Michael R. Hawks

Passive ranging of dynamic rocket plumes using infrared and visible oxygen attenuation

Atmospheric oxygen absorption bands in observed spectra of boost phase missiles can be used to accurately estimate range from sensor to target. One method is to compare observed values of band averaged absorption to radiative transfer models. This is most effective using bands where there is a single absorbing species. This work compares spectral attenuation of two oxygen absorption bands in the near-infrared (NIR) and visible (Vis) spectrum, centered at 762 nm and 690 nm, to passively determine range. Spectra were observed from a static test of a full-scale solid rocket motor at a 900m range. The NIR O2 band provided range estimates accurate to within 3%, while the Vis O2 band had a range error of 15%. A Falcon 9 rocket launch at an initial range of 13km was also tracked and observed for 90 seconds after ignition. The NIR O2 band provided in-flight range estimates accurate to within 2% error for the first 30 seconds of tracked observation. The Vis O2 band also provided accurate range estimates with an error of approximately 4%. Rocket plumes are expected to be significantly brighter at longer wavelengths, but absorption in the NIR band is nearly ten times stronger than the Vis band, causing saturation at shorter path lengths. An atmospheric band is considered saturated when all the in-band frequencies emitted from the rocket plume are absorbed before reaching the sensor.

Flight test of an imaging O2(X-b) monocular passive ranging instrument

An instrument for monocular passive ranging based on atmospheric oxygen absorption near 762 nm has been designed, built and deployed to track emissive targets. An intensified CCD array is coupled to variable band pass liquid crystal filter and 3.5 - 8.8 degree field of view optics. The system was first deployed for a ground test viewing a static jet engine in afterburner at ranges of 0.35 - 4.8 km, establishing a range error of 15%. The instrument was also flight tested in a C-12 imaging an the exhaust plume of another aircraft afterburner at ranges up to 11 km.

Passive ranging of boost-phase missiles

The depth of absorption bands in observed spectra of distant, bright sources can be used to estimate range to the source. Previous efforts in this area relied on Beers Law to estimate range from observations of infrared CO2 bands, with disappointing results. A modified approach is presented that uses band models and observations of the O2 absorption band near 762 nm. This band is spectrally isolated from other atmospheric bands, which enables direct estimation of molecular absorption from observed intensity. Range is estimated by comparing observed values of band-average absorption, (see manuscript), against predicted curves derived from either historical data or model predictions. Accuracy of better than 0.5% has been verified in short-range (up to 3km) experiments using a Fourier transform interferometer at 1cm-1 resolution. A conceptual design is described for a small, affordable passive ranging sensor suitable for use on tactical aircraft for missile attack warning and time-to-impact estimation. Models are used to extrapolate experimental results (using 1 cm-1 resolution data) to analyze expected performance of this filter-based system.

Passive ranging using mid-wavelength infrared atmospheric attenuation

Methods of estimating range to an emissive target based on the depth of an atmospheric absorption band are presented. The present work uses measurements of the CO2 absorption band centered at 2.0 μm where signal-to-background ratios are maximum for many applications. Observed spectra are compared to model spectra to estimate range. Spectral regions with minimal attenuation are used to estimate source parameters in order to isolate atmospheric transmission. The spectra of 21 high explosive events were used to test this technique. A simple technique treating the fireball as a blackbody consistently underestimated true range by approximately 13%. A more realistic source model using some order-of-magnitude assumptions of fireball composition reduces range error to 3%. The technique produces accurate results without requiring detailed knowledge of source parameters or atmospheric conditions.

Passive ranging of emissive targets using atmospheric oxygen absorption lines

Highly-energetic targets such as rocket plumes and detonation fireballs are difficult to locate and track effectively with active sensors that rely on reflection, but traditional passive sensors cannot determine range. Development of a passive ranging sensor will enable accurate target location and tracking while simultaneously improving covertness. Recent work is presented on development of a passive sensor to estimate range using spectroscopic measurements of atmospheric absorption. In particular, advantages of measuring absorption on the O 2(bX) transition near 762nm are discussed. Theoretical predictions are compared with experimental results to verify model performance at short ranges (up to 200m). Range accuracy better than 1% has been demonstrated using a Fourier transform spectrometer at short range. Extension of the theory to long range (in excess of 100km) is also discussed.

Nondestructive evaluation of aircraft coatings with infrared diffuse reflectance spectra

Aircraft coatings degrade over time, but aging can be difficult to detect before failure and delamination. We present a method to evaluate aircraft coatings in situ using infrared diffuse reflectance spectra. This method can detect and classify coating degradation much earlier than visual inspection. The method has been tested on two different types of coatings that were artificially aged in an autoclave. Spectra were measured using a hand-held diffuse reflectance infrared Fourier transform spectrometer (DRIFTS). One set of 72 samples can be classified as either aged or unaged with 100% accuracy. A second sample set contained samples that had been artificially aged for 0, 24, 48 or 96 hours. Several classification methods are compared, with accuracy better than 98% possible.

Short-Range Demonstrations of Monocular Passive Ranging Using O2 (X3Σg− → b1Σ+g) Absorption Spectra:

The depth of absorption bands in observed spectra of distant, bright sources can be used to estimate range to the source. Experimental results are presented based on observations of the O2 X(v″ = 0) → b(v′ = 0) absorption band centered around 762 nm and the O2 X(v″ = 0) → b(v′ = 1) band around 689 nm. Range is estimated by comparing observed values of band-average absorption against predicted curves derived from either historical data or model predictions. Accuracy of better than 0.5% was verified in short-range (up to 3 km), static experiments using a high-resolution (1 cm−1) spectroradiometer. This method was also tested against the exhaust plume of a Falcon 9 rocket launched from Cape Canaveral, Florida. The rocket was launched from an initial range of 13 km and tracked for 90 s after ignition. Range error was below 2% for the first 30 s and consistent with predicted error throughout the track.

Analytic treatment of high power diode pumped lasers with unstable resonator in a flowing medium

Diode-pumped alkali metal vapor lasers (DPAL) offer significant promise for high average power. The DPAL system has high gain and will high output coupling and an unstable resonator to achieve excellent beam quality. We analyze the Rb-He system using average equations for the pump, laser and populations, including amplified spontaneous emission. We extend the formulation to include flow and temperature release and study its effects on the laser efficiency and beam quality. The design and analysis of the DPAL resonator and the influence of spatial variations in gain medium on far field beam quality are developed. A systematic study of the influence of gain medium aberrations, flow geometry, and resonator design on far field beam quality is reported. The relative advantages of longitudinal and transverse flow geometries to beam quality are evaluated. Finally, coupling of the pump and laser radiation fields is dramatic in the DPAL system. The standard approaches to merging CFD analysis of the gain medium with wave optics resonator simulations will require new techniques.

Fresnel zone plate light field spectral imaging simulation

Through numerical simulation, we have demonstrated a novel snapshot spectral imaging concept using binary diffractive optics. Binary diffractive optics, such as Fresnel zone plates (FZP) or photon sieves, can be used as the single optical element in a spectral imager that conducts both imaging and dispersion. In previous demonstrations of spectral imaging with diffractive optics, the detector array was physically translated along the optic axis to measure different image formation planes. In this new concept the wavelength-dependent images are constructed synthetically, by using integral photography concepts commonly applied to light field (plenoptic) cameras. Light field cameras use computational digital refocusing methods after exposure to make images at different object distances. Our concept refocuses to make images at different wavelengths instead of different object distances. The simulations in this study demonstrate this concept for an imager designed with a FZP. Monochromatic light from planar sources is propagated through the system to a measurement plane using wave optics in the Fresnel approximation. Simple images, placed at optical infinity, are illuminated by monochromatic sources and then digitally refocused to show different spectral bins. We show the formation of distinct images from different objects, illuminated by monochromatic sources in the VIS/NIR spectrum. Additionally, this concept could easily be applied to imaging in the MWIR and LWIR ranges. In conclusion, this new type of imager offers a rugged and simple optical design for snapshot spectral imaging and warrants further development.

Unstable resonators for high power diode pumped alkali lasers

The high gain Diode Pumped Alkali Laser (DPAL) system will require an unstable resonator with high Fresnel number and high output coupling to achieve excellent beam quality. Coupling of the diode pump and laser radiation fields is dramatic in the DPAL system. Merging flow field analysis of the gain medium with wave optics resonator simulations requires new techniques. We develop a wave-optics simulation of confocal, positive-branch unstable resonators for the DPAL gain media to assess the limitations on far field beam quality. The design and analysis of the DPAL resonator and the influence of spatial variations in gain medium on far field beam quality are developed. The relative advantages of longitudinal and transverse flow geometries to beam quality are evaluated. A systematic study of the influence of gain medium aberrations, flow geometry, magnification, and resonator design on far field beam quality is reported.