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Spectral imagery data (2.0 to 5.4 micrometer) was collected of plumes of ships by the NATO Special Working Group 4. It provides the means to study the signature of a target spectrally, spatially, and temporally. This experimental data has been used to validate the infrared signature of the plume of a ship as computed by NATO's flow-field program NPLUME v1.6 and the NATO Infra-Red Air Target Model NIRATAM v3.1. Two spatial positions in the spectral imagery data cube were selected. One which represents the background spectrum, and one which represents the spectrum of the plume of the ship. Theoretical spectra were computed by means of NPLUME v1.6 and NIRATAM v3.1. A computed background spectrum was fitted to the experimental background spectrum using a user-defined atmosphere in accordance with the meteorological conditions during the trial. A computed plume spectrum was fitted to the observed plume spectrum in order to determine the chemical composition of the exhaust gas. Since NIRATAM only takes into account plume radiation from CO, CO2, H2O, and soot, the analysis is necessarily limited to these species. Using the derived fitting parameters from the experimental data we make predictions about the infrared signature of the plume in two wavelength bands (mid-wave infrared and the long-wave infrared). The average transmission through the plume in the mid-wave infrared (3.0 to 5.0 micrometer) ranges from 65% close to the exit plane, to 100% where the plume dissolves in the ambient atmosphere. For the long-wave infrared (8.0 to 10.0 micrometer) the range in transmission is 90% to 100%. The active species in the mid-wave infrared and the long-wave infrared are the same for the plume as for the intervening atmosphere. The main difference is that the absorption features are deeper and wider for the plume. Based on this work we arrive at the conclusion that spectral imagery data of the plume of a ship can be adequately modeled using NIRATAM v3.1 in conjunction with NPLUME v1.6. Alternatively, the experimental data validates NIRATAM v3.1 and NPLUME v1.6. Some modifications to the NIRATAM source code have been proposed as a result of this study. A new release of NIRATAM and NPLUME which incorporates some of these changes is expected shortly (NIRATAM v3.2).
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