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 nun 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 P-2(3/2,1/2) - 4 S-2(1/2) doublet. Indeed, the entire O-2 (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.