Bryan J. Steward

Visible and Near-Infrared Spectra of the Secondary Combustion of a 152 mm Howitzer

Visible and near-infrared spectra were collected from 53 firings of three different propellants from a 152 mm howitzer. The observable muzzle flashes had 50–100 ms durations and were collected with approximately 0.75 nm spectral and 10 Hz temporal resolutions. Atomic and molecular emissions were primarily limited to contaminant species including K, Na, Li, CaOH, CuOH, and CuO. A relatively strong continuum baseline was observed from hot particulate matter and plume-scattered solar radiation. A radiative transfer model was used to demonstrate self-absorption in an optically thick plume as the likely source of significant broadening of the potassium 4 2P3/2,1/2 – 4 2S1/2 doublet. Indeed, the entire O2 (X-b) absorption band is evident in the blue wing. This feature is used to estimate range to source to within 4–9% for individual firings and 0.5% with multiple observations. Ratios of atomic line intensities were used to differentiate munitions configurations, yielding ratios of between-class variance to within-class variance of 8.6 to 18.2 using 1 to 4 atomic lines. Further reduction of the number of features by characterizing the relative atomic line intensities by a temperature parameter (T = 7900–8900 K) significantly degrades class discrimination.

Simulating systematic scene-change artifacts in Fourier-transform spectroscopy

Improved understanding of midwave infrared (1-5μm) spectral emissions from detonation fireballs is needed to develop a battle space optical forensics capability. While Fourier-transform spectrometers (FTS) are an attractive tool, interferometer-based spectroscopic measurements can be corrupted when the observed scene intensity systematically varies during the measurement time. Approximating a detonation fireball as a blackbody radiator with a time-varying temperature T and modified by atmospheric attenuation τ(~ν), double-sided interferograms from an ideal FTS were calculated and converted to measured spectra Lm(~ν) to characterize the nature and magnitude of scene-change artifacts. T(x) decreased exponentially with optical path difference x, -xm ≤ x ≤ xm, at various rates relative to the Michelson mirror speed so that changing scene spectra could be simulated on 1700 ≤ ~ν ≤ 7900cm-1 at δ ~ν = 3.64cm-1 resolution (xm = 0.25cm, Hamming apodization). The real part of Lm(~ν), Re{Lm(~ν)}, is well approximated by the instantaneous spectrum at zero path difference, L(~ν,x = 0). In regions where τ(~ν) is highly structured, both the imaginary component Im{Lm(~ν)} and the differences between Re{Lm(~ν)} and L(~ν,0) exhibit spectral features, and in general |Im{Lm(~ν)}|>>|Re{Lm(~ν)}-L(~ν,0)|. In a region of highly structured absorption, 2800 ≤~ν ≤ 3500cm-1, a 600K decrease in temperature produced RMS values of 62 and 5μW/(cm2 • sr •cm-1) in Im{Lm(~ν)} and Re{Lm( ~ν)-L(~ν,0)}, respectively, compared with an RMS value of 1924μW/ (cm2 • sr • cm-1) in Re{Lm(~ν)}. A method based on theoretical expressions developed by Kick et al. is devised to interpret Lm(~ν) and provide estimates of the temporal evolution T(x) when its functional formis not known a priori.

Reduction of optically observed artillery blast wave trajectories using low dimensionality models

Muzzle blast trajectories from firings of a 152 mm caliber gun howitzer were obtained with high-speed optical imagers and used to assess the fidelity with which low dimensionality models can be used for data reduction. Characteristic flow regions were defined for the blast waves. The near-field region was estimated to extend to 0.98 - 1.25 meters from the muzzle and the far-field region was estimated to begin at 2.61 - 3.31 meters. Blast wave geometries and radial trajectories were collected in the near through far-fields with visible imagers operating at 1,600 Hz. Beyond the near-field the blast waves exhibited a near-spherical geometry in which the major axis of the blast lay along the axis of the gun barrel and measured within 95% of the minor axis. Several blast wave propagation models were applied to the mid and far-field data to determine their ability to reduce the blast wave trajectories to fewer parameters while retaining the ability to distinguish amongst three munitions configurations. A total of 147 firings were observed and used to assess within-configuration variability relative to separation between configurations. Results show that all models perform well, and drag and point blast model parameters additionally provide insight into phenomenology of the blast.

Characterization and discrimination of large caliber gun blast and flash signatures

Two hundred and one firings of three 152 mm howitzer munitions were observed to characterize firing signatures of a large caliber gun. Muzzle blast expansion was observed with high-speed (1600 Hz) optical imagery. The trajectory of the blast front was well approximated by a modified point-blast model described by constant rate of energy deposition. Visible and near-infrared (450 - 850 nm) spectra of secondary combustion were acquired at ~0.75 nm spectral resolution and depict strong contaminant emissions including Li, Na, K, Cu, and Ca. The O2 (X→b) absorption band is evident in the blue wing of the potassium D lines and was used for monocular passive ranging accurate to within 4 - 9%. Timeresolved midwave infrared (1800 - 6000 cm-1) spectra were collected at 100 Hz and 32 cm-1 resolution. A low dimensional radiative transfer model was used to characterize plume emissions in terms of area, temperature, soot emissivity, and species concentrations. Combustion emissions have ~100 ms duration, 1200 - 1600 K temperature, and are dominated by H2O and CO2. Non-combusting plume emissions last ~20 ms, are 850 - 1050 K, and show significant continuum (emissivity ~0.36) and CO structure. Munitions were discriminated with 92 - 96% classification accuracy using only 1 - 3 firing signature features.

Development and validation of the AFIT scene and sensor emulator for testing (ASSET)

ASSET is a physics-based model used to generate synthetic data sets of wide field of view (WFOV) electro-optical and infrared (EO/IR) sensors with realistic radiometric properties, noise characteristics, and sensor artifacts. It was developed to meet the need for applications where precise knowledge of the underlying truth is required but is impractical to obtain for real sensors. For example, due to accelerating advances in imaging technology, the volume of data available from WFOV EO/IR sensors has drastically increased over the past several decades, and as a result, there is a need for fast, robust, automatic detection and tracking algorithms. Evaluation of these algorithms is difficult for objects that traverse a wide area (100-10,000 km) because obtaining accurate truth for the full object trajectory often requires costly instrumentation. Additionally, tracking and detection algorithms perform differently depending on factors such as the object kinematics, environment, and sensor configuration. A variety of truth data sets spanning these parameters are needed for thorough testing, which is often cost prohibitive. The use of synthetic data sets for algorithm development allows for full control of scene parameters with full knowledge of truth. However, in order for analysis using synthetic data to be meaningful, the data must be truly representative of real sensor collections. ASSET aims to provide a means of generating such representative data sets for WFOV sensors operating in the visible through thermal infrared. The work reported here describes the ASSET model, as well as provides validation results from comparisons to laboratory imagers and satellite data (e.g. Landsat-8).

Remote discrimination of large-caliber gun firing signatures

Abstract. To investigate discrimination of firing signatures in the battlespace, blast, and flash signatures were collected from two large-caliber guns. Signatures included blast wave trajectories from high-speed (1600 Hz) imagery, visible and near-infrared (450 to 850 nm) spectra, and mid-wave infrared (1800 to 6000     cm − 1 ) spectra. Thirty empirical and 12 phenomenological features were extracted from the signatures over 25 observations. Multiple discriminant analysis was used for feature selection and dimensionality reduction, and discrimination was based on Bayesian probability theory. When discriminating between three munitions configurations fired from a 152 mm howitzer, leave-one-out testing with the empirical features demonstrated classification accuracies of 92% to 96%. Phenomenological features (for example, blast energy, combustion and excitation temperatures, and species concentrations) accurately classified munitions less consistently (72% to 96%). The two most salient features for differentiating between two weapons (a single 120 mm tank firing and all 152 mm howitzer firings) were soot emissivity and H 2 O concentration. Empirical features consistently discriminated the weapons best when more than two features were used. Moderate to strong correlations ( r 2 = 0.5 to 1.0) were observed amongst blast, visible, and infrared features. A limited examination indicated that strongly correlated features may be substituted for one another.