The Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) is an imaging spectrometer that measures spatial images of the total up welling spectral radiance from 400 to 2500 nm at 10 nm spectral intervals. Quantitative research and application objectives for surface investigations require conversion of the measured radiance to surface reflectance or surface leaving radiance. To calculate apparent surface reflectance an estimation of aerosol optical depth is required for compensation of aerosol scattering and absorption across the spectral range. Determination of other atmospheric characteristics such as atmospheric water vapor and surface pressure is also required. In this paper we describe a set of algorithms to estimate aerosol optical depth, atmospheric water vapor, and surface pressure height from the AVIRIS measured radiance. Based upon these determined atmospheric parameters we described an algorithm to calculated apparent surface reflectance from the AVIRIS measured radiance using a radiative transfer code.

Remote sensing of water status and biochemical components of vegetation can have important applications in the fields of agriculture and forestry. Reflectance of fresh, green vegetation in the 1.0 - 2.5 micrometers region is dominated by liquid water absorption and also weakly affected by absorption due to biochemical components, such as lignin and cellulose. We have developed both the nonlinear and linear least squares spectrum- matching techniques for deriving equivalent water thickness (EWT) of vegetation from AVIRIS data in the 1.0 and 1.6 micrometers regions. Seasonal variations of EWTs over an agricultural area in Greeley, Colorado are determined. EWTs from 1.0 micrometers region are generally greater than those from 1.6 micrometers region because of the deeper light penetration into the canopy. After fitting the AVIRIS data with water spectrum alone, a weak lignin-cellulose absorption feature centered at 1.72 micrometers is seen in the residual spectra. We map the depth of the 1.72-micrometers feature, which can be considered as an index of component abundance in the canopy.

An analysis, based on the inversion of a simple non-linear model of the ground reflectance, was conducted on several AVIRIS scenes. The scenes were acquired during the MAC EUROPE 91 campaign on the 5th and 22nd of July, over two test sites (Black Forest and Freiburg). The model consists in a linear mixing of the soil reflectance and a green vegetation reflectance described with a Kubelka-Munk formula containing the chlorophyll and water specific absorption coefficients. Its inversion provides a Green Vegetation Fraction of the pixel and two parameters related respectively to chlorophyll and water. The model can then be used to evaluate the magnitude of the 1.7 micrometers absorption feature which is thought to be a signature of the vegetation biochemical components. The spatial and temporal variability of this feature over the scenes is commented.17

Bidirectional reflectance factors (BRF) for a pecan orchard have been studied using Advanced Solid-State Array Spectrometer (ASAS) data acquired in the solar principal plane at altitudes of 2300 m and 5300 m above ground. In particular, the angular dependency of the BRF of different targets such as sunlit and shaded portions of the pecan tree, orchard floor, and soil (road) have been studied for viewing directions between -45 degrees and +45 degrees. The results indicate in general an increasing reflectance from the forward scattering direction to the backscattering direction. In addition, an increase in pixel size has significant effects on the surface BRFs.

Derivative spectroscopy has been widely used in chemistry and physics for signal analysis. This technique was applied to the analysis of high spectral-resolution Airborne Visible and Infrared Imaging Spectrometer (AVIRIS) data from series of plots containing varying quantities of green vegetation. A derivative- based green vegetation index for high spectral-resolution AVIRIS data, DGVI (Derivative Green Vegetation Index), has been developed. The DGVI has proven to be effective in the estimation of green vegetation cover in areas having discontinuous plant canopies.

An inversion technique for estimating snow grain size using imaging spectrometer data has been developed that appears to be accurate and insensitive to the effects of noise. Using a radiative transfer model, the method relates the area of an ice absorption feature centered at about (lambda) equals 1.03 micrometers to the equivalent ice particle size that produces the same absorption band area for a modeled reflectance spectrum. An Airborne Visible and Near-infrared Imaging Spectrometer (AVIRIS) image was used to create a map of snow surface layer grain size for the Mammoth Mountain, California region. When reflectance spectra were infused with up to 10 percent random, Gaussian noise, the band area mapping technique was able to accurately predict grain sizes.

Optical case-2 waters near an ocean outfall were examined, using a combination of AVIRIS imagery and ship-based surface and profile bio-optical measurements. Bio-optical mooring data were useful in determining the hydrodynamics of the area. After correcting the image to units of water-leaving radiance (L_w), excellent agreement was achieved between remotely-sensed and in- situ measurements. Spectra from visibly different areas were extracted and compared to the in-water measurements and to each other. Near-shore spectra were dominated by the presence of suspended sediment from beach erosion. Spectra from the central part of the image had a characteristic signature from particulates from either the outfall or resuspension of bottom material or both. At the offshore edge of the image elevated levels of chlorophyll had the greatest influence on spectral shape. Backscatter at 660 nm was calculated from the AVIRIS data and a backscatter image was produced which clearly showed the distribution of the two types of sediments.

According to Kirk's as well as Morel and Gentili's Monte Carlo simulations, the popular simple expression, R approximately equals 0.33 b_b/a, relating subsurface irradiance reflectance (R) to the ratio of the backscattering coefficient (b_b) to absorption coefficient (a), is not valid for b_b/a > 0.25. This means that it may no longer be valid for values of remote-sensing reflectance (above-surface ratio of water-leaving radiance to downwelling irradiance) where Rrs4/ > 0.01. Since there has been no simple Rrs expression developed for very turbid waters, we developed one based in part on Monte Carlo simulations and empirical adjustments to an R_rs model and applied it to rather turbid coastal waters near Tampa Bay to evaluate its utility for unmixing the optical components affecting the water- leaving radiance. With the high spectral (10 nm) and spatial (20 m²) resolution of Airborne Visible-InfraRed Imaging Spectrometer (AVIRIS) data, the water depth and bottom type were deduced using the model for shallow waters. This research demonstrates the necessity of further research to improve interpretations of scenes with highly variable turbid waters, and it emphasizes the utility of high spectral-resolution data as from AVIRIS for better understanding complicated coastal environments such as the west Florida shelf.

This paper reports the assessment of water reflectance spectra measured under controlled condition for analysis of the imaging spectrometry data of inland waters. The application of imaging spectrometry to the study of inland waters depends on a better understanding of the optical properties of the water components. To increase this understanding, a collection of water spectra has been measured under laboratory and ground control conditions as a part of an EOS interdisciplinary investigation. The radiometric measurement derived from those experiments is known as the Bidirectional Reflectance Function (BRDF). R was simulated using variations of a model derived from the two stream approximation of the radiative transfer equation (RTE) and compared to the BRDF. BRDF was also measured 'in situ' from a boat based spectrometer (BoaR) and from an helicopter based spectrometer (HeIR). Modelled R (MoIR), laboratory measured (BRDF) (LabR) and ground measured BRDF (BoaR and HeIR) were compared. They were also used to unmix the spectra of two different aquatic systems. Preliminary results show that at low chlorophyll and gilvin concentration, there is a good agreement between ModR and LabR, regardless of the model variation used. For both low and high inorganic matter concentration, the agreement between ModR and LabR is poor. All sets of data present the main features which characterize the high chlorophyll concentration reflectance spectra, but the amount of reflectance displayed by LabR and ModR is 50 times smaller than that measured in situ. When the different sets are used as input to unmixing algorithms they also yield different results.

The Tucurui reservoir was formed by damming the Tocantins river and flooding around 2430 Km² of forest land. Its average depth is 18 meters with a dendritic shoreline which is responsible for a large variety of water masses with distinctive properties. This aquatic system therefore offers an unique opportunity for collection of wide variety of spectral data representative of the conditions usually found in Amazon reservoirs. These could be used as an spectral library for interpretation of imaging spectrometric data. This paper reports an experiment performed in April 1992 in which spectrometric data were collected over the Tucurui reservoir concurrently to water sampling at surface, at 50% light penetration depth, and at Secchi depth. Water samples were analyzed to obtain chlorophyll pigment, total suspended solids concentration and yellow substance attenuation. The preliminary results show two different types of optical waters, influenced mainly by total suspended solids concentration.

The unprecedented data volume in the era of NASA's Mission to Planet Earth (MTPE) demands innovative information extraction methods and advanced processing techniques. The neural network techniques, which are intrinsic to distributed parallel processings and have shown promising results in analyzing remotely sensed data, could become the essential tools in the MTPE era. To evaluate the information content of data with higher dimension and the usefulness of neural networks in analyzing them, measurements from the GER 63-channel airborne imaging spectrometer data over Cuprite, Nevada, are used. The data are classified with 3-layer Perceptron of various architectures. It is shown that the neural network can achieve a level of performance similar to conventional methods, without the need for an explicit feature extraction step.

Conventional sensors for the VIS spectrum are limited in their performance disregarding the spectral characteristics of the objects. Knowing these characteristics for soil, water and vegetation optimal bandwidth, distances and number of bands can be designed for each special monitoring case, taking atmospheric and flight conditions into consideration.

We describe a novel cryogenically cooled, spatially modulated, imaging, Fourier transform interferometer spectrometer for spectral measurements in the 1 - 5 micrometers range. Using spatial modulation and a detector array to sample the interferogram, the instrument employs no moving parts to obtain spectra. It is extremely robust and potentially more reliable than other interferometers in addition to taking advantage of the multiplexing afforded by array detectors. The instrument technology possesses a unique combination of characteristics which forms a niche for spectral measurement not widely known but of great potential value. These characteristics include broad wavelength range, wide field of view if desired, simultaneous measurement of all spectral channels, compactness, no moving parts, and moderate resolution. We present a small amount of test data derived from the instrument.

During the year 1992 a prototype airborne imaging spectrometer is developed in Finland. The instrument is in the first place developed for technology demonstration, performance verification and algorithm development. The first tests of the AISA (Airborne Imaging Spectrometer for different Applications) are performed during the end of 1992 and the beginning of 1993. This paper describes the instrument design concept and will list the first test results. The instrument has 288 spectral channels, a spatial resolution of 384 pixels across track and is through software flexible programmable. Targets during the development have been simplicity and robustness. Applications can be found in forestry, ecology, hydrology, geology, agriculture etc.

The compact airborne spectrographic imager (casi) is a pushbroom imaging spectrograph intended for acquisition of VNIR multispectral imagery from light aircraft. An ongoing development program has resulted in improvements to the radiometric calibration procedures, and the capability for roll correction and geocorrection of imagery acquired with casi. A variety of monitoring and research missions have been undertaken for aquatic and terrestrial applications and development of remote sensing methodologies.

The European Community and DLR are funding a 79-channel airborne imaging spectrometer (DAIS-7915) to be used for remote sensing applications such as environmental monitoring of land and marine ecosystems, vegetation stress research, agriculture and forestry resource mapping, geological mapping, mineral exploration and supply of data for geographic information systems. The DAIS sensor covers the spectral range from the visible to thermal infrared wavelengths at variable spatial resolutions from 2 - 30 m. Therefore, DAIS can also be used for the investigation of specifications for future airborne and spaceborne optical instruments for specific applications.

HYDICE (the Hyperspectral Digital Imagery Collection Experiment) is a program to build and operate an advanced airborne imaging spectrometer. Scheduled to be operating in 1994, it will provide high quality hyperspectral data for use by a number of US civil agencies in determining its utility for a wide range of applications, as well as in support of basic research. The current status of the system under construction and plans for its operation are reviewed.

AVIRIS operations at the Jet Propulsion Laboratory consist primarily of a sensor task and a data facility task. These two activities are supported by an experiment coordination, a calibration and a management effort. The sensor task is responsible for AVIRIS sensor maintenance, laboratory calibration, and field operations. The AVIRIS data facility is responsible for data archiving, data calibration, quality monitoring and distribution. In this paper we describe recent improvements in these two primary AVIRIS tasks. The inflight performance of AVIRIS in 1992 and 1993 that resulted from these improvements is also presented.

The Modular Optoelectronic Scanner MOS is a spaceborne imaging spectrometer in the VIS/NIR range of optical spectrum. It was especially designed for remote sensing of the atmosphere-ocean system providing 17 channels at high radiometric resolution and high absolute calibration accuracy. It will be launched to the Russian space station MIR on board of the PRIRODA remote sensing module in the mid of 1994. The paper presents the sensor concept of an atmospheric and a biospheric spectrometer blocks and the scientific goals of the German participation within PRIRODA as well as main aspects of the entire PRIRODA mission.

HRIS is proposed as a spaceborne, high-resolution imaging spectrometer designed to image a variable (+/- 30 degree(s)) 30 km swath with 40 m SSP pixel size in the spectral range from 450 to 2340 nm with an average 10 nm spectral bandwidth. HRIS is conceived as a push-broom imager with two-dimensional detector arrays for spectral and spatial coverage. The challenging requirements for this instrument will be discussed as well as the concept derived against these requirements. Emphasis is on the optical definition, particularly the spectrometer optics, the focal plane assembly--here mostly the hybrid SWIR CMT detector array--and the calibration concept which includes two external references, ratioing radiometers and an internal reference. The other subunits will be described briefly only. The presentation will conclude with a preliminary development plan.

The design and construction of new types of imaging systems has become feasible with the availability of reliable, relatively low cost focal plane arrays (FPA's). This paper presents experimental and modeling investigations of an imaging grating spectrometer utilizing a 128 X 128 InSb FPA for measurements in the 3 - 5 micrometers region. The modeling efforts verify the conceptual feasibility and identify practical limitations in sensitivity and dynamic range by considering system throughput, significant instrument thermal self-emission, and system noise. The model includes a detailed examination of the optical parameters, the focal plane noise sources, and the electronics. The design concepts and performance were verified experimentally by building and testing a prototype imaging spectrometer using commercially available optics, FPA, electronics, and computer equipment. Data is presented which illustrate the simultaneous spectra and spatial measurement features and the versatility of the sensor system. Both the model and the measurement results show the impact of instrument self-emissions, FPA noise, and FPA nonuniformities on the sensor system. It is very apparent that cryogenic optics, improved FPA non-uniformity correction, and an upgraded data acquisition system will significantly improve the performance of the prototype imaging spectrometer system and are important considerations for future designs.

An imaging spectrometer design has been demonstrated which utilizes a plano blazed diffraction grating in divergent light. It offers important advantages over traditional collimated light spectrometers using either a grating or a prism. This design in a simple, compact configuration has linearity in wavelength and field and uniform spacing of wavelengths with no geometric distortion (smile). In addition to these features, polarization sensitivity and radiometric efficiency of the design will be discussed and compared with the requirements for the High Resolution Imaging Spectrometer (HIRIS) for the Earth Observing System.

A prototype imaging interferometer called DASI (digital array scanned interferometer) is under development at our laboratories. Our objective is to design an instrument for remote sensing of Earth's atmosphere and surface. This paper describes the unusual characteristics of DASIs which make them promising candidates for ground and aircraft-based terrestrial measurements. These characteristics include superior signal-to-noise, design simplicity and compactness, relative to dispersion based imaging spectrometers. Perhaps one of the most notable features of DASIs is their ability to acquire an entire interferogram simultaneously without any moving optical elements. We also describe selected laboratory and ground based field measurements using the prototype DASI. A CCD detector array was placed at the DASI detector plane for wavelength coverage from 0.4 to 1.0 micrometers . A NICMOS MCT detector was used for coverage from 1.1 to 2.2 micrometers . The DASI was configured to have a spectral resolution of about 300 cm^-1, a spatial field of view of 5 degrees, and a constant number of transverse spatial elements (detector dependent) for each exposure frame. Frame exposure rates were up to 0.6 Hz with the potential to achieve 5 Hz. Image cube measurements of laboratory targets and terrestrial scenes were obtained by multiple frame scanning over the field of view. These data sets reveal the potential science yields from obtaining simultaneous high resolution spatial and spectral information.

A demonstration imaging spectrometer using a liquid crystal tunable filter (LCTF) was built and tested on a hot air balloon platform. The LCTF is a tunable polarization interference or Lyot filter. The LCTF enables a small, light weight, low power, band sequential imaging spectrometer design. An overview of the prototype system is given along with a description of balloon experiment results. System model performance predictions are given for a future LCTF based imaging spectrometer design. System design considerations of LCTF imaging spectrometers are discussed.

An imaging spectrograph with high spectral resolution (< 0.55 nm) operating in the UV region between 300 - 320 nm is presented. The instrument uses Differential Optical Absorption Spectroscopy (DOAS) to monitor the SO₂ total content in the earth's atmosphere from a sun synchronous orbit. The design of the entire instrument including wide-angle optics (+/- 57.5 degree(s)), opto- mechanics and sensor electronics (low light CCD application) and the in-flight calibration unit are described. The requirements on stability and calibration accuracy of the instrument caused by the DOAS method are outlined.

The operating principles of an Imaging Fourier Transform Spectrometer (IFTS) are discussed. The advantages and disadvantages of such instruments with respect to alternative imaging spectrometers are discussed. The primary advantages of the IFTS are the capacity to acquire more than an order of magnitude more spectral channels than alternative systems with more than an order of magnitude greater etendue than for alternative systems. The primary disadvantage of IFTS, or FTS is general, is the sensitivity to temporal fluctuations, either random or periodic. Data from the IRIFTS (ir IFTS) prototype instrument, sensitive in the infrared, are presented having a spectral sensitivity of 0.01 absorbance units per pixel, a spectral resolution of 6 cm^-1 over the range 0 to 7899 cm^-1, and a spatial resolution of 2.5 mr.