Eric R. Keim

First use of an airborne thermal infrared hyperspectral scanner for compositional mapping

In May 1999, the airborne thermal infrared hyperspectral imaging system, Spatially Enhanced Broadband Array Spectrograph System (SEBASS), was flown over Mormon Mesa, NV, to provide the first test of such a system for geological mapping. Several types of carbonate deposits were identified using the 11.25-μm band. However, massive calcrete outcrops exhibited weak spectral contrast, which was confirmed by field and laboratory measurements. Because the weathered calcrete surface appeared relatively smooth in hand specimen, this weak spectral contrast was unexpected. Here we show that microscopic roughness not readily apparent to the eye has introduced both a cavity effect and volume scattering to reduce spectral contrast. The macroroughness of crevices and cobbles may also have a significant cavity effect. The diminished spectral contrast is important because it places higher signal-to-noise ratio (SNR) requirements for spectroscopic detection and identification. This effect should be factored into instrumentation planning and interpretations, especially interpretations without benefit of ground truth. SEBASS had the required high SNR and spectral resolution to allow us to demonstrate for the first time the ability of an airborne hyperspectral thermal infrared scanner to detect and identify spectrally subtle materials.

Dyson spectrometers for high-performance infrared applications

The Dyson spectrometer form has the potential to deliver good imaging performance, high throughput, and low distortion in a compact configuration suitable for cryogenic infrared applications. The three main requirements for a practical implementation—availability of the required concave diffraction grating, availability of the Dyson lens material, and clearance for slit and focal plane packaging—are now within the state of the art, opening the Dyson form to serious consideration. Several high-performance Dyson designs for the long-wavelength infrared are presented.

MAKO: a high-performance, airborne imaging spectrometer for the long-wave infrared

We report progress on a high-performance, long-wavelength infrared hyperspectral imaging system for airborne research. Based on a f/1.25 Dyson spectrometer and 128x128 arsenic doped silicon blocked impurity band array, this system has significantly higher throughput than previous sensors. An agile pointing/scanning capability permits the additional signal to be allocated between increased signal-to-noise and broader area coverage, creating new opportunities to explore LWIR hyperspectral phenomenology.

MAGI: A New High-Performance Airborne Thermal-Infrared Imaging Spectrometer for Earth Science Applications

A new airborne facility instrument for Earth science applications is introduced. The Mineral and Gas Identifier (MAGI) is a wide-swath (programmable up to ±42° off nadir) moderate spectral resolution thermal-infrared (TIR) imaging spectrometer that spans the 7.1- to 12.7-μm spectral window in 32 uniform and contiguous channels. Its spectral resolution enables improved discrimination of rock and mineral types, greatly expanded gas-detection capability, and generally more accurate land-surface temperature retrievals. The instrument design arose from trade studies between spectral resolution, spectral range, and instrument sensitivity and has now been validated by flight data acquired with the completed sensor. It offers a potential prototype for future space-based TIR instruments, which will require much higher spectral resolution than is currently available in order to address more detailed climate, anthropogenic, and solid Earth science questions.

Mako airborne thermal infrared imaging spectrometer: performance update

The Aerospace Corporation’s sensitive Mako thermal infrared imaging spectrometer, which operates between 7.6 and 13.2 microns at a spectral sampling of 44 nm, and flies in a DeHavilland DHC-6 Twin Otter, has undergone significant changes over the past year that have greatly increased its performance. A comprehensive overhaul of its electronics has enabled frame rates up to 3255 Hz and noise reductions bringing it close to background-limited. A replacement diffraction grating whose peak efficiency was tuned to shorter wavelength, coupled with new AR coatings on certain key optics, has improved the performance at the short wavelength end by a factor of 3, resulting in better sensitivity for methane detection, for example. The faster frame rate has expanded the variety of different scan schemes that are possible, including multi-look scans in which even sizeable target areas can be scanned multiple times during a single overpass. Off-nadir scanning to ±56.4° degrees has also been demonstrated, providing an area scan rate of 33 km2/minute for a 2-meter ground sampling distance (GSD) at nadir. The sensor achieves a Noise Equivalent Spectral Radiance (NESR) of better than 0.6 microflicks (μf, 10-6 W/sr/cm2/μm) in each of the 128 spectral channels for a typical airborne dataset in which 4 frames are co-added. An additional improvement is the integration of a new commercial 3D stabilization mount which is significantly better at compensating for aircraft motions and thereby maintains scan performance under quite turbulent flying conditions. The new sensor performance and capabilities are illustrated.

Urban-industrial emissions monitoring with airborne longwave-infrared hyperspectral imaging

The advantages of airborne hyperspectral longwave-infrared imaging for emissions monitoring are described in the context of urban-industrial environments. These benefits are illustrated by means of several case studies.

Dyson spectrometers for infrared earth remote sensing

The Dyson spectrometer form is capable of providing high throughput, excellent image quality, low spatial and spectral distortions, and high tolerance to fabrication and alignment errors in a compact format with modest demands for weight, volume, and cooling resources. These characteristics make it attractive for hyperspectral imaging from a space-based platform. After a brief discussion of history and basic principles, we present two examples of Dyson spectrometers being developed for airborne applications. We conclude with a concept for an earth science instrument soon to begin development under the Instrument Incubator Program of NASAs Earth Science Technology Office.

Infrared remote sensing of Mars and the Mars astrobiology exploration strategy

The Mars exploration strategy calls first for the detection from orbit of minerals indicative of environments conducive to the support of life or the preservation of biomarkers. That information would then be used for astrobiology landing site selection. The near-term search will be conducted by the 1996 Global Surveyor Thermal Emission Spectrometer (TES) and the 2001 Mars Odyssey 9-band radiometer Thermal Emission Imaging System (THEMIS). This places the productivity of TES and THEMIS in the critical path of the Mars astrobiology strategy. Most predictions of mineral detection limits for TES and THEMIS are based on laboratory spectra of fresh mineral surfaces. However, standard laboratory measurements of fresh mineral surfaces generally do not reproduce all the spectral effects of weathering and surface roughness that are very apparent in field spectra, and these differences can critically affect interpretations of TES and THEMIS data. Here we examine causes of variations in spectral contrast, and differences in spectral signatures recorded in the field and in typical laboratory measurements, and show what the results indicate for the search for minerals and landing sites using TES and THEMIS. We conclude that for TES and THEMIS to attain their predicted mineral detection limits, minerals must be present under specific conditions: well-crystalline, smooth-surfaced at several scales, and low atmospheric downwelling radiance contribution. As a result, TES and THEMIS should not necessarily be used to exclude landing sites that are of interest for other reasons (e.g. geomorphology), but that exhibit no clear detections of minerals of interest to astrobiologists.

High areal rate longwave-infrared hyperspectral imaging for environmental remote sensing

The Mako airborne longwave-infrared hyperspectral sensor is a whiskbroom imager operating in the 7.6-13.2 μm region with 44-nm spectral sampling and <30 mK noise-equivalent differential temperature (NEDT). It has undergone progressive development since its inaugural flights in 2010 and is capable of acquiring 112° swaths with an areal rate of 33 km2 min-1 at 2-m ground sampling distance. The sensor performance envelope allows for a number of operational modes that can be deployed against a variety of acquisition scenarios. Its suitability for environmental remote sensing applications is illustrated with reference to a number of representative case studies drawn from several years of airborne collections within the Los Angeles Basin and beyond.