Realization of SI-traceable infrared radiance measurements from space for achieving benchmark climate observations

We have developed a prototype satellite instrument to make SI-traceable measurements of thermal infrared radiance for the purpose of monitoring the Earth's climate. The instrument employs a Fourier transform spectrometer with two detectors operating in the frequency range 600-2000 cm -1 . A blackbody calibration system was built to allow traceable reference measurements of infrared radiance. SI-traceable thermometry and reflectometry are used to determine the parameters in the Planck equation accurately. Diagnostics were tested that will allow the evaluation of these parameters on-orbit, during the lifetime of the satellite mission, as they are subject to drift resulting from exposure to the launch and low Earth orbit environments. The laboratory-based prototype demonstrates that infrared radiance can be measured with an accuracy of ±0.1 K on a 270 K background at 1000 cm -1 . This corresponds to a radiometric uncertainty of 2 parts in 10 3 , which is lower than is achieved by existing space-based radiometers. The observations from such a satellite instrument will reveal the longwave forcing of the climate, the climate's response, and the longwave feedbacks involved in that response. These constitute an effective benchmark climate record that can be used to detect and at tribute climate change. Furthermore, the measurements can be used to test general circulation models. The longwave water vapor feedback, cloud feedback and temperature change are uniquely discernable in a time series of thermal infrared spectra. By comparing the observed and modeled climate sensitivity, general circulation models can be constrained according to their predictive capabilities.