Electronic Military & Defense Annual Resource

6th Edition

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 reference, as well as for signal strength, are set to a level that prevents false alarms. Figure 3 compares the fitted reference spectrum of ethylene to the measurement of a stack that is approximately 1.4 km away. The identification image shows the presence of ammonia and indicates the direction of propagation of the target compound. Examples of measurements with spilled liquids shown in this article were performed from shorter distances to reduce waste of the sample compounds. Identification of liquid target compounds also works from greater distances, as the influence of distance between the measurement device and sample on the signal is small, and detection is mainly limited by the spot size. For a positive identification, an area viewed by at least some pixels needs to be entirely covered by the target compound — a minimum size that naturally increases with distance. At the experimental distance of several meters, this is provided by a spot of about one centimeter in diameter. Figure 4 shows the identification of a few µl of dimethyl methylphosphonate (DMMP, a simulant for the nerve gas agent sarin) from a distance of six meters. It shows that, on the entire spot, DMMP could be identified with no false alarms for other compounds. On the right, the measured and calculated spectra of a selected pixel are displayed. The same sample carries other target liquids as well, and ethyl methylphosphonate was applied to check for cross-correlations. Figure 5 shows how all target compounds could be identified and distinguished. As stated earlier, the identification routines use massive parallelization. This allows identification and mapping in real time. For the standard library, with more than 40 compounds and a spatial resolution of 128 x 128 pixels, the measurement and identification of gases are completed in less than two seconds. Figure 6 shows a series of consecutive ammonia- identification images, each with people walking past the release spot. Liquid detection is slower due to the need for forward simulation, but the analysis still can be performed in less than one second per reference database entry. Hyperspectral Imaging As An Early Warning Technique Protecting a facility from airborne chemicals means gathering information about the presence and composition of a gas cloud in a wide radius around the site. The common approach — using an array of detectors — presents several difficulties. While working with point sensors allows the use of highly sensitive techniques, a single detector can give information only about a local concentration. This setup requires a mesh of measurement devices placed around the area of interest in a way that guarantees detection of any gas cloud in any height above ground. Furthermore, information about a gas cloud's propagation and dimension requires an enormous number of measuring points. Besides, single detectors placed outside a guarded site are difficult to maintain and to calibrate, both of which are frequent necessities. Passive remote sensing via FTIR spectroscopy is a key technique for a variety of surveillance tasks. To identify compounds in both liquid and gaseous phase the measurement technique is ideal for an early warning system for CWAs. Working in the fingerprint region of the IR spectrum allows measurements at distances of up to several kilometers away from the target. The technique allows the detection of several hundred chemicals while remaining extraordinarily sensitive. For airborne chemicals and a typical cloud Electronic Military & Defense Annual Resource, 6th Edition 32 Figure 4: Identification of DMMP. Left: Identification image of approximately 20 µl of DMMP from a distance of about six meters. Top right: Comparison between the measurement (blue) and the simulated and fitted brightness temperature spectrum (orange). Lower right: Comparison between the reflection coefficient calculated from the fitted spectrum (black) and the reference (red). This comparison is one criterion for a positive identification. The other is signal strength as difference between minimum and maximum of the fitted simulated spectrum, compensated for atmospheric interferents. Figure 5: Left to right: Identification of DMMP, diisopropyl methylphosphonate (DIMP), and triethyl phosphate (TEP), all of which are simulants for CWAs. Note that there are no false identifications for each compound, even when other chemicals are present.

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