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Designing an imaging spectrometer using and AOTF can be a difficult task since there is no software that can simulate the bulk diffraction that takes place in the AOTF material.
The Moon Mineralogy Mapper's (M3) is a high uniformity and high signal-to-noise ratio NASA imaging spectrometer that is a guest instrument on the Indian Chandrayaan-1 Mission to the Moon. The laboratory measured spectral, radiometric, spatial, and uniformity characteristics of the M3 instrument are given. The M3 imaging spectrometer takes advantage of a suite of critical enabling capabilities to achieve its measurement requirement with a mass of 8 kg, power usage of 15 W, and volume of 25X18X12 cm. The M3 detector and spectrometer are cooled by a multi-stage passive cooler. This paper presents early M3 performance assessment results.
The Moderate-Resolution Imaging Spectrometer (MODIS), as presently conceived, is a system of two imaging spectroradiometer components designed for the widest possible applicability to research tasks that require long-term (5 to 10 years), low-resolution (52 channels between 0.4 and 12.0 micrometers) data sets. The system described is preliminary and subject to scientific and technological review and modification, and it is anticipated that both will occur prior to selection of a final system configuration; however, the basic concept outlined is likely to remain unchanged.
A spectrometric imager and a thermal imager, which achieve multiplexing by the use of binary optical encoding masks, were developed. The masks are based on orthogonal, pseudorandom digital codes derived from Hadamard matrices. Spatial and/or spectral data is obtained in the form of a Hadamard transform of the spatial and/or spectral scene; computer algorithms are then used to decode the data and reconstruct images of the original scene. The hardware, algorithms and processing/display facility are described. A number of spatial and spatial/spectral images are presented. The achievement of a signal-to-noise improvement due to the signal multiplexing was also demonstrated. An analysis of the results indicates both the situations for which the multiplex advantage may be gained, and the limitations of the technique. A number of potential applications of the spectrometric imager are discussed.
One design for a geostationary Total Ozone Mapping Spectrometer (TOMS) with many desirable features is an imaging spectrometer. A preliminary study makes use of a 0.25 m Czerny-Turner spectrometer with which the Earth is imaged on a charge-coupled device (CCD) in dispersed light. The wavelength is determined by a movable grating which can be set arbitrarily by ground control. The signal integration time depends on wavelength but this system allows arbitrary timing by command. Special circumstances such as a requirement to track a low-lying sulfur dioxide cloud or a need to discriminate high level ozone from total ozone at midlatitudes could be obtained by adding a particular wavelength to the normally pre-programmed time sequence. The incident solar irradiance is measured by deploying a diffuser plate in the field of view. Individual detector elements correspond to scene elements in which the several wavelengths are serially sampled and the Earth radiance is compared to the incident sunlight. Thus the problem of uncorrelated drift of multiple detectors is removed.
HIRIS, a facility instrument on the Earth Observing System Platform, will enable scientists to both identify and interpret changes, in the global environment, at a level of detail unavailable with previous instrumentation. This instrument will make localized measurements of biological, ecological, climatological, hydrological, and geological properties, and, with its finer wavelength sampling and full spectral coverage over the wavelength region of interest, will allow these properties to be identified and studied quantitatively from space. The data obtained will be downlinked from space, processed, catalogued, stored in active archives for easy access, and distributed to users.
Radiance data were acquired over part of the Clark Mountain Range, California in 1990 by the Airborne Visible Infrared Imaging Spectrometer (AVIRIS). From these calibrated radiance data, the lateral distribution of water vapor in precipitable mm was recovered through analysis of the 940 nm water absorption. Surface pressure height was calculated from the 760 nm oxygen absorption band in the AVIRIS data. Surface slopes are estimated from relative elevations derived from the equivalent water vapor elevation for a standard atmosphere. Atmospheric optical depths measured at the Mountain Pass site in these data are extended to the entire region and varied based on the calculated surface pressure elevation. These parameters, derived from every AVIRIS spatial element, are used to constrain the LOWTRAN 7 radiative transfer code for the recovery of surface reflectance. Once transformed to reflectance, the imaging spectrometer data may be analyzed quantitatively for determination of surface composition, and compared with reflectance measurements acquired from different locations, times, and instruments.
A proposed visible-light imaging spectrometer on a chip would be based on the concept of a heterostructure comprising multiple layers of silicon-based photodetectors interspersed with long-wavelength-pass optical filters. In a typical application, this heterostructure would be replicated in each pixel of an image-detecting integrated circuit of the active-pixel-sensor type (see figure). The design of the heterostructure would exploit the fact that within the visible portion of the spectrum, the characteristic depth of penetration of photons increases with wavelength. Proceeding from the front toward the back, each successive long-wavelength-pass filter would have a longer cutoff wavelength, and each successive photodetector would be made thicker to enable it to absorb a greater proportion of incident longer-wavelength photons. Incident light would pass through the first photodetector and encounter the first filter, which would reflect light having wavelengths shorter than its cutoff wavelength and pass light of longer wavelengths. A large portion of the incident and reflected shorter-wavelength light would be absorbed in the first photodetector. The light that had passed through the first photodetector/filter pair of layers would pass through the second photodetector and encounter the second filter, which would reflect light having wavelengths shorter than its cutoff wavelength while passing light of longer wavelengths. Thus, most of the light reflected by the second filter would lie in the wavelength band between the cutoff wavelengths of the first and second filters. Thus, further, most of the light absorbed in the second photodetector would lie in this wavelength band. In a similar manner, each successive photodetector would detect, predominantly, light in a successively longer wavelength band bounded by the shorter cutoff wavelength of the preceding filter and the longer cutoff wavelength of the following filter.
Two important sensor design drivers are the requirement for spatial registration of the spectral components and the implementation of the advanced multispectral capability, including spectral band width, number of bands and programmability. The dispersive approach, fundamental to the imaging spectrometer concept, achieves these capabilities by utilizing a spectrometer to disperse the spectral content while preserving the spatial identity of the information in the cross-track direction. Area array detectors in the spectrometer focal plane detect and store the spatial and multispectral content for each line of the image. The choice of spectral bands, image IFOV and swath width is implemented by programmed readout of the focal plane. These choices in conjunction with data compression are used to match the output data rate with the telemetry link capability. Progress in the key technologies of optics, focal plane detector arrays, onboard processing, and focal plane cooling supports the viability of the imaging spectrometer approach.
The Airborne Visible/Infrared Imaging Spectrometer (AVIRIS), flown aboard the NASA ER-2 aircraft in 1987 and 1989, used four linear arrays and four individual spectrometers to collect data simultaneously from the 224 bands in a scanned 614 pixel-wide swath perpendicular to the aircraft direction. The research had two goals. One was to evaluate the AVIRIS data. The other was to look at the subtle lithological variation at the two test sites to develop a better understanding of the regional geology and surficial processes. The geometric characteristics of the data, adequacy of the spatial resolution, and adequacy of the spectral sampling interval are evaluated. Geologic differences at the test sites were mapped. They included lithological variation caused by primary sedimentary layering, facies variation, and weathering; and subtle mineralogical differences caused by hydrothermal alterations of igneous and sedimentary rocks. The investigation used laboratory, field, and aircraft spectral measurements; known properties of geological materials; digital image processing and spectrum processing techniques; and field geologic data to evaluate the selected characteristics of the AVIRIS data.
High resolution achieved with wide field of view. Imaging Spectrometer features off-axis reflecting optics, including reflecting "slit" that also serves as field flattener. Only refracting element is prism. By scanning slit across object or scene and timing out signal, both spectral and spatial information in scene are obtained.
The Airborne Imaging Spectrometer (AIS) is a high spectral resolution (9.6-nm-wide bands between 0.9 and 2.4 microns) instrument. Analysis of AIS data revealed significant differences in characteristics of the spectral radiance curves of four types of wetland vegetation canopies (trees, broadleaf herbaceous, Spartina alterniflora, and S. patens/Distichlis spicata) in Delaware, enabling them to be distinguished. The single most useful spectral region was that between 1.40 and 1.90 microns. Differences in radiance values at various wavelengths between samples of the same vegetation type could potentially be used to estimate biomass. Thus, high spectral resolution spectrometry appears to have significant value for remote sensing studies of wetland vegetation.
Information on an expert system for imaging spectrometer analysis results is outlined. Implementation requirements, the Simple Tool for Automated Reasoning (STAR) program that provides a software environment for the development and operation of rule-based expert systems, STAR data structures, and rule-based identification of surface materials are among the topics outlined.
The data-processing problems associated with imaging spectrometer data are reviewed; new algorithms and optical processing solutions are advanced for this computationally intensive application. Optical decision net, directed graph, and neural net solutions are considered. Decision nets and mineral element determination of nonmixture data are emphasized here. A new Fisher/minimum-variance clustering algorithm is advanced, initialization using minimum-variance clustering is found to be preferred and fast. Tests on a 500-class problem show the excellent performance of this algorithm.
Science requirements for a moderate resolution imaging spectrometer (MODIS) data system are outlined, as well as the operations concept and system specifications based on these requirements. Two systems are considered - MODIS-N for a 40-channel nadir-viewing scanner and MODIS-T for a 64-channel tiltable scanner. The first system collects data from 15 thermal-infrared channels at all times and from 25 reflected-energy channels during daytime, while MODIS-T will take data from 64 channels on a 100-percent duty cycle during daytime only. Standard data products, data acquisition and processing, and requirements on data are discussed. Emphasis is placed on the interfaces between the science data processing facilities and the scientific users.
The Moderate Resolution Imaging Spectrometer (MODIS) is a facility instrument to be flown on the first Earth Observing System (scheduled launch in the late 1990s). The MODIS has two components. One component is a 110-deg-scan-angle instrument called MODIS-N (nadir). This instrument has 40 selected bands supporting observations of the land surface, the oceans, and atmosphere in the visible, NIR, short-wave IR (1.0-3.0 microns), and thermal IR (3.0-15.0 microns). The other component is a 90-deg-scan-angle scanning instrument that can tilt fore and aft along the satellite track, called MODIS-T (tilt). Both MODIS-N and MODIS-T are nearing the end of detailed design studies. The driving scientific requirements include absolute calibration accuracy 2 percent, instrument-induced polarization less than 2 percent, SNR reaching 800:1 for observing ocean color at large solar zenith angles, and dynamic range allowing observations of cloud characteristics and snow-covered areas.
An upgraded version of AVIRIS, an airborne imaging spectrometer based on a whiskbroom-type scanner coupled via optical fibers to four dispersive spectrometers, that has been in operation since 1987 is described. Emphasis is placed on specific AVIRIS subsystems including foreoptics, fiber optics, and an in-flight reference source; spectrometers and detector dewars; a scan drive mechanism; a signal chain; digital electronics; a tape recorder; calibration systems; and ground support requirements.
Recent laboratory calibrations of the Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) include new methods for the characterization of the geometric, spectral, temporal and radiometric properties of the sensor. New techniques are desired in order to: (1) increase measurement accuracy and precision, (2) minimize measurement time and expense, (3) prototype new field and inflight calibration systems, (4) resolve measurement ambiguities, and (5) add new measurement dimensions. One of the common features of these new methods is the use of the full data collection and processing power of the AVIRIS instrument and data facility. This allows the collection of large amounts of calibration data in a short period of time and is well suited to modular data analysis routines.