An overview of the ASAS data acquired in support of the 1989 field experiments is presented and data quality is discussed. The precision of the ASAS data is considered through the presentation of SNRs derived from both field and laboratory data. ASAS is an airborne, off-nadir pointing, imaging spectroradiometer that acquires digital image data for 29 visible and near-infrared spectral bands (465 to 871 nm) with a spectral resolution of 15 nm. Surfaces observed for the field experiments include volcanic surfaces and a playa within a sparsely vegetated semiarid ecosystem, grass canopies within a prairie ecosystem, and tree canopies within a northern forest ecosystem. It is shown that calibrated ASAS data are sufficiently exact for investigations of the directional distribution of radiation scattered from terrestrial surfaces.

The modifications made to both the AVIRIS instrument and the data processing facility since the instrument made its initial flight in the summer of 1987 are described. Historical development of the data system is discussed and attention is given to enhancements to instrument stability and noise performance. AVIRIS data facility objectives include rapid and automated decommutation and archiving of data, the ability to assess the quality of the data and health of the instrument, and the provision of an automated procedure for applying radiometric corrections to the data and providing responsive processing of data requests from investigators. The modifications described have resulted in an overall radiometric stability of better than 10 percent, with stability of only a few percent during a single flight.

Through an in-flight calibration experiment at Rogers Dry Lake, California on September 20, 1989, the radiometric and spectral properties of AVIRIS were determined. In-flight spectral channel positions and the spectral response function in 10 regions of the AVIRIS spectral range, taking in all four spectrometers, are shown to agree closely with the corresponding parameters measured in the laboratory. The intraflight stability for the Rogers Dry Lake calibration site is better than 2 percent with the exception of the strong atmospheric water absorptions where the measured radiance is close to zero. This experiment has provided both direct generation of an in-flight spectral and radiometric calibration and validation of the laboratory calibration at the reported level accuracy.

The laboratory procedures, algorithms, measurements, and uncertainties associated with generation of the spectral and radiometric calibration of data acquired by AVIRIS are described. AVIRIS is an airborne sensor that obtains high-spatial-resolution image data of the earth in 224 spectral channels in four spectrometers covering the range from 400 to 2450 nm. The spectral calibration of AVIRIS agrees with the in-flight data to within two nanometers, and the absolute radiometric calibration is consistent with the in-flight verification to 10 percent over the spectral range. In-flight radiometric stability as measured by five consecutive passes over the surface calibration site is reported to be between three and five percent.

Analysis of the Geoscan Mk II scanner imagery obtained over the past twelve months in Australia and in the USA has shown that this advanced system can effect direct mineral identification (DM1) with only a minimum of processing and in an operational commercial mode of use. This paper attempts to show some of these results with imagery flown over goldmineralizaon (Leonora W. Australia) porphyry-copper mineralization and associated higher-level advanced argiffic alteration (Yerington NV--Ann Mason and Buckskin Ra. ) and a copper-gold skarn nearby (Ludwig NV). In all cases simple banddifferencing is all the processing that is required and this can be effected minutes after landing from a flight using a proprietary image-display system (GIPSy) which accepts the optical disks directly. [3 1.

Two Geophysical Environmental Research Corp. (GER) scanners are discussed in this paper. The GER 63 channel airborne imaging spectrometer covers 0.4-2.5 micrometer. There are 24 channels from 500-1000 nm, 7 channels from 1-1.8 micrometer and 32 channels from 2-2.5 micrometer. The thermal scanner has one channel in 0.4-1.1 micrometer, one channel in 3-5 micrometer, 3 channels with 1 micrometer bandwidth in 8-12 micrometer, 6 channels with half micrometer bandwidth in 8-12 micrometer and one channel in 8-12 micrometer. The GER thermal scanner was first flown in January 1990. The initial results indicate that this type of scanner gives the lowest noise equivalent temperature available.

A very flexible, sophisticated, low-cost and powerful commercial imaging spectrograph has been designed and developed that can provide resolutions from under one meter to several meters depending upon aircraft altitude and ground speed. The instrument uses a two-dimensional frame transfer CCD array to image a line beneath the aircraft and to sense the spectrum for each point in the scene. To keep data rates compatible with the built-in digital cassette recorder, a flexible scheme allows the operator to select the spectral bands and spatial information to be recorded. This compact airborne spectrographic imager can be employed to examine water quality, vegetation stress, fish schools, and to distinguish camouflage from vegetation.

Through the use of a special optical filter, the Thermal Infrared Imaging Spectrometer, an airborne multispectral IR imaging instrument operating in the thermal emission region (7.5-14 microns), will achieve signal-to-noise ratios greater than 600 with ambient temperature optics. This instrument will be used to do compositional surface mapping of the terrain, and will refine the ability to categorize rock families and types by providing much higher spectral resolution in the emission region than was previously available. Details of the optical system, the detector, the cooler system, and the support electronics are described.

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.

MODIS is intended to provide daily global surveys for the oceans, the atmosphere, and land. To achieve this capability, this instrument requires at least a 1700 km swath width and provides geometric-instantaneous-fields-of-view that are either 214 m, 428 m, or 856 m in size with reference to a 705 km satellite altitude. The range of products to be supported by MODIS includes ocean chlorophyll concentration, ocean primary productivity, dissolved organic matter in the oceans, global SST, total suspended solids, land cover type, land surface temperatures, vegetation indices, and volcanic and fire events. Details of the MODIS-N and MODIS-T instrument concepts are discussed.

MERIS is designed primarily as an ocean color sensor with potential extension to land observations, which will also be utilized for atmospheric investigations. As a moderate spatial resolution, wide swath sensor, this instrument will provide data to be used as inputs to climatological, global and ecosystem models on a long-term basis. An overview is presented of the mission plan, the system design as being studied by industry, and the application objectives as derived from the user requirements.

The wedge spectrometer design concept incorporates a tapered multilayer thin-film dielectric interference filter mounted in close proximity to a two-dimensional detector array such that each row of detectors receives energy from a different spectral band corresponding to the filter's bandpass at that position. This design concept is applicable to a broad range of spectral regimes, from the visible and near infrared through the longwave IR. Spectrometer design alternatives, performance characteristics, and development status are discussed.

A new concept for a field portable spectrometer designed to meet the needs of the remote sensing community is presented. This instrument uses acoustooptic tunable filters (AOTFs) as wavelength sorters, allowing the design of a rugged, compact, light-weight tool that provides broad spectral coverage, great versatility, and ease of utilization. The spectrometer provides continuous spectral coverage from 0.4 to 2.5 microns with two channels defined by detector technology, while a visible channel covering the 0.4 to 1.0 micron spectral range uses silicon PV photodiodes. The short-wavelength IR channel covers the 0.9 to 2.5 micron special range with thermoelectrically cooled lead sulfide PC detectors.

Column atmospheric water vapor amounts at high spatial resolution were derived from spectral data collected by the airborne visible-infrared imaging spectrometer (AVIRIS). The quantitative derivation is made by curve fitting observed spectra with calculated spectra in the 0. 94 jim and 1. 14 jim water vapor band absorption regions using an atmospheric model a narrow band spectral model and a nonlinear least squares fitting technique. The derivation is also made using a band ratioing technique. These techniques are applicable for retrieving water vapor values from AVIRIS data measured on clear days with visibilities 20 km or greater. The precision of the retrieved column water vapor amounts is 5 or better. It now appears feasible to derive high spatial resolution column water vapor amounts over land areas from satellite altitude with the proposed high resolution imaging spectrometer (HIRIS). Curve fitting of spectra near 1 jim from areas covered with vegetation using an atmospheric model and a simplified vegetation reflectance model indicates that both the amount of atmospheric water vapor and the moisture content of vegetation can be retrieved simultaneously because the band centers of liquid water in vegetation and the atmospheric water vapor are offset by approxinuitely 0. 05 jim. 1.

A comparison is made between two ground-based atmospheric water-vapor measurement techniques, each of which uses data from a solar-pointing radiometer. One technique uses visible wavelength channels to retrieve aerosol loading, surface pressure readings for Rayleigh scattering analyses, and the 0.94 micron channel to extract water vapor from the residual of total versus scattering opacity. The other technique requires only the ratio of channels centered at 0.94 and 0.87 micron. Results are given for the April 13, 1989 AVIRIS in-flight calibration.

The capability to predict the response of ecosystems to change relies on our ability to understand and model the effective functioning of biotic processes at large scales and the transport functions of the atmospheric/hydrospheric processes. To successfully evaluate changes in ecological processes at the required spatial and temporal scales remote sensing technology and ecosystem theory must be considered jointly. A review of developments in remote sensing analysis using high spectral resolution sensors has led to the selection of a potential set ofparameters to be used in ecosystem models. These parameters quantify the light interception properties that scale from leaf to landscape. Spectral mixture analysis forms a framework for the systematic separation of both vegetative and non-vegetative components at sub-pixel spatial resolution. The spectral concentrations of the vegetative components defined by the spectral mixture analysis are then used to drive canopy radiative transfer models from which the ecosystem parameters are inferred. 1.

Near infrared reflectance spectra were used to estimate nitrogen lignin and cellulose concentrations in fresh foliage samples. The spectra of sixty samples from 8 species (4 deciduous 4 conifer) were measured from llOO-2500nxn when the samples were both fresh and dried. Existing regression equations relating near infrared reflectance to sample composition were used to measure concentrations in the dry ground samples. Multiple linear regressions were then used to derive equations relating spectral measurements of the fresh samples to constituent concentration. Results suggest that these constituents can be measured in fresh foliage samples using high resolution near infrared reflectance spectroscopy. 1.

The reflectance of six vegetated areas was tracked from spring through early summer and early fall using three dates of groundreflectance calibrated AVIRIS data sets acquired in 1989. Annual vegetation types exhibit green vegetation spectral features in the spring. The annual plants are largely senesced by early summer and have decreased chlorophyll pigment and leaf water absorption. With the loss of leaf water lignin-cellulose absorptions emerge at 2. 09 and 2. 27 jim. AVIRIS spectra from two forest types show a slight increase in the magnitude ofthe chlorophyll red edge in early summer when compared to the spring data. In the early fall data there is a major decline in the magnitude of the chlorophyll red edge and leafwater absorptions at 0. 95 and 1. 15 jim in tree and shrub dominated areas in drought induced dormancy or undergoing senescence. 1.

Several types of damage occur in the coniferous forests on Whiteface Mountain NY including foliar loss and mortality in balsam fir (Abies balsamea) due to synchronized aging and wind- generated dessication and a similar foliar loss and mortality or forest decline damage in red spruce (Picea rubens) thought to be related to air pollution. The damage in fir forms characteristic patterns of alternating living and dead " waves" of trees on the upper-elevation northwest-facing slopes while forest decline damage in spruce forms isolated patches of dead and dying trees on upper-elevation slopes. Previous studies have identified spectral reflectance characteristics associated with forest decline damage in red spruce including chiorosis and a subtle blue shift of the chlorophyll red edge. The present study provides ground- based spectral characteristics of fir wave damage and an analysis of calibrated Fluorescence Line Imager (FLI) data acquired along the same fir wave used for the in situ measurements. In addition both in situ and FLI reflectance measurements are compared for white birch (Betula papyrifera) and balsam fir. Derivative curve data were produced from both in situ and FLI reflectance measurements for the red edge spectral region for birch and for various portions of a fir wave. These results suggest that with proper atmospheric correction of airborne imaging spectrometer datasets the derivative curve approach will provide an accurate means of assessing red edge parameters and that such will allow for identification of specific types of forest damage made on the basis of spectral fine features. 1.

An AVIRIS image was obtained for Mono Lake California on 26 May 1989 a day with excellent visibility. Aospherically-corrected reflectance spectra derived from the image indicate a spectral signature for chlorophyll a the dominant photosynthetic pigment in the phytoplankton of the lake. Chlorophyll a concentrations in the lake were about 22 mg m3 and the upwelling radiance was low with a peak reflectance at about 570 nm of about 5. Coherent noise appeared in the image as regular variations of 0. 1 to 0. 2 p. W cm2 t1 oriented diagonally to the flight line. A simple ratio of two spectral bands removed the conspicuous undulations but modifications of the shielding within the instrument are needed to improve the signal especially over dark targets such as lakes. 1.

Spectral radiance imagery measured by the Airborne Visibleflnfrared Imaging Spectrometer (AVIRIS) are reduced to reflectance through compensation for atmospheric scattering water vapor absorption absorption of the well mixed gases solar irradiance and the solar zenith angle. The LOWTRAN 7 (Kneizys et al. 1989) radiative transfer code form the basis for this retrieval. LOWTRAN 7 is constrained with water vapor determinations from the AVIRIS radiance data through an algorithm operating on the 940 urn atmospheric water band for every spatial element. In situ measurements of atmospheric optical depths are used to constrain the LOWTRAN 7 aerosol models. Accuracy of the retrieved reflectance spectra is evaluated with respect to surface spectra measured at the time of the overflight. The mineral bastnaesite is identified in the analysis of the retrieved reflectance imagery. These reflectance data provide a means to map the subtle mineral gradients in the PreCambrian block of the Clark Mountain range in southeastern California for geological analysis. 1. 0

The development of imaging spectrometers heralds a revolution in our ability to use remote sensing in the earth sciences. The combined high spectral and spatial resolution of these new sensors should motivate a shift in interpretation techniques towards more deterministic and quantitative methods. Geophysical inversion theory provides the proper framework for the development of these new approaches to remote sensing interpretation. Models relating the observable radiance to the controlling parameters are inverted along with auxiliary data and error models to arrive at quantitative estimates and uncertainties of the properties of interest. Linear spectral unmixing is a simple but useful example of one of the many inversion procedures that can be envisioned for these new remote sensing data. 1.

Spatial and spectral mode FLI airborne imaging spectrometer data obtained from a metal-stressed Norway spruce forest exhibiting subtle stress symptoms correlate poorly with changes in canopy chlorophyll content and biomass with respect to changes or " shifts" in the wavelength positions of the red chlorophyll absorptance well and the transition region between the red reflectance minimum and near-infrared reflectance maximum (the so-called " red edge" ) with " shift" directions being highly variable and displaying no clear trends relative to canopy stress conditions. The lack of strong relationships between canopy parameters that provide a measure of its health and vitality and canopy spectral features that are purported to give a reliable measure of changes in these features does not bode well for the use of the " red edge shift" as a consistent and unambiguous indicator of stress in coniferous forests especially for low or incipient levels of stress. 1.