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Far-infrared Spectroscopy of the Troposphere (FIRST): sensor calibration performance

The radiative balance of the troposphere, and hence global climate, is dominated by the infrared absorption and emission of water vapor, particularly at far-infrared (far-IR) wavelengths from 15-50 μm. Water vapor is the principal absorber and emitter in this region. The distribution of water vapor and associated far-IR radiative forcings and feedbacks are widely recognized as major uncertainties in our understanding of current and the prediction of future climate. Cirrus clouds modulate the outgoing longwave radiation (OLR) in the far-IR. Up to half of the OLR from the Earth occurs beyond 15.4 μm (650 cm-1). Current and planned operational and research satellites observe the midinfrared to only about 15.4 μm, leaving space or airborne spectral measurement of the far-IR region unsupported. NASA has now developed the sensor required to make regular far-IR measurements of the Earth's atmosphere possible. Far InfraRed Spectroscopy of the Troposphere (FIRST) was developed for NASA's Instrument Incubator Program under the direction of the Langley Research Center. The objective of FIRST is to provide a balloon-based demonstration of the key technologies required for a space-based sensor. The FIRST payload will also be proposed for science flights in support of validation of the various experiments on the Earth Observing System (EOS). We discuss the FIRST Fourier transform spectrometer system (0.6 cm-1 unapodized resolution), along with its radiometric calibration in the spectral range from 10 to 100 µm (1000 to 100 cm-1). FIRST incorporates a broad bandpass beamsplitter, a cooled (~180 K) high throughput optical system, and an image type detector system. We also discuss the actual performance of the FIRST instrument relative to its performance goal of a NE(delta)T of 0.2 K from 10 to 100 μm.