The far-infrared spectroscopy of the troposphere (FIRST) instrument is a Fourier transform spectrometer developed to measure the Earth's thermal emission spectrum with a particular emphasis on far-infrared (far-IR) wavelengths greater than 15 μm. FIRST was developed under NASA's Instrument Incubator Program to demonstrate technology for providing measurements from 10 to 100 μm (1000 to 100 cm~(-1)) on a single focal plane with a spectral resolution finer than 1 cm~(-1). Presently no spectrometers in orbit are capable of directly observing the Earth's far-IR spectrum. This fact, coupled with the fundamental importance of the far-IR to Earth's climate system, provided the impetus for the development of FIRST. In this paper the FIRST instrument is described and results of a detailed absolute laboratory calibration are presented. Specific channels in FIRST are shown to be accurate in the far-IR to better than 0.3 K at 270 K scene temperature, 0.5 K at 247 K, and 1 K at 225 K.
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