Radiometric calibration of IR Fourier transform spectrometers: solution to a problem with the High-Resolution Interferometer Sounder

TLDR

The High‑Resolution Interferometer Sounder (HIS) is a calibrated Fourier transform spectrometer flown on NASA U‑2 aircraft to measure Earth’s infrared emission, demanding radiometric precision of about 0.1 °C for temperature and 1 °C for humidity. HIS performs inflight two‑point radiometric calibration with hot and cold blackbody references, and the updated analysis corrects for a dual phase response caused by emission from the interferometer beam splitter. Laboratory tests uncovered brightness‑temperature errors up to 15 °C in one spectral band caused by a source‑dependent phase response, which were eliminated by minor calibration‑equation adjustments that traced the dual‑phase effect to emission from the beam splitter.

Abstract

A calibrated Fourier transform spectrometer, known as the High-Resolution Interferometer Sounder (HIS), has been flown on the NASA U-2 research aircraft to measure the infrared emission spectrum of the earth. The primary use-atmospheric temperature and humidity sounding-requires high radiometric precision and accuracy (of the order of 0.1 and 1 degrees C, respectively). To meet these requirements, the HIS instrument performs inflight radiometric calibration, using observations of hot and cold blackbody reference sources as the basis for two-point calibrations at each wavenumber. Initially, laboratory tests revealed a calibration problem with brightness temperature errors as large as 15 degrees C between 600 and 900 cm(-1). The symptom of the problem, which occurred in one of the three spectral bands of HIS, was a source-dependent phase response. Minor changes to the calibration equations completely eliminated the anomalous errors. The new analysis properly accounts for the situation in which the phase response for radiance from the instrument itself differs from that for radiance from an external source. The mechanism responsible for the dual phase response of the HIS instrument is identified as emission from the interferometer beam splitter.