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ScaRaB is a radiometer built to observe the Earth Radiation budget at the top of the atmosphere. This instrument has been produced by means of a joint program that gathers France, Russia and Germany. SCARAB has been planned to fill the gap between the flights of NASAs instruments ERB and CERS. Thus SCARAB mission is similar to the one fulfilled by ERB and CERES. SCARAB FM1 has been launched in January 1994. At the very beginning of the flight, a critical problem occurred on-board: three calibration lamps over 6 failed and were unusable for the rest of the mission. An alternative way of calibration was found that saved the mission. This method, explained in a previous paper, and its accuracy are briefly recalled in the article. FM2 has been launched in July 1998. The first data show a perfect functioning of the on-board calibration devices. It is an opportunity to test the 3 in-flight calibration methods developed: a method using on-board lamps and black-bodies as previously planned on FM1, another one using the dependence of gains to temperature as used on FM1, and a last one using only black body-simulators. After describing more in detail these different methods, a comparison of their performances is established. These results are compared with FM1 performances. Future prospects of the calibration are given in conclusion.

The outgoing longwave radiation is an indicator of the overall state of the Earthatmosphere system and its diurnal variation reveals how the system responds to the solar diurnal forcing. The Earth Radiation Budget Experiment (ERBE) is the first attempt to use a multi-satellite system to obtain improved time sampling necessary for determining diurnal variation. We have used 3-hourly Meteosat data to improve the time sampling still further as compared to the four observations per 24 hours obtained during most of the ERBE lifetime. Regression techniques and radiative transfer calculations allow us to convert the relatively narrow-band Meteosat infrared window (IRW: 11 pm) and " water vapor" (WV: 6. 3 m) channel data into estimates of broad-band longwave (LW) radiant exitances. We compare the diurnal variations obtained from these data with the ERBE diurnal cycle estimates. Results show that the ERBE diurnal modelling algorithms generally perform very well and that most of the discrepancies that are observed would not have occurred if the system had been complete with scanners functioning together on 3 satellites as originally planned. 1.© (1990) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.