Jean-Marc Thériault

MODTRAN2: suitability for remote sensing

MODTRAN2 (1992) is the most recent version of MODTRAN, the Moderate Resolution Atmospheric Radiance and Transmittance Model, first released by the Geophysics Directorate, Phillips Laboratory, in 1990. It encompasses all the capabilities of LOWTRAN 7, the historic 20 cm-1 resolution radiance code, but incorporates a much more sensitive molecular band model with 2 cm-1 resolution. For inversion algorithm applications, MODTRAN2 must prove to be sufficiently accurate when calculating layer- specific perturbations. First steps in establishing this capability have recently been accomplished. DREV (Defence Research Establishment Valcartier, Canada), in conjunction with the Geophysics Directorate, has taken measurements with a surface-based Bomem interferometer (approximately 1 cm-1 resolution), with full supporting sonde profiles (z, T, p, and relative humidity). This suggests that the derivative matrices, typically required for inversion algorithms, may be readily (and rapidly) calculated using MODTRAN whenever its spectral resolution is adequate.

Passive standoff detection of chemical warfare agents on surfaces

Results are presented on the passive standoff detection and identification of chemical warfare (CW) liquid agents on surfaces by the Fourier-transform IR radiometry. This study was performed during surface contamination trials at Defence Research and Development Canada-Suffield in September 2002. The goal was to verify that passive long-wave IR spectrometric sensors can potentially remotely detect surfaces contaminated with CW agents. The passive sensor, the Compact Atmospheric Sounding Interferometer, was used in the trial to obtain laboratory and field measurements of CW liquid agents, HD and VX. The agents were applied to high-reflectivity surfaces of aluminum, low-reflectivity surfaces of Mylar, and several other materials including an armored personnel carrier. The field measurements were obtained at a standoff distance of 60 m from the target surfaces. Results indicate that liquid contaminant agents deposited on high-reflectivity surfaces can be detected, identified, and possibly quantified with passive sensors. For low-reflectivity surfaces the presence of the contaminants can usually be detected; however, their identification based on simple correlations with the absorption spectrum of the pure contaminant is not possible.

Passive standoff detection of Bacillus subtilis aerosol by Fourier-transform infrared radiometry

An analysis is presented on the passive standoff detection and identification of Bacillus subtilis (BG) clouds with the Compact ATmospheric Sounding Interferometer (CATSI) sensor. This research is based on recent spectral measurements obtained during the Technology Readiness Evaluation trial held July 2002 at Dugway Proving Ground, Utah. Results obtained from three trial BG cloud episodes are used to explain and demonstrate the detection capability of the CATSI sensor. The BG clouds were measured at a distance of 3 km from the sensor in a near-horizontal path scenario. It was found that the low thermal contrast of approximately 0.2 K between the BG cloud and the background yielded weak but observable spectral signatures. The processing of the spectral signatures with the GASeous Emission Monitoring (GASEM) algorithm has provided a rough estimate of BG cloud column densities. The results of a series of simulations with the FASCOD3 transmission model have shown that the detection sensitivity for BG can be greatly improved for both slant path uplooking and downlooking scenarios.

Passive remote monitoring of chemical vapors by differential Fourier-transform infrared radiometry: results at a range of 1.5 km

A method for the passive remote monitoring of chemical vapors by differential Fourier-transform infrared radiometry is presented to determine the characteristics of a chemical vapor plume from a stack located at a distance of more than 1 km from the sensor. This measurement technique is based on the use of a double-beam Fourier-transform infrared spectrometer that is optimized for optical subtraction. A description of the interferometer (compact atmospheric sounding interferometer) is given along with the algorithm (GASEM) that has been developed for the on-line detection, identification, and quantification of chemical vapor plumes. The detection method is described with a particular emphasis placed on its monitoring capabilities. The analysis focuses on the experimental results obtained at a recent open-air experiment for vapor plume mixtures of dimethyl methyl phosphonate and SF6 probed at a distance of 1.5 km. The accuracy of a simplified plume radiance model implemented in the detection algorithm is specifically addressed. The measurement technique has been successfully used to detect and identify low, medium, and high concentrations of vapor mixtures but appears to have limited quantification capabilities in its present form.

Radiometric calibration of FT-IR remote sensing instrument

Recently, Bomem developed CATSI, a small FTIR system referred to as the Compact ATmospheric Sounding Interferometer. In order to meet the highest radiometric precision and accuracy, the instruments performs calibrations, using observations of hot and cold blackbody reference sources as the basis for a two-point calibration at each wavenumber. This paper presents the results of the analysis of the radiometric calibration of this instrument, with emphasis on the temporal behavior of the instrument response.

Gaseous emanation detection algorithm using a Fourier transform interferometer operating in differential mode

Enactment of the Clean Air Act Amendments of 1990 has resulted in an increased ambient air monitoring needs for industry, some of which may be met efficiently using open- path optical remote sensing (ORS) techniques. Among the most promising of these techniques, we note the Fourier transform spectrometry (FTS). This technique is well suited for the detection of organic and inorganic chemicals since most of them have characteristic absorption bands in one or both of the IR atmospheric window regions. The need for reliable atmospheric pollution monitoring has motivated the development of a number of new chemical analysis approaches. This paper presents an approach for the spectral remote sensing of gaseous emanations from chemical agents, based on the measurements of their weak emission spectra via a passive IR FTS sensor. The method is implemented using such a sensor operating in differential mode between the two inputs, allowing the background to be optically subtracted. A specific algorithm has been developed jointly by Bomem and the Defence Research Establishment of Valcartier to take advantage of this particular setup and allow identification of trace gases in real time. A software research tool named GASEM implements this algorithm and is used with CATSI, a double-beam remote sensing interferometer operating in differential mode in the 3.5-17 micrometers spectral range.

Passive Standoff Detection of SF6 at a Distance of 5.7 km by Differential Fourier Transform Infrared Radiometry

Recent results are presented on the passive detection, identification, and quantification of a vapor cloud of SF6 measured at a horizontal standoff distance of 5.7 km using a dual-beam interferometer optimized for background signal suppression. The measurements were performed at Defense Research and Development Canada (DRDC)–Valcartier during a number of recent open-air experiments. The measurement approach is based on the differential passive standoff detection method that has been developed by DRDC Valcartier during the past few years. This work represents the first such measurement reported in the open literature for a standoff distance as large as 5.7 km. These results clearly demonstrate the capability of the differential radiometry approach to the detection, identification, and quantification of chemical vapor clouds located at long distances from the sensor.

Estimating atmospheric extinction for eyesafe laser rangefinders

Extinction of laser rangefinder (LRF) pulses by the atmosphere depends on the laser wavelength, weather conditions, and the aerosol concentration along the optical path. The total atmospheric extinction α is the sum of the molecular and aerosol contributions α m and α a . We present simple expressions for α m and α a for wavelengths near 1.44, 1.54, 2.1, and 10.6 μm, which are eyesafe for most LRF applications. Also included are results for 1.06 μm, which although not an eyesafe wavelength is used extensively for LRF applications. The expressions allow the extinction coefficient to be estimated as a function of standard meteorological parameters, assuming horizontal beam propagation at sea level and a homogeneous atmosphere. Measurements of the signal-to-noise ratio of LRF returns from a calibrated target are presented for various weather conditions and wavelengths.

Differential detection with a double-beam interferometer

First steps in the development of a new method for the remote detection of gaseous emanations by FTIR are presented. The method is based on the use of a double beam interferometer-spectrometer (CATSI) optimized for optical subtraction. The instrument is described with a particular emphasis on its capabilities for differential detection and background suppression. Radiometric equations applicable to the differential detection of gaseous emanations by FTIR are established for the general case of slant path scenarios containing any type of background scenes. Finally the potential of the method is illustrated through several laboratory measurements done with the CATSI system.

Standoff detection of explosives: a challenging approach for optical technologies

Standoff detection of explosives residues on surfaces at few meters was made using optical technologies based on Raman scattering, Laser-Induced Breakdown Spectroscopy (LIBS) and passive standoff FTIR radiometry. By comparison, detection and analysis of nanogram samples of different explosives was made with a microscope system where Raman scattering from a micron-size single point illuminated crystal of explosive was observed. Results from standoff detection experiments using a telescope were compared to experiments using a microscope to find out important parameters leading to the detection. While detection and spectral identification of the micron-size explosive particles was possible with a microscope, standoff detection of these particles was very challenging due to undesired light reflected and produced by the background surface or light coming from other contaminants. Results illustrated the challenging approach of detecting at a standoff distance the presence of low amount of micron or submicron explosive particles.