Two different approaches to achieve vicarious calibration without a priori on the aerosol model: application to SPOT5 over the test site of La Crau, France

Abstract

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 <i>in-situ </i>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.