An aircraft version of the high-resolution interferometer sounder (HIS), a Fourier transform spectrometer designed for meteorological applications, has been used to measure the upwelling infrared emission of the earth with a resolving power on the order of 1000. HIS measurements from high-altitude NASA research aircraft have demonstrated that the high radiometric accuracies required for atmospheric temperature and humidity sounding (1°C absolute brightness temperature and 0.1° C RMS reproducibility) can be achieved. Calibration is accomplished using periodic views of two onboard high-emissivity blackbodies, servo controlled to 300 K and 240 K. For an interferometer, this approach relies on careful optical design and alignment to avoid unknown dependence of the responsivity on optical path difference. The aircraft model is a successful prototype for spacecraft versions for weather and climate monitoring.