干旱区生态环境知识资源中心

In the characterization of a space-borne wide field-of-view sensor, like Vegetation, the multi-angular calibration is strongly complementary to the absolute calibration. It is defined as the process of estimating the sensitivity variations at different points of the Vegetation wide field-of-view. This effect has to be integrated in the data processing. Pre-flight measurements were performed before launch, but because of heavy irradiations and aging of the different part of the sensor, it is necessary after launch to check and/or adjust the multi-angular calibration coefficients, g(p). Far this, the g(p) coefficients were split into three terms which required different methods: i/ first, the low-frequency term (g(pLF)) which refer to variation of the optic transmission which slightly decreases when viewing angle increases. The g(pLF) were verified using acquisitions over 20 desert sites for which TOA reflectances are accurately characterized (from ground measurements and POLDER/ADEOS-1 measurements). No in-flight variation of the g(pLF) were detected, ii/ second, the high-frequency term (g(pHF)) which refer to variation of the sensitivity of the elementary detectors. The g(pH)F were verified statistically using acquisitions over the Antarctica site and were accurately checked for the 4 spectral bands. ii/ third, the medium-frequency term (g(pMF)) which refer to various kinds of variation (optics, detectors...). The g(pMF) were verified during the Vegetation like using the on-board calibration device (lamp profiles) and some small variations were identified (< 0.5% for B0, B2, B3 and 1% for MIR). This aspect is still under investigation using acquisitions over Antarctica.