ScaRaB: absolute calibration and results of the inter-comparison campaigns with CERES-Terra

ScaRaB (SCAnner for RAdiation Budget) is an Indo-French satellite onboard MEGHA-TROPIQUES launched on October 12th 2011. This radiometer has been designed to fill the gap between the ERBS and CERES missions to study the water cycle and energy exchanges in the tropics. ScaRaB is fit with four parallel and independent channels: channel-2 and channel-3 being considered as the main ones, channel-1 is dedicated to measure solar radiance while channel-4 is an infrared window. The absolute calibration of ScaRaB is achieved by internal calibration sources (black bodies and a lamp for channel-1). The radiometric properties of deserts sites and more especially their stable spectral response over time made them very good candidates to perform temporal monitoring of ScaRaB channel-1. This paper deals with the corresponding results. High altitude clouds are observed by ScaRaB to survey the balance between channel 2 and channel 3: the earth long-wave radiance is isolated by subtracting the short-wave channel to the total channel. Radiometric cross calibration of Earth observation sensors is a crucial need to guarantee or quantify the consistency of measurements from different sensors. CERES and ScaRaB Earth Radiation Budget missions have the same specification: to provide an accuracy of similar to 1% in the measurement of short-wave and long-wave radiances and an estimation of the short-wave and long-wave fluxes less than 10 W/m(2). Taking advantage of the "equatorial" orbit of Megha-Tropiques, NASA proceeded to manoeuvers on CERES-Terra in order to ease an inter-comparison between both instruments over common targets. Actually, The CERES PAPS mode was used to align its swath scan in order to increase the collocated pixels between the two instruments. The experience lasted 3 months from March 22th and May 31st 2015. A previous similar campaign has already been led in 2012. This article presents the results of these inter-comparisons, providing an indication on the temporal stability of the calibration between 2012 and 2015.