Electronic Military & Defense magazine was developed for engineers, program managers, project managers, and those involved in the design and development of electronic and electro-optic systems for military, defense, and aerospace applications.
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Techniques Electronic Military & Defense Annual Resource, 6th Edition 31 mination by an active IR-source is needed. The signal comes from the natural temperature differences between a target and whatever background exists. Identification Routines The identification method is based on the approximation of a measured spectrum with a combination of reference spectra. First, the spectrum of the brightness temperature (TBr) is calculated: The brightness temperature spectrum is analyzed sequentially for the target compounds contained in the system's spectral library. The signatures of the target compound and atmospheric gases are fitted to the resulting spectrum. Then, to decide if the target compound is present, a correlation analysis between the measured spectrum and the signature of the target compound is performed. If the coefficient of correlation and the signal- to-noise ratio are greater than compound-specific threshold values, the target compound is identified. Remember that the algorithm does not require background spectra/images, so there is no need for a measurement of the scenery with no target compounds present. This is important in many situations, such as accidents or chemical attacks, when forces are called to the scene after the event. For gaseous target compounds, measured and corrected spectra can be directly compared to absorption spectra in the library. For the liquid target compounds, the sample's physical properties (e.g., the liquid's layer thickness and the background material's reflection properties) influence the shape of the signal. Therefore, forward simulation is performed to simulate the brightness temperature spectrum for each of the target compounds. The calculation is based on the liquid's optical properties, the background material's reflection spectrum, and the incident radiation's spectrum. 5,6,7,8 Spectra of the incident radiation and, if possible, of the unstained carrier material are automatically gathered along with the measurement. If the latter spectra cannot be acquired, a typical profile is used. The simulated spectra for each target compound are then fitted to the measured spectra, and spectral features of atmospheric gases are subtracted. Eventually, from the fitted spectrum — compensated for interferents — a reflection spectrum can be calculated that resembles the pure reflection spectrum of the liquid target compound, which can then be compared to the spectral library. Figure 2 shows the identification of ammonia measured from a distance of approximately one kilometer. Pixels where a chemical is identified are depicted with a red overlay atop the video image of the scenery; the user then is warned optically and acoustically. Compound-specific thresholds for coefficient of correlation between measurement and Figure 2: Release experiment of ammonia: At left, the screen of the measurement PC is shown with the point of release in the distance. At right, the identification image is depicted as a red overlay on a video image of the scenery. Figure 3: Measurement of a stack emitting ethylene. The identification image is displayed at left, and, at right, the fitted reference spectrum of ethylene (orange) is compared to the measurement (blue).