Far-infrared sensor for cirrus (FIRSC): an aircraft-based Fourier-transform spectrometer to measure cloud radiance.

We describe an aircraft-based Fourier-transform spectrometer (FTS) designed to measure the Earth outgoing radiance spectrum in the far-infrared-submillimeter spectral range. The instrument features include a rapid-scan FTS to obtain high spatial resolution from a moving aircraft platform, a sensitive two-channel detector, and a CCD camera for recording the nadir cloud scene with each scan record. Such measurements demonstrate the sensitivity of Earth radiance to high clouds and provide spectral data for improving techniques for remote sensing and retrieval of atmospheric and cloud properties.

Accurate frequency measurements for H(2)O and (16)O(3) in the 119-cm(-1)OH atmospheric window.

We report frequency measurements for relatively weak H(2)O and (16)O(3) rotational transitions in the ground state and in the nu(2) = 1 vibrationally excited state. We obtained the frequency measurements by using the laboratory technique of tunable far-infrared spectroscopy with the objective of improving H(2)O and O(3) line parameters required for modeling the important atmospheric spectral window near 119 cm(-1). New sets of molecular constants are calculated from the (16)O(3) data, and improved values are reported for the frequencies of the H(2)O lines. The improvement in atmospheric simulations obtained with the new results is illustrated by comparison with recent high-resolution balloon-based atmospheric measurements. These new data significantly improve simulations of high-resolution atmospheric emission spectra.

Portable (3)He detector cryostat for the far infrared.

A portable (3)He cryostat for far infrared detection applications is described. The cryostat has been used for a number of years in aircraft- , observatory- , and laboratory-based research. Some related studies of various bolometric materials are also reported.

Infrared passbands for clear-air-turbulence detection.

Clear air turbulence (CAT) ahead of an aircraft can be detected in real time by an infrared radiometer. The alert time and reliability depend on the passband of the infrared filter used and the altitude of the aircraft. Preliminary results and analyses show that a nominal passband of 26 to 35 microm appears optimal to alert CAT from 1.5 to 6.0 min ahead of the encounter. The alert time increases with higher altitude as the atmospheric absorption, determining the horizontal weighting, is reduced.

A double pass spectrometer for the far infrared.

A far ir spectrometer has been built which uses a double pass optical system to increase the spectral dispersion. Because of the energy limitations in this spectral range, this means of increasing the dispersion is an efficient way to improve the resolving power of a grating monochromator. The same optical system is also used to advantage in a lamellar grating interferometer where the increased dispersion improves the performance at higher frequencies. Consideration is also given to instrument errors and signal noise which can affect, and in some cases limit, the present performance of a far ir Fourier interferometer.