Remote Sensing from Space

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The Sun-earth Imbalance radiometer for a direct measurement of the net heating of the earth

Although it is generally accepted that the climate on earth is changing due to a radiative energy imbalance at the top of the atmosphere, up to now this radiation imbalance has not been measured directly. The measurement is challenging both in terms of space-time sampling of the radiative energy that is leaving the earth and in terms of accuracy. The incoming solar radiation and the outgoing terrestrial radiation are of nearly equal magnitude – of the order of 340 W/m² – resulting in a much smaller difference or imbalance of the order of 1 W/m². The only way to measure the imbalance with sufficient accuracy is to measure both the incoming solar and the outgoing terrestrial radiation with the same instrument.

By reanalyzing data from the NASA LARC Earth Radiation Budget Experiment, we have been able to demonstrate that the sampling problem can be overcome, even with the low resolution Wide Field of View radiometer. We have combined the measurements of the precessing ERBS satellite for midlatitude and equatorial regions with measurements of the sun synchronuous NOAA9 satellite for the polar regions.

For the accuracy requirement an improved instrument design is needed. We propose a new instrument, which we call the Sun-earth IMBAlance (SIMBA) radiometer. It is an improved wide field of view cavity radiometer based on our long experience with the DIARAD type of instrument for the measurement of Total Solar Irradiance. Currently we have two DIARAD instruments in space, on SOHO and on the ISS, and a third one will be launched this year on the Picard microsatellite.