Far InfraRed Spectroscopy of the troposphere (FIRST): Sensor concept

FIRST (a NASA Instrument Incubator Program) is a balloon-based demonstration of a space-based sensor to measure the Earth's thermal infrared at high spatial and spectral resolution. The radiative balance of the troposphere, and hence climate, is dominated by the infrared absorption and emission of water vapor, particularly at far-infrared (far-IR) wavelengths longer than 15 pm. Because half of the Earth's outgoing longwave radiation occurs beyond 15.4 mum (650 cm(-1)), the distribution of water vapor and its far-IR radiative forcings and feedbacks are major uncertainties in understanding and predicting future climate. However, far-IR emission (spectra or band-integrated) has rarely been directly measured from space platforms. FIRST will be a Fourier Transform Spectrometer (FTS) with radiometric calibration in the spectral range from 10 to 100 mum (1000 to 100 cm(-1)) at 0.6 cm(-1) unapodized resolution. It will incorporate a broad bandpass bearnsplitter and a high-throughput optical and detector system. FIRST has a NEAT performance goal of 0.2 K from 10 to 100 mum. The spectral resolution will allow simultaneous retrievals of temperature and water vapor profiles. A 10 x 10 array of 10 km IFOVs is desired isolate clear and cloudy fields of view, while providing daily global coverage capability.