L. L. Smith

Comparison between infrared Martian disk spectra and optical properties of terrestrial analogs

Abstract Medium spectral resolution (20 cm−1) infrared measurements of the Martian disk made between 2900 and 5600 cm−1 from the NASA Lear Airborne Observatory have been successfully compared with predictions derived from a model of the Martian soil and atmosphere. Modeling of the Martian atmosphere permitted the extraction of Martian soil reflectance in the CO2 bands centered at 3657 cm−1. Three previously considered acceptable Martian soil analogs, limonite, montmorillonite, and basalt, were analyzed to determine the optical complex indices of refraction in the same range as the airborne observations, for mathematical modeling. A characteristic surface particle size ∼1 to 3 μm diameter is indicated. It is concluded that the Martian soil surface near-infrared optical properties are consistent with a soil composition similar to montmorillonite or limonite, mixed with a basalt.

High-resolution Martian atmosphere modeling

Abstract A multilayer radiative transfer, high-spectral-resolution infrared model of the lower atmosphere of Mars has been constructed to assess the effect of scattering on line profiles. The model takes into account aerosol scattering and absorption and includes a line-by-line treatment of scattering and absorption by CO 2 and H 2 O. The aerosol complex indices of refraction used were those measured on montmorillonite and basalt chosen on the basis of Mars ir data from the NASA Lear Airborne Observatory. The particle sizes and distribution were estimated using Viking data. The molecular line treatment employs the AFGL line parameters and Voigt profiles. The modling results indicate that the line profiles are only slightly affected by normal aerosol scattering and absorption, but the effect could be appreciable for heavy loading. The technique described permits a quantitative approach to assessing and correcting for the effect of aerosols on lineshapes in planetary atmospheres.

Advanced plume analysis software for gas measurement

Passive remote determination of the presence and quantity of gaseous chemicals is of interest for a variety of applications, but has been technically difficult to accomplish because of challenging measurement and analysis difficulties. This paper describes progress in the development of infrared plume analysis software. To unravel the atmosphere, background, and thermal contrast relationships it has been found necessary to obtain both spatial and spectral information and to have a set of codes which take these phenomena into account. Measured data were used along with model analyses to develop advanced plume analysis software (APAS) applicable to calculating how accurately amounts of gas ben be determined from overhead with current and possible future sensor technology. The APAS suite of codes incorporates sensor noise, plume contrast and atmospheric effects modules to arrive at a comparative measure of gas plume detection accuracy. The application of the APAS codes to prediction of detection accuracy for several candidate sensors is shown.

Development and testing of a MWIR Fabry-Perot interferometer for hyperspectral imaging

This paper describes the theory of design, operation, and testing of a tunable MWIR Fabry-Perot interferometer operating in low orders. This device is called the agile bandpass tunable filter (ABTF) due to the fact that the spectral bandwidth can be changed by a large factor by changing the order. In first order the system can be tuned over the entire 3.5-5 micrometers spectral region with only a single order sorting filter. We provide a short introduction to tunable filters an then briefly discuss the requirements that low order operation places on the Fabry-Perot dielectric mirrors. Operation in low orders forces one to abandon the classical Fabry-Perot approximation that the mirrors are negligibly thin compared to the plate separation. Rather, one must now account for the phase properties of the dielectric stack mirrors as they produce phase effects comparable to the plate separation. We next address the issue of control of the Fabry-Perot. This is accomplished through a closed-loop system using capacitive sensor on the Fabry-Perot flats to measure the separation of the plates. Additionally we describe how the ABTF is characterized using a FTIR to measure the bandpass shape and position, and we show some examples of measurements made with the ABTF used as a hyperspectral imaging system with a 256 X 256 HgCdTe camera. We conclude with a discussion of potential applications and future work.

High-Resolution Lower Atmospheric Transmission Predictions Over Long Paths

For the development of long range infrared sensors, it is important to have an accurate assessment of the transmission of IR radiation in the atmosphere. Measured experimental data to date have been obtained over tens of kilometers. In this paper a theoretical analysis of factors affecting clear band atmospheric transmission over ranges exceeding 200 km is presented. The analysis considers altitudes below 10 km (important for detection of low altitude targets) in the IR and uses a Grumman line-by-line code, LINETRAN, which incorporates the AFGL Line Parameter Atlas. Multiple atmospheric layers in LINETRAN account for the effects of refraction and altitude-dependent absorbers. The absorption contribution from each layer is calculated and summed to give the total transmission between the source and the observer. LINETRAN molecular absorption (smoothed to 20 cm-1 resolution)is compared to the molecular absorption predicted by the band-model code LOWTRAN for several clear bands and several ranges. The relative effects of humidity and visibility on band integrated transmission are then shown. Molecular line absorption, molecular continuum absorption, and aerosol extinction are included for two clear bands. The need for measurement of atmospheric parameters such as humidity and visibility during long range transmission measurements is seen to be essential for further progress in long range atmospheric modeling.

Recommended Modification Of LOWTRAN 4 To Include First Order Solar Scattering

The atmospheric transmittance and radiance code LOWTRAN 4 has been modified to include first order scattering of sunlight into the line of sight. This scattering becomes important for wavelengths shorter than 5μm under daytime conditions. The radiance due to both aerosol and molecular scattering is combined with atmospheric extinction and thermal emission already contained in the model. A mean phase function is defined by averaging over a given aerosol size distribution, composition, and wavelength in order to simplify the calculations. A comparison of the model with observational data is presented.