During the "Optic-Congo" oceanographic survey which took place in 2005 on board the "Beautemp-Beaupré" SHOM
vessel, different optical measurements of the surface water were acquired using a TRIOS radiance sensor fixed onboard a
mini-catamaran. Hydrological measurements (CTD, fluorescence, attenuation, scattering) and water samples were
simultaneously collected in order to measure SPM, Chlorophyll-a and CDOM concentrations. Four types of surface
water colours (blue, green-yellow, dark and brown) were identified. The main characteristics of these waters were the
very low Chlorophyll-a concentrations for this period of the year (March), and the very high CDOM concentrations
along the Congo coast, and particularly in the turbid plume of the Congo River. The attenuation and scattering
measurements highlighted the predominance of organic matter at the water surface. These observations were documented
using a beam electron microscope and by microanalysis. This data set was used to classify the water bodies along the
Gabon and Congo coasts. We propose here to use the remote sensing reflectance (Rrs) measurements to invert the IOP
(absorption (a) and backscattering (bb)) using the WASI numerical bio-optical model. The model is iterative: the Rrs
WASI simulations are computed given initial values of ocean constituents' concentrations and iteratively adjusted to the
Rrs in-situ measurements. The IOP computations are satisfying when the correlations between simulated and measured
Rrs are optimized. Then, the attenuation coefficients (Kd) are computed from the IOP coefficients. These results are
compared with measurements of Ku carried out during the survey.
An original data set has been acquired on the 5th of April 2008 during the international field experiment ECORS-Truc
Vert 2008 (SW France) in the nearshore zone over a complex bathymetry and in moderate turbid waters (SPM ⪅10
mg/l). This data set synchronizes in-situ reflectance measurements onboard a jet-ski, bathymetric surveys and a
Formosat-2 high-resolution satellite acquisition.
The jet-ski provides an interesting mean to gather optical data in shallow waters and in environments hard to sample with
traditional coastal ships. An experimental device has been implemented on the jet-ski, equipped with two TRIOS
RAMSES sensors which measure simultaneous atmospheric downwelling irradiances Ed and in-water upwelling
radiances Lu in the 350-950nm range. Water samples have also been collected at different stages of the jet-ski trajectory
(3-25m water depth) in order to assess the concentrations of the ocean constituents (SPM and Chl-a).
In the current study we present a methodology to validate FORMOSAT-2 high-resolution ocean color data using "jetski"
reflectance measurements, which first require a detailed analysis. The reflectance spectra measurements are shown
to be consistent: (i) they are typical of the presence of mineral particles with light absorption at short wavelengths; (ii)
their shape and magnitude depend on the depth and the water type (turbidity); (iii) some of them, especially in low
turbid waters, are similar to other reflectance spectra measured northward from a ship (Gironde mouth). Thus, the use of
"jet-ski" ocean color measurements appears to be adequate for remote sensing calval activities in shallow case-2 waters.
Spatial remote sensing atmospheric correction algorithms validation remains a challenge particulary over land and coastal environment. To assess this type of algorithm in the case of MERIS scheme, we propose a methodology based on the use of in-situ extinction and sky measurements from the world-wide Sun radiometer network AERONET. The spectral dependency in the blue and red derived from the extinction measurements is used to parameterize an aerosol model defined by the Jung power law size distribution in a first step and a chemical composition represented by a refractive index. This model is used to compute the phase function, a main input to a radiative transfer code (successive order of diffusion based) that allows to simulate the atmospheric parameters (radiances, transmittances). A comparison between the diffuse transmittance from sky measurements and that simulated allow to check the validity of the proposed method. The context of the study is calibration and validation in remote sensing using only the radiative properties of the atmosphere. A sensitivity study of the method to various parameters and an error budget will be reported.
In this paper, we propose two approaches to achieve calibration of the SPOT5 satellite, both based on the use of ground-based measurements achieved with a CIMEL sun-photometer. These approaches present the originality not to require any hypothesis on the aerosol model, on the contrary of the standard SPOT5 calibration. The principle of one of them relies indeed on the inversion of the aerosol phase function - thus atmospheric - from the sky diffusion measurements in the principal plane. The radiance-based method, fully described in Santer and Martiny (2003), allows the retrieval of the phase function with an accuracy of less than 1% using an iterative mode. We use such phase function, optical thickness and surface reflectance as inputs of a radiative-transfer-code for computations of SPOT5 top-of-atmosphere radiances. A second approach, inspired by Biggar et al. (1990), relies more directly on the sky and surface radiances measured by the CIMEL instrument. In this paper, we remind the principles of the two methods and the radiance-based method is applied as an example on 20 July, 2002. Discrepancies up to 11% are found out with the standard calibration coefficients. To conclude on the efficiency of the SPOT5 calibration methods, we recommend applying them to a huge and adapted dataset, spread on a longer period. Moreover, if the methods are accurate at 2-3%, we know that they are weakly sensitive to the radiance calibration of the sun-photometers. Standard calibration methods using integrating spheres do not give satisfactory results especially at short wavelengths (accuracy up to 10%). We present thus in the first part of the paper in-situ radiance calibration methods, based on the Rayleigh scattering knowledge and we show up that these methods lead to an improvement of the accuracy of 5%. The study is conducted over the inland site of La Crau, South of France.
The SeaWiFS sensor is required to provide spectral water leaving radiances with 5% absolute accuracies in the open ocean. This is extremely demanding because first the signal coming from the water body represents only 10% of the measured signal and second, calibration procedure of bands 7 and 8 is not direct like the vicarious calibration applied to bands 1 through 6. As a change of 5% in the sensitivity of these bands can imply errors of 10 to 50% on the water leaving radiance, it is suggested to revisit more accurately the SeaWiFS calibration in the near infrared. In this paper, we propose to apply a calibration method based on the use of CIMEL ground-based measurements. The radiance-based method, fully described in Santer and Martiny (2003), aims at the inversion of the atmospheric phase function from diffusion measurements in the principal plane with an accuracy of less than 1% using an iterative mode. Phase function, optical thickness and wind speed are the inputs of a radiative-transfer-code for computations of SeaWiFS top-of-atmosphere radiances. The method is quite sensitive to the CIMEL radiance calibration and to assumptions regarding the sea surface roughness. Nevertheless, its accuracy is of 2-3% depending on SeaWiFS geometry. Applications were conducted on Venice (Italy) and Lanai (Hawaii) datasets. The results depict an overestimate of the SeaWiFS calibration of 6.3% at 865 nm and 3% at 765 nm.
The aerosol climatology used in the atmospheric correction algorithms may not be representative enough of the coastal areas. We take advantage of a network of ground-based measurements of aerosol optical properties (the CIMEL instrument in AERONET) to validate these aerosol models. From space, the selection of the aerosol model is based on the spectral dependency of the aerosol path radiances between the red and infrared. A similar scheme is applied on the CIMEL extinction measurements. Knowing the standard aerosol model and the optical thickness, we simulated the sky radiances and the atmospheric transmittances. An extensive comparison between these simulations and the CIMEL measurements is conducted for two sites in the European waters. The impact of the use of the standard aerosol climatology on the retrieval of the water leaving radiances over coastal areas is also evaluated.
SeaDAS proposes the water leaving radiance as a level 2 product for the SeaWiFS sensor. This radiance results from the Gordon & Wang atmospheric correction scheme, applied on the measured Top Of Atmosphere radiance. We have access to ground-based measurements acquired by the CIMEL instrument at the SeaWiFS time overpass in different locations over European coastal waters.
An in-house software package has been developed in order to derive from the CIMEL data the required inputs (optical thickness and phase function) to a radiative transfer code (RTC). Computations allow to retrieve the SeaWiFS water leaving radiances with the “actual” atmospheric optical properties. An extended comparison between these values and those proposed by SeaDAS is conducted.
We use ground-based measurements in order to validate the SeaWiFS calibration as well as the SeaWiFS atmospheric correction scheme over coastal waters. Ground-based extinction measurements provide the required information to compare to the SeaWiFS aerosol products. For most of the cases, the aerosol optical thicknesses at 865 nm agree while large discrepancies occur in the blue. The SeaWiFS algorithm quite systematically selects the maritime model while the actual aerosol models correspond to a broader variety offered by the complexity of the sources near by the European coastal areas. The radiometer also measures at several wavelengths the atmospheric downwelling radiances in the almucantar and in the principal plane (PPL). Knowing by measurements the aerosol optical thickness (tau) a, a simple scheme was developed in order to derive the aerosol phase function Pa from the PPL radiances. We then have the required inputs to a radiative transfer code to properly account for the aerosols. In a forward mode, it is then applied to simulate the signal at SeaWiFS time of overpass in order to express the TOA radiances for comparison to the SeaWiFS measurements. In a backward mode, it can be used to convert the top of atmosphere radiance into a water leaving radiance. We present here some preliminary results for 3 days over the Venice site in Italy.
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