Airglow due to upper atmospheric molecular emissions has been known since the late 1960's. Recent interest in
extending high altitude low light imaging capability has created an interest in exploiting the airglow phenomenon. We
discuss the feasibility of developing a sensor that operates above the atmospheric layer responsible for airglow. We
examine the implications to the design of the focal plane array and electronics caused by viewing the ground through
the radiant layer.
This paper discusses recently developed algorithms for the classification of pixels in hyperspectral images, used in conjunction with a library of hyperspectral hemispherical reflectance data measured in the laboratory and partitioned into usable classes of materials. The algorithms are based upon functions of the principal components of the class covariances and the corresponding null spaces, and the underlying measures used in the classification statistics are similar to Mahalanobis distances. The algorithms can be used as stand-alone processing or combined with spatial and temporal algorithms n a higher level system of hyperspectral image processing. The nature of the classification algorithms and the database will be discussed, with particular attention being paid to issues specific to this approach. The basic performance of the classifier algorithms will be demonstrated using modified laboratory data. The applicability of orthogonal subspace projection methods to problems inherent in remote sensing using hyperspectral invisible and IR data will be emphasized, while specifically dealing with the compensation for inaccuracies in necessary estimates of atmospheric attenuation and target temperature. Preliminary results of classification of field collected hyperspectral data will also be presented, and ongoing and future work in hyperspectral classification described.
Robert Ryan, Richard Madonna, Theodore Hilgeman, John Hulsmann, William Van Nostrand, Lawrence Lesyna, Nils Fonneland, J. Michael, Lewis Smith, S. Mielke
This paper describes the theory of design, operation, and testing of a tunable MWIR Fabry-Perot interferometer operating in low orders. This device is called the agile bandpass tunable filter (ABTF) due to the fact that the spectral bandwidth can be changed by a large factor by changing the order. In first order the system can be tuned over the entire 3.5-5 micrometers spectral region with only a single order sorting filter. We provide a short introduction to tunable filters an then briefly discuss the requirements that low order operation places on the Fabry-Perot dielectric mirrors. Operation in low orders forces one to abandon the classical Fabry-Perot approximation that the mirrors are negligibly thin compared to the plate separation. Rather, one must now account for the phase properties of the dielectric stack mirrors as they produce phase effects comparable to the plate separation. We next address the issue of control of the Fabry-Perot. This is accomplished through a closed-loop system using capacitive sensor on the Fabry-Perot flats to measure the separation of the plates. Additionally we describe how the ABTF is characterized using a FTIR to measure the bandpass shape and position, and we show some examples of measurements made with the ABTF used as a hyperspectral imaging system with a 256 X 256 HgCdTe camera. We conclude with a discussion of potential applications and future work.
The agile bandpass tunable filter (ABTF) is a new instrument with variable resolution and center wavelength that allows MWIR and LWIR infrared cameras to recognize spectral images that are characteristic of specific materials, including gases. Benefits of this new technology include the ability to: (1) Task infrared camera-based sensors to search for specific materials, and (2) Transform infrared camera data into signatures easily understood by untrained users. The application of a working prototype hyperspectral ABTF is described. The prototype is capable of rapidly selecting bandpasses from among several spectral resolutions, and is easily tunable over the MWIR spectral band. The application of the ABTF to signature imaging of gases is shown.
Guillermo Loubriel, Fred Zutavern, Gary Denison, Wesley Helgeson, Dan McLaughlin, Marty O'Malley, C. Sifford, L. Beavis, Carl Seager, Arye Rosen, Richard Madonna
We present the results of experiments aimed at improving the lifetime (longevity) of Si photoconductive semiconductor switches (PCSS). Because damage at the metal-semiconductor interface is the primary damage mechanism in most PCSS, we have tested different contact metallizations. The test setup utilizes: a Nd:YAG laser that operates at 540 Hz with 50 mJ, 10 ns FWHM pulses; a circuit that charges a 50 (Omega) line in 800 ns and discharges it in 20 ns through a 50 (Omega) load; and a lateral switch geometry and 0.25 cm by 0.25 cm switches. The contacts examined include: Cr(diffused)-Cr-Mo-Au, Al(diffused)-Cr-Mo-Au, 31P(ion implanted)-Ti-Pt, Al(diffused)-Pt-Ti-Pd-Au, and edge contacts. In the case of the Cr contacts we have tried thicker Mo or Au layers. For the Al contacts we have tried 1 micrometers and 0.1 micrometers thick depositions. Most contacts survived 107 pulses when switching 32 kV/cm (8 kV over 0.25 cm). The Al diffused went up to 44 kV/cm (1 X 105 pulses). The implanted P switch was switched 2.2 X 107 times at 44 kV/cm and 0.9 X 106 times at 48 kV/cm.
Diffraction plays an important role in determining the RCS of a body. At some frequencies, typically low frequencies, diffraction can dominate the scattering process. This work examines the effects of diffraction on ultrawide band (UWB) radar pulses. We measured the RCS of three different size square aluminum plates using a UWB pulse and broadband sampling oscilloscope. We used a bistatic measurement configuration with a bistatic angle of 10.58 deg. The plates were rotated from -90 deg to +90 deg in 2 deg increments (angles relative to the center line of the bistatic configuration). Diffraction patterns for each plate are plotted and the time domain wave forms are analyze. We find that the smallest plate has a significant amount of diffracted energy even in its specular reflection lobe. The largest plate is relatively free from diffraction. Travelling waves and late time resonances are observed for the plates.
This paper describes the performance of a broadband (0.3-10 GHz) cavity backed TEM horn with an exponential flare in two planes. The antenna pattern was measured using a pulsed source and found to have a 3 dB width of 50 deg. The two-way transfer function of a pair of these horns was determined using pulsed measurements. The two way gain of the horns was found to be 6 dB. Diffraction of the low frequency components of the radiated pulse was observed and the effects on waveform shape noted. In particular, we have observed that the back and side lobes of this antenna are dominated by the low frequency components, and the corresponding temporal waveform has a different shape than the waveform found in the main lobe.
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