The aircraft HIS is a Fourier Transform Spectrometer which views directly downward through an open port in a pod mounted under the center line of the fuselage. From altitude, its instantaneous FOV is 2 km directly below the plane. Many of the important design parameters of the HIS are summarized in the table.
Calibration is accomplished by viewing two high-emissivity blackbodies, servo-controlled to 300 K and about 240 K. After collecting 12 spectra of the earth, a 45-degree scene-switching mirror rotates the field-of-view from the open Earth viewing port to give four spectra of the hot and four spectra of the cold blackbodies. At about 6 seconds per spectrum, this calibration cycle takes 2 minutes. The blackbodies, built and tested by Eppley Labs, are blackened cavities with thermoelectric cooler/heaters for temperature control and platinum resistance thermometers for monitoring. The temperature of the interferometer optics is not actively controlled.
A linear-plane mirror Michelson interferometer from BOMEM of Canada provides double sided interferograms in both scan directions. Its auto-alignment system makes it possible to operate in the ambient thermal environment of the pod and in very close proximity to the aircraft jet engine. The optical bench isshock-mounted to dampen high-frequency vibration and the interferometer is evacuated to protect the beamsplitter during descent. The three spectral bands, covering most of the region from 3.8 to 16.6 microns, are split inside a single liquid helium dewar, which contains three sets of bandpass cold filters, focusing optics, and arsenic-doped silicon detectors. The preamplifiers are external and operate near the ambient pod temperature of about 260 K. The gain of each channel is fixed, and the signals are digitized with a 16-bit A/D converter. Onboard numerical filtering is used to reduce the sample from the HeNe laser rate by factors of 14, 8, and 8 in bands I, II and III. The data system is controlled with a 6809 microprocessor-based system built at the University of Denver. The three bands of interferometer data and housekeeping parameters are combined and recorded on formatted cassette tapes. Two drives with a capacity of 67 megabytes each are used to provide 9 hours of continuous recording time.
Processing of selected data in the field is performed on IBM-compatible personal computers. Data are transferred to hard disk and is processed with custom software, which displays the measured interferograms and corresponding spectra, and performs calibration to yield radiance or brightness temperature spectra. The calibration procedure uses full complex spectra to avoid errors that can arise from radiance emitted by the warm interferometer. Software is also available to produce vertical cross sections of retrieved atmospheric state parameters, thus permitting science analysis of the results in the field.
The absolute radiometric accuracy is better than 1 degree K brightness temperature (at 260 K) and the noise is a few tenths of a degree K. Temperature retrieval accuracy approaches 1K. The block diagram illustrates the functional overview of the HIS major subsystems, including the Aircraft Experiment Interferometer (AEI), the Onboard Recording System (OBRS), which uses two 3M HCD-75 tapes drives, and the System Control and Monitor (SCAM) System.
