Ronald J. Huppi

Large-scale, persistent latitude structures in the mesospheric temperature during ANLC-93

Michelson interferometer measurements of the hydroxyl (OH) Meinel night-airglow emissions were made from the NCAR aircraft during the ANLC-93 Campaign. The spectra have been analysed to give the OH band radiance and rotational temperature, which is taken to be the neutral temperature at the peak of the OH emissions near 87 km. On two occasions, once in the presence of Noctilucent Clouds (NLC), the research flights spanned ∼10° of latitude and reached as high as 66°N. The mean latitudinal structure of the measured mesospheric temperature field is in good agreement with the CIRA-86 model. In addition, both measurements show large-amplitude (∼15 K), large-scale (2–4° of latitude), quasi-periodic oscillations about the model mean. Finally, the data support the hypothesis that these oscillations are temporally stable, and that they, along with the air mass characterized by the mean temperature field, can be advected with the tidal winds, and play a role in NLC formation.

Spectral measurements of high temperature 13 C 16 O 2 and 13 C 16 O 18 O in the 4.3-μm region

High resolution (0.007-cm(-1)) spectral measurements of a hot (800 K) CO(2) sample consisting of 88% (13)C(16)O(2), 11% (13)C(16)O(18)O, and 1% various other isotopes were made in the 4.3-microm region using the Air Force Geophysics Laboratory Fourier transform spectrometer. Vibration-rotation constants which predict line positions to better than 0.001 cm(-1) are presented for several bands of these two isotopes.

Comparison of infrared imaging hyperspectral sensors for military target detection applications

Recent studies have demonstrated the potential for exploring spectral discriminates in the thermal infrared for day/night surveillance and targeting of military targets in situations where the thermal contrast is low. Although the spectral discriminates have been found to be very subtle in most cases, good detection performance is achievable due to the generally high band-to-band spectral correlation of the background. This, however, presents a challenging set of requirements for infrared multispectral and hyperspectral sensors designed for this application. In this paper, we examine the merits and limitations of various design approaches, including imaging Michelson interferometers, dispersive spectrometers, and spatial Fourier transform spectrometers. The comparison is based on detailed sensor modeling as well as laboratory and field measurements of state-of-the-art instruments: a dispersive spectrometers and a n imaging Fourier transform spectrometer. The primary emphasis of this paper is the design of a hyperspectral sensor for tower-based and subsequent airborne data collection. Implications for operational multispectral sensor designs are also given.

A geosynchronous imaging Fourier transform spectrometer (GIFTS) for hyperspectral atmospheric remote sensing: instrument overview and preliminary performance results

The Geosynchronous Imaging Fourier Transform Spectrometer (GIFTS) was developed for the NASA New Millennium Program (NMP) Earth Observing-3 (EO-3) mission. This paper discusses the GIFTS measurement requirements and the technology utilized by the GIFTS sensor to provide the required system performance. Also presented are preliminary results from the recently completed calibration of the instrument. The GIFTS NMP mission challenge was to demonstrate new and emerging sensor and data processing technologies to make revolutionary improvements in meteorological observational capability and forecasting accuracy using atmospheric imaging and hyperspectral sounding methods. The GIFTS sensor is an imaging FTS with programmable spectral resolution and spatial scene selection, allowing radiometric accuracy and atmospheric sounding precision to be traded in near-real time for area coverage. System sensitivity is achieved through the use of a cryogenic Michelson interferometer and two large-area, IR focal plane detector arrays. Due to funding limitations, the GIFTS sensor module was completed as an engineering demonstration unit, which can be upgraded for flight qualification. Capability to meet the next generation geosynchronous sounding requirements has been successfully demonstrated through thermal vacuum testing and rigorous IR calibration activities.

Aurorally enhanced infrared emissions

Aurorally enhanced IR emissions in the 2.9-microm region have been measured from a jet aircraft with a zenith looking radiometer. Overtone chemiluminescent emission from chemically produced NO is the postulated source.

Update on the imaging sensor for GIFTS

Remote temperature sounding from the vantage point of Earth Orbit improves our weather forecasting, monitoring and analysis capability. Recent advances in the infrared hyperspectral sensor technology promise to improve the spatial and temperature resolution, while offering relatively quick re-look times to witness atmospheric dynamics. One approach takes advantage of a two-dimensional, imaging Fourier transform spectrometer to obtain a data cube with the field of view along one plane and multiple IR spectra (one for every FPA pixel) along the orthogonal axis. Only the pixel pitch in the imaging focal plane and the optics used to collect the data limit the spatial resolution. The maximum optical path difference in the Michelson FTS defines the spectral resolution and dictates the number of path-length interferogram samples (FPA frames required per cube). This paper discusses the unique challenges placed on the focal plane by the Geosynchronous Imaging Fourier Transform Spectrometer (GIFTS) approach and how advanced focal plane technology is applied to satisfy these challenges. The instrument requires a midwave spectral band from 4.4 to 6.1m to capture the C02 and H20 absorption bands, and an optional VLWIR spectral band to cover from 8.85-14.6m. The paper presents performance data of Liquid Phase Epitaxy (LPE) fabricated HgCdTe detectors and design details of the advanced readout integrated circuit necessary to meet the demanding requirements of the imaging sensor for the GIFTS instrument. Point defects are removed by using a unique super-pixel approach to improve operability for the VLWIR focal plane. Finally, early focal plane performance measurements are reported, including Noise Equivalent Input, responsivity uniformity, output offset stability and 1/f noise knee.

Experimental and modeling investigation of a 3-5 μm imaging spectrometer

The design and construction of new types of imaging systems has become feasible with the availability of reliable, relatively low cost focal plane arrays (FPAs). This paper presents experimental and modeling investigations of an imaging grating spectrometer utilizing a 128 X 128 InSb FPA for measurements in the 3 - 5 micrometers region. The modeling efforts verify the conceptual feasibility and identify practical limitations in sensitivity and dynamic range by considering system throughput, significant instrument thermal self-emission, and system noise. The model includes a detailed examination of the optical parameters, the focal plane noise sources, and the electronics. The design concepts and performance were verified experimentally by building and testing a prototype imaging spectrometer using commercially available optics, FPA, electronics, and computer equipment. Data is presented which illustrate the simultaneous spectra and spatial measurement features and the versatility of the sensor system. Both the model and the measurement results show the impact of instrument self-emissions, FPA noise, and FPA nonuniformities on the sensor system. It is very apparent that cryogenic optics, improved FPA non-uniformity correction, and an upgraded data acquisition system will significantly improve the performance of the prototype imaging spectrometer system and are important considerations for future designs.

Cryogenically Cooled Fourier Transform Spectrometers

Very sensitive Fourier transform spectrometers (FTSs) have been developed for infrared measurements utilizing advanced technology to achieve stable operation at very low temperatures. All of the structural, optical, and detector components used to construct these spectrometers are cryogenically cooled to improve the sensitivity of their detectors and to eliminate unwanted background emission signals from the components. Various designs of cooled FTS systems which have been successfully used and tested by USU and AFGL are presented in the paper. The performance specifications, the advantages, and the limitations of each technique are discussed. Also, general evaluations of the advantages and the limitations of cooling an FTS are given.

High resolution fourier spectroscopy of nitrous oxide at elevated temperatures

The Air Force Geophysics Laboratory high resolution interferometer has been used to measure the infrared absorption spectrum in the 8-μm region of a nitrous oxide sample heated to temperatures up to 800 K. A least-squares-fit was then used to obtain effective molecular constants for 18 rotation-vibration bands. These constants predict the position of spectral lines originating from excited rotational states with an accuracy considerably greater than previously available constants.

Rocket-Borne Cryogenic Michelson Interferometer

A nitrogen-cooled Michelson interferometer was fabricated using a unique flexural pivot mirror translation system which allows a proportionally large aperture and is ideally suited for operation at cryogenic temperatures. Cooling the entire interferometer yields a sensitivity sufficient to measure weak atmospheric emissions from an electron-gun induced artificial aurora. The spectral range 2.0 to 5.6 µm is scanned at a repetition rate of 1.8 seconds with an apodized resolution of 2 cm-1. Piezoelectric elements in the fixed mirror mount allow realignment at cryogenic temperatures. Laser (sampling) and white light (absolute position) reference channels are run antiparallel to the main channel using the same optics. When launched aboard a Talos Castor rocket as part of the EXCEDE payload the interferometer maintained alignment within 20 percent of launch modulation efficiency.