Currently, the HIS instrument is flown aboard NASA ER2 aircraft, mounted within a pod (about 3m long and 0.5m wide) either under the center line of the fuselage or under the wing. Flight level is at approximately 20 km (65,000 ft) or 50 mb. The instrument is shock mounted to dampen high-frequency vibrations from the aircraft.
The HIS instrument is based upon Michelson interferometric principles (Smith, 1979, 1980, 1983, Huang, 1989), generating interferograms as initial products.
Interferograms are produced at the rate of one every six seconds, with twelve interferograms produced before on-board calibration is performed. One interferogram consists of the forward or backward transverse of the Michelson moving mirror across the full optical path length, with separate detectors for each spectral band of incoming radiation. Calibration is performed by rotating a 45^ scan mirror from nadir, twice sampling two high emissivity blackbodies, servo controlled at altitude to 300K and 240K. The instrument's auto-alignment system makes it possible to operate in the ambient thermal environment of the pod and close proximity to aircraft engines.
The three spectral bands, covering most of the region from 3.8 to 16 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 (about 240K). The gain of each channel is fixed and the signals are digitized with a 16 bit A/D. On-board numerical filtering is used to reduce the sample rate from the HeNe laser rate by factors of 14, 8, and 8 in Bands I, II, and III, respectively.
The data system is controlled with a 6809 microprocessor-based system built at the University of Denver. The three spectral bands of interferometer data and housekeeping parameters are combined and recorded on formatted cassette tapes. Two drives with a combined capacity of 134 megabytes provide nine hours of continuous recording time.
Processing of the data is performed at the University of Wisconsin-Madison - Cooperative Institute for Meteorological Satellite Studies (UW-CIMSS) in conjunction with the National Environmental Satellite Data and Information Service (NESDIS), utilizing custom software to perform calibration and produce radiance and brightness temperature spectra. Details are provided in Revercomb, 1989 and Smith, 1991.
Since the HIS aircraft instrument has been proven by extensive meteorological and mathematical research to be a highly accurate and reliable instrument, the next step to take with the HIS program is to place the instrument aboard a geosynchronus satellite. As shown in the Meteorological Information section, the improvement of the HIS over GOES in vertical and horizontal resolution is substantial, and this improvement would greatly increase mesoscale weather prediction accuracy, which are dependent upon the current mesoscale analysis system for information.
The current United States radiosonde network has a very poor spatial and temporal resolution, with spacing between stations from 50 to 500 km, with launches only twice daily. In addition, the Doppler radar wind profilers, however accurate to within 1 m s-1 with a vertical resolution of 1 km, also are insufficient for mesoscale analysis, since the horizontal spatial resolution of the network is on the order of 500 km. However, increased accuracy and resolution can be achieved utilizing the GHIS. Horizontal and vertical resolutions of the GHIS are expected to be near 1 to 2 km, with measurements taken every six seconds (excluding calibration scans). These resolutions are a drastic improvement over current GOES sounders, which have horizontal and vertical resolutions of 7 and 5 km, respectively.
The AERI system is the ground based version of the HIS. This system measures downwelling atmospheric emitted radiation, as opposed to the HIS/GHIS, which measures upwelling atmospheric emitted radiation. A full description of the AREI instrument and its capabilities are provided within the AERI section of the CIMSS Homepage.