Far-infrared spectroscopy of the troposphere: instrument description and calibration performance

Abstract

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.

Far-infrared spectroscopy of the troposphere: calibration with a cold background

“…The absolute accuracy goal of FIRST was established to be identical to the precision, acknowledging the intent to average many spectra to substantially reduce random noise in the overall error budget. The prior calibration [7] and this calibration demonstrate that FIRST does meet these goals for scene temperatures of ∼270-325 K.…”

“…FIRST was calibrated for absolute radiance in the balloon-observing mode by having the on-board WBB attached to one port of the scene-select assembly, a space view simulator (SVS) blackbody on the second port, and the long wave infrared calibration source (LWIRCS) [7,10,11] blackbody on the third port of the scene-select assembly. LWIRCS (Table 2, Fig.…”

“…That calibration was performed for the ground-based observation mode of FIRST, which uses a warm and an ambient blackbody for on-board calibration and showed FIRST to be accurate in the far-IR to better than 0.3 K for 270-325 K scene temperatures, 0.5 K at 247 K and 1 K at 225 K [7]. The reduced accuracy at lower temperatures is due to magnification of errors when extrapolating away from the temperatures of the two calibrator blackbodies.…”

“…This paper describes the results of the cold background calibration of FIRST performed in 2013. The lessons learned from this calibration activity and from the prior warm source calibration activity [7] will be extremely valuable in developing space-flight missions designed to measure the far-IR spectrum [8].…”

“…Table 1 lists the basic FIRST instrument parameters. A complete description of the instrument and basis for the design is provided elsewhere [7,9]. The noiseequivalent temperature difference NEΔT in the table is for a single spectrum of a target.…”