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ScaRaB (Scanner for Radiation Budget) is an optical scanning radiometer devoted to the measurements of radiative fluxes at the top-of-atmosphere (TOA) in the shortwave and longwave domain. The optical radiometer is composed of 4 parallel and independent telescopes focusing the reflected solar and emitted thermal radiation of the earth atmosphere on 4 detection channels with high absolute accuracy (1% - 2%) owing to onboard calibration modules.

Channel 2 and channel 3 are considered as the main channels, channel 2 providing directly the solar energy reflected by the earth-atmosphere, channel 3 measuring the total energy (solar and thermal). During night-time, the longwave radiance is directly given by the channel 3. During daylight, however, the longwave radiance is given by a difference between the channel 3 and the channel 2 radiance measurements. Since the ScaRaB 2 and 3 channels have very similar spectral response in the shortwave spectral domain, no additional spectral correction is necessary to determine the longwave radiance from such a difference. However, as for the previous ERBE (Earth Radiation Budget Experiments) an excellent cross-calibration between the shortwave (channel 2) and the total (channel 3) channel is required.

Channel 1 and channel 4 are narrow band channels used for scene identification in the visible (channel 1) and in the Infrared (channel 4) domains. The main channels characteristics are listed in the table hereafter:

Wavelength Channel
1-Visible 0.55-0.65 µ
2-Solar 0.2-4 µ
3-solar .2-100 µ
4-IR Window 10.5-12.5 µ

At the altitude of 866 km, the instantaneous ScaRaB field of view (FOV) of 48 mili-radians corresponds to a geographical footprint of approximately 40 km squared at nadir to 200 km on the edge. To ensure a swath of about 2200km, a cross track scanning of the instrument is performed. The cross track scanning is obtained by the rotation of the telescopes and associated detectors in the nadir plane, which is perpendicular to the satellite speed vector. The procedures of calibration and processing of the data in order to derive fluxes from the original radiances have been set up and tested by CNES and LMD.

Source: ISRO-CNES Megha-Tropiques Web Site