the surface reflectance. The Rayleigh reflectance is computed from a...

In-flight calibration of large field of view sensors at short wavelengths using Rayleigh scattering

Abstract Satellite observations over the ocean in the backscatter direction are dominated by Rayleigh scattering. We use this predictable radiance for in-flight calibration of the visible SPOT channels. Two methods are evaluated. The first method directly relates the measured numerical signal in a short wavelength channel to the predicted reflectance. In the second method, we use a second channel centred at a longer wavelength, to correct the short wavelength channel for the effect of the atmospheric aerosol contribution. These two methods are examined for channel B0 , centred at 045 μm planned for launch on SPOT-4 VEGETATION, and for channel B1 centred at 0-55 mu;mm, currently on-board SPOT-1 HRV. In both cases, the channel B3 , centred at 0-85 mu;mm is used for aerosol correction. Error analysis shows that accuracies of 3 and 5 per cent respectively can be achieved for B0 and B1. The last section of the paper is devoted to a validation of the error analysis using SPOT-1 HRV data.

MERIS in-flight spectral calibration

This paper will describe the spectral calibration activities conducted during the MERIS commissioning phase and during operation since orbit 12000. MERIS is a medium resolution (300–1200m) push‐broom imaging spectrometer covering the spectral domain 390–1040nm with 15 bands, programmable in position and width down to steps of 1.25nm. The onboard spectral calibration hardware is based on the use of an Erbium doped‐diffuser panel presenting well‐defined absorption peaks. In the spectral calibration mode, MERIS is configured with narrow bands centred on an Erbium absorption feature (two are used). The first orbit, the instrument is calibrated by viewing the “white” radiometric diffuser plate and the following orbit the “pink” Erbium diffuser plate is deployed. This method allows each of the MERIS detectors involved to be characterized in wavelength. The Fraunhofer absorption lines were used to complement these results by providing additional measurements in the violet and near infrared part of the spectrum. For this method, MERIS was configured both for Earth and diffuser observations and acquired data for only a limited number of orbits. This procedure was repeated for different band settings covering a number of Fraunhofer absorption lines. Finally, using Oxygen (O2A) absorption Earth observation data, two different approaches were developed, one based on the retrieval of surface pressure and one based on the shape of the O2A absorption band. Both methods were developed for clear sky land observations, but their performances are improved over bright land targets. Both methods agree to within an accuracy of 0.02 nm. The results from the different methods are analyzed in order to propose a spectral model for the MERIS instrument. Preliminary results of the spectral variation with time are reported. Except camera 4, the instrument is quite stable with time. Camera 4 needs further investigations to better understand its behaviour. Except for the use of the MERIS oxygen band, the spectral characterization of the other MERIS bands is achieved within the nominal accuracy (1 nm).

Retrieval of aerosol single-scattering albedo from ground-based measurements: Application to observational data

The single-scattering albedo ω0 of atmospheric aerosols is a key parameter concerning the effect of the particles on the Earths radiative budget. Retrieval of this parameter from in situ measurements is a difficult task. We address the possibility to derive ω0 from ground-based measurements of solar transmission and diffuse skylight, independent of the knowledge of the aerosol properties or the absorption mechanism. The proposed method is based on the normalization of the aerosol phase function that is derived from scannings of the skylight. The method has been applied to a series of observations conducted during a few years over the french site of La Crau, near the industrial area of Fos sur Mer in the southeast of France. The aerosol albedo, retrieved near the 870 nm wavelength, ranges from 0.60 to 0.95. The estimated error is Δω0 ≈ ±0.05. To test their reliability, the retrieved ω0 have been used to calculate the diffuse/total solar fluxes at ground level. These estimates are consistent within about 10% with independent direct/diffuse flux measurements performed simultaneously.

Surface pressure estimates from satellite data in the oxygen A-band: Applications to the MOS sensor over land

A fast method for the Apparent Pressure Retrieval (APR method) over land from satellite data, based on a two band ratio in the oxygen A-band (759-770 nm), is described. This method is devoted to the cloud detection and atmospheric corrections. Parameterizations are performed from line-by-line calculations assuming a pure absorbing medium. Moreover, we defined a corrective factor to account for scattering effects of the atmosphere. We validated this method with measurements of the MOS sensor (Modular Optoelectronic Scanner), whose spectral characteristics are appropriate. Comparisons with ECMWF (European Centre for Medium-Range Weather Forecasts) pressures showed the need to perform in-flight calibrations over a reference scene to account for spectral shifts of filter responses. Therefore we selected bright surfaces for the calibration, such as deserts, because of their major contribution to the satellite signal. After calibration the accuracy of the method is about 10 hPa over bright surfaces. Comparisons for various meteorological and geographical conditions showed that deviations between ECMWF pressures and MOS apparent pressures are generally less than 30 hPa using scattering corrections. These deviations are multiplied by 2 without correction. The APR method has been included in the cloud detection and atmospheric correction algorithms for the MOS data processing over land. Theoretical studies showed that the APR method is suitable for the cloud discrimination and that an error of 30 hPa on the surface pressure retrieval has no noticeable effect on the geophysical products of these algorithms, such as aerosol optical thickness or surface reflectance. Consequently, this method is potentially applicable to the retrieval of the apparent pressure over land with the MOS algorithms, as well as other similar satellite sensors such as the Medium Resolution Imaging Spectrometer.

Operational Remote Sensing of Aerosols over Land to Account for Directional Effects.

The assumption that the ground is a Lambertian reflector is commonly adopted in operational atmospheric corrections of spaceborne sensors. Through a simple modeling of directional effects in radiative transfer following the second simulation of the satellite signal in the solar spectrum (6S) approach, we propose an operational method to account for the departure from Lambertian behavior of a reflector covered by a scattering medium. This method relies on the computation of coupling terms between the reflecting and the scattering media and is able to deal with a two-layer atmosphere. We focus on the difficult problem of aerosol remote sensing over land. One popular sensing method relies on observations over dense dark vegetation, for which the surface reflectance is low and quite well defined in the blue and in the red. Therefore a study was made for three cases: (1) dark vegetation covered by atmospheric aerosols, (2) atmospheric aerosols covered by molecules, and finally (3) dark vegetation covered by atmospheric aerosols covered by molecules. Comparisons of top-of-the-atmosphere reflectances computed with our modeling and reference computations made with the successive-order-of-scattering code show the robustness of the modeling in the blue and in the red for aerosol optical thicknesses as great as 0.6 and solar zenith angles as large as 60 degrees . The model begins to fail only in the blue for large solar zenith angles. The benefits expected for aerosol remote sensing over land are evaluated with an aerosol retrieval scheme developed for the Medium-Resolution Imaging Spectrometer. The main result is a better constraint on the aerosol model with inclusion of directional effects and a weaker effect on the optical thickness of the retrieved aerosol. The directional scheme is then applied to the aerosol remote-sensing problem in actual Indian Remote Sensing Satellite P3/Modular Optoelectronic Scanner images over land and shows significant improvement compared with a Lambertian algorithm. Moreover, it confirms our main theoretical conclusion.

Atmospheric correction for inland waters—application to SeaWiFS

Inland waters are an increasingly valuable natural resource that has major impact and benefits for population and environment. The new generation of ocean colour sensors has better spatial resolution, and hence are suitable for monitoring water quality of lakes. As an alternative to standard algorithms developed for oceans, which often fail over inland waters, we propose here a scheme based on aerosol remote sensing over land. The ocean colour sensors have spectral bands that allow characterization of aerosols over dark land pixels (vegetation in the blue and in the red spectral bands). It is then possible to use a representative aerosol model (aerosol optical thickness and aerosol type) for atmospheric correction over inland waters after validating the spatial homogeneity of the aerosol model in the lake vicinity. The performance of this new algorithm is shown in Sea‐viewing Wide Field‐of‐view Sensor (SeaWiFS) scenes of Lakes Balaton (Hungary) and Constance (Germany). We demonstrate the good spatial homogeneity of the aerosols and the meaningfulness of the water‐leaving reflectances derived over these two lakes. We also addressed the particularity of Fresnel reflection computation. The direct to diffuse term of this Fresnel contribution is reduced because of the limited size of the lake. Based on the primary scattering approximation, we propose a simple formulation of this component. A specific Fresnel correction needs to be developed to fulfil the accuracy requirements.

A surface reflectance model for aerosol remote sensing over land

MERIS aerosol remote sensing over land is based on the use of pixels covered by vegetation. Dense Dark Vegetation pixels are selected using the Atmospheric Resistant Vegetation Index as spectral index. Above an ARVI threshold, TARVI, at a given wavelength, a standard DDV reflectance ρDDV is set to a constant value. Initially, 11 biomes and 20 DDV models have been selected from the POLDER 1 imagery. A clear limitation to the initial process was the limited spatial coverage of DDV pixels. That the reason why, the DDV concept has been extended to include less dark pixels. Preliminary results indicated that a simple linear regression between surface reflectance ρ and ARVI applies. The goal of this paper is to investigate the scope for such linear relationship. The first step is to start from the current DDV models and to complementary define the slope χ which describes the linear decrease of ρ with ARVI and the ARVI range on which it applies.An extensive archive of MERIS images has been fully corrected from the atmosphere using ground based measurements. The outputs are the surface reflectances from which the reflectance model can be built. An independent validation on the surface reflectance model has been conducted on a large SeaWiFS archive on which the MERIS algorithm has been processed. The outputs of this work are new monthly Look Up Tables in the spectral bands used for aerosol remote sensing: 412 nm, 443 nm and 670 nm. For each of the 11 biomes used in MERIS, the slope χ and the ARVI domain on which the linear fit applied, are used as auxiliary data in the new version of the MERIS ground segment. The second MERIS processing includes this new reflectance model. We investigate the limitations of this model in two aspects. First, locally, we use ground based solar extinction measurements to validate the aerosol products. The aerosol optical thicknesses are quite well retrieved in the blue while MERIS underestimate by a factor two the aerosol optical thickness at 670 nm. We also analysed MERIS level 3 aerosol products on which the spatial continuity of the DDV models is poor between biomes. In both cases, the new surface reflectance model does not appear responsible but instead the initial DDV model has to be consolidated through more accurate values of the DDV surface albedo and through a better global mapping.