Advanced Solid-state Array Spectroradiometer Research Articles

Detection of Spatio-Temporal Changes of Norway Spruce Forest Stands in Ore Mountains Using Landsat Time Series and Airborne Hyperspectral Imagery

The study focuses on spatio-temporal changes in the physiological status of the Norway spruce forests located at the central and western parts of the Ore Mountains (northwestern part of the Czech Republic), which suffered from severe environmental pollution from the 1970s to the 1990s. The situation started improving after the pollution loads decreased significantly at the end of the 1990s. The general trends in forest recovery were studied using the tasseled cap transformation and disturbance index (DI) extracted from the 1985–2015 time series of Landsat data. In addition, 16 vegetation indices (VIs) extracted from airborne hyperspectral (HS) data acquired in 1998 using the Advanced Solid-State Array Spectroradiometer (ASAS) and in 2013 using the Airborne Prism Experiment (APEX) were used to study changes in forest health. The forest health status analysis of HS image data was performed at two levels of spatial resolution; at a tree level (original 2.0 m spatial resolution), as well as at a forest stand level (generalized to 6.0 m spatial resolution). The temporal changes were studied primarily using the VOG1 vegetation index (VI) as it was showing high and stable sensitivity to forest damage for both spatial resolutions considered. In 1998, significant differences between the moderately to heavily damaged (central Ore Mountains) and initially damaged (western Ore Mountains) stands were detected for all the VIs tested. In 2013, the stands in the central Ore Mountains exhibited VI values much closer to the global mean, indicating an improvement in their health status. This result fully confirms the finding of the Landsat time series analysis. The greatest difference in Disturbance Index (DI) values between the central (1998: 0.37) and western Ore Mountains stands (1998: −1.21) could be seen at the end of the 1990s. Nonetheless, levelling of the physiological status of Norway spruce was observed for the central and western parts of the Ore Mountains in 2013 (mean DI values −1.04 (western) and −0.66 (central)). Although the differences between originally moderately-to-heavily damaged, and initially damaged stands generally levelled out by 2013, it is still possible to detect signs of the previous damage in some cases.

Structure Analysis and Classification of Boreal Forests Using Airborne Hyperspectral BRDF Data from ASAS

A new approach is presented for deriving vegetation canopy structural characteristics from hyperspectral bidirectional reflectance distribution function (BRDF) data. The methodology is based on the relationship between spectral variability of BRDF effects and canopy geometry. Tests with data acquired with the Advanced Solid-State Array Spectroradiometer (ASAS) over Canadian boreal forests during the BOREAS campaign show that vegetation structural characteristics can be derived from the spectral variability of BRDF effects. In addition, the incorporation of both BRDF effects and hyperspectral resolution data substantially improve the classification accuracy. Best classification results are obtained when hyperspectral resolution and BRDF data are combined, but the improvement is not consistent for all classes. For example, adding BRDF information to hyperspectral data increases the overall classification accuracy for a six-class fen site from 37.8% to 44.7%. The addition, however, reduces the accuracy for the jack pine class from 43.6% to 28.8%. These new findings provide evidence for improved capabilities for applications of MISR and MODIS data. The spectral resolution of MODIS is expected to be sufficient to derive canopy structural information based on the spectral variability of BRDF effects, and for MISR a significant improvement of classification accuracies can be anticipated from the combination of nadir reflectance and off-nadir data.

Determination of aerosol optical depth and land surface directional reflectances using multiangle imagery

Spectral aerosol optical depths, surface hemispherical‐directional reflectance factors, and bihemispherical reflectances (albedos) are retrieved for an area of Glacier National Park using spectral, multiangle imagery obtained with the airborne advanced solid state array spectroradiometer (ASAS). The retrieval algorithms are described and are identical in principle to those being devised for use by the multiangle imaging spectroradiometer (MISR) which will fly on the EOS‐AM1 spacecraft in 1998. As part of its science mission, MISR will produce global coverage of both aerosol amounts and land surface reflection properties. The results in this paper represent the initial effort in applying the MISR algorithms to real data. These algorithms will undergo additional testing and validation as more multiangle data become available.

Deconvolution of remotely sensed spectral mixtures for retrieval of LAI, fAPAR and soil brightness

Linear mixture models have been used to invert spectral reflectances of targets at the Earth's surface into proportions of plant and soil components. However, operational use of mixture models has been limited by a lack of biophysical interpretation of the results. The main objectives of this study were (1) to relate the deconvolved components of a mixture model with biophysical properties of vegetation and soil at the surface and (2) to apply the mixture model results to remotely sensed imagery. A radiative transfer model (SAIL: Scattering by Arbitrarily Inclined Leaves) was used to generate reflectance ‘mixtures’ from leaf and bare soil spectral measurements made at HAPEX-Sahel (Hydrological Atmospheric Pilot EXperiment) study sites. The SAIL model was used to create canopy reflectances and fractions of absorbed photosynthetically active readiation (fAPAR) for a range of mixed targets with varying leaf area index (LAI) and soils. A spectral mixture model was used to deconvolve the simulated reflectance data into component fractions, which were then calibrated to the SAIL-generated LAI, fAPAR and soil brightness. The calibrated relationships were validated with observational ground data (LAI, fAPAR and reflectance) measured at the HAPEX Sahel fallow bush/grassland, fallow grassland and millet sites. Both the vegetation and soil component fractions were found to be dependent upon soil background brightness, such that inclusion of the soil fraction information significantly improved the derivation of vegetation biophysical parameters. Soil brightness was also shown to be a useful parameter to infer soil properties. The deconvolution methodology was then applied to a nadir image of a HAPEX-Sahel site measured by the Advanced Solid State Array Spectroradiometer (ASA). Site LAI and fAPAR were successfully estimated by combining the fractional estimates of vegetation and soils, obtained through deconvolution of the ASAS image, with the calibrated relationships between vegetation fraction, LAI and fAPAR, obtained from the SAIL data.

Estimating land surface albedo in the HAPEX-Sahel southern super-site: inversion of two BRDF models against multiple angle ASAS images

The provision of spatially and temporally disaggregated values of albedo is critical to an improved understanding of energy interactions at the Earth surface. Remotely sensed estimates of land surface albedo can best be obtained by inverting models of the Bidirectional Reflectance Distribution Function (BRDF) against bidirectional reflectance factor measurements sampled at different sensor view angles and solar illumination angles. This paper describes the preliminary results obtained using such an approach over the HAPEX-Sahel (Hydrological and Atmospheric Pilot Experiment) southern super-site in Niger. Two BRDF models, one empirical (modified-Walthall) and one semi-empirical (a linear kernel-driven model, employing isotropic, geometric and volume scattering kernels), are inverted analytically against each pixel in a set of co-registered multispectral images acquired by NASA's Advanced Solid-state Array Spectroradiometer (ASAS). The paper describes the methods used to register these images to sub-pixel accuracy, to perform radiometric and first-order atmospheric correction of the data, and to invert the BRDF model against the pre-processed image data to yield spatially referenced estimates of the model parameters and angularly integrated terms related to albedo. Spatial patterns, closely related to variations in land cover type, are clearly evident in these data. It is shown that, in this instance, the simple empirical model provides a better fit to the measured data, particularly at red and near-infrared wavelengths. The poorer performance of the semi-empirical model at this particular study site is discussed in terms of the assumptions that the model makes about energy interaction with the land surface. The impacts of changes in the projected instantaneous field-of-view as a function of sensor view angle and of residual image-to-image mis-registration on the derived BRDF model parameters and estimated albedo values are also examined.

Estimating the fraction of absorbed photosynthetically active radiation (ƒAPAR) at FIFE with airborne bidirectional spectral reflectance data

The relationship between the fraction of absorbed photosynthetically active radiation (ƒAPAR) and spectral vegetation indices (SVIs) is affected by various factors, including background effects and the view zenith angle of the sensor. This study investigated the effects of estimating ƒAPAR from multiple off‐nadir airborne measurements acquired by the advanced solid‐state array spectroradiometer (ASAS) during the First International Satellite Land Surface Climatology Project Field Experiment. Using atmospherically corrected data, ƒAPAR was estimated with both directional SVIs (normalized difference vegetation index (NDVI) and simple ratio (SR)) and PAR hemispherical reflectance (RPAR) calculated from PAR bidirectional reflectance factors for the seven viewing angles of ASAS. Very weak linear relationships were found between the ground‐measured total ƒAPAR values and ASAS measurements, assumed to be due to the presence of varying amounts of senescent plant materials in the canopy. Exclusion of data collected in October when the canopy was fully senescent somewhat improved these relationships. However, correction of total ƒAPAR values by the fraction of green vegetation present (green ƒAPAR) provided substantial improvement. For both SVIs the highest coefficients of determination (r2 = 0.839 for NDVI; r2 = 0.890 for SR) were found at 15° in the backscattering direction decreasing to minimum values at ±45°, a trend thought to be associated with the effect of canopy structure on the SVIs. Although RPAR was not strongly correlated with either the total or the green ƒAPAR, it was related to the independently measured PAR flux reflected from the canopy indicating a potential for deriving PAR hemispherical reflectance from directional measurements for natural cover types such as grasslands.

The behavior of the anisotropic reflectance of a pecan orchard derived from multiview and multiscale imaging spectrometer data

Bidirectional reflectances for a pecan orchard have been studied using Advanced Solid-state Array Spectroradiometer (ASAS) data acquired in the solar principal plane and perpendicular to it at altitudes of 2300 m and 5300 m above ground. In particular, the angular dependency of the bidirectional reflectance of two targets, sunlit portions of the pecan tree and composite (tree crowns and orchard floor), have been studied for viewing directions between − 45° and + 45°. The results generally indicate an increasing reflectance from the forward scattering direction to the backscattering direction for image sequences acquired in the solar principal plane and a symmetrical behavior about nadir with decreasing reflectance towards the extreme viewing angles for site passes perpendicular to the solar principal plane. These off-nadir effects vary with solar zenith angle, target type (composition and structure), and spatial scale. The largest variations were usually observed in the backscatter directions. It was also found that a combination of these factors, especially with the target characteristics, had a significant impact on the surface's bidirectional reflectance. In addition, these reflectance changes are wavelength-dependent, with the largest difference in relative reflectance occuring between the visible and infrared bands. In general, the overall behavior of the bidirectional reflectance derived from airborne data follows the trends observed in ground-based investigations.

Multiple view zenith angle observations of reflectance from ponderosa pine stands

Reflectance factors (RF(lambda)) from dense and sparse ponderosa pine (Pinus ponderosa) stands, derived from radiance data collected in the solar principal plane by the Advanced Solid-State Array Spectro-radiometer (ASAS), were examined as a function of view zenith angle (theta(sub v)). RF(lambda) was maximized with theta(sub v) nearest the solar retrodirection, and minimized near the specular direction throughout the ASAS spectral region. The dense stand had much higher RF anisotropy (ma)dmurn RF is minimum RF) in the red region than did the sparse stand (relative differences of 5.3 vs. 2.75, respectively), as a function of theta(sub v), due to the shadow component in the canopy. Anisotropy in the near-infrared (NIR) was more similar between the two stands (2.5 in the dense stand and 2.25 in the sparse stand); the dense stand exhibited a greater hotspot effect than 20 the sparse stand in this spectral region. Two common vegetation transforms, the NIR/red ratio and the normalized difference vegetation index (NDVI), both showed a theta(sub v) dependence for the dense stand. Minimum values occurred near the retrodirection and maximum values occurred near the specular direction. Greater relative differences were noted for the NIR/red ratio (2.1) than for the NDVI (1.3). The sparse stand showed no obvious dependence on theta(sub v) for either transform, except for slightly elevated values toward the specular direction.

Validation of bidirectional and hemispherical reflectances from a geometric-optical model using ASAS imagery and pyranometer measurements of a spruce forest

Aircraft imagery and ground measurements acquired for a spruce forest stand in Howland, Maine as part of the 1990 Forest Ecosystem Dynamics Multisensor Aircraft Campaign (FEDMAC) are used to validate the Li—Strahler geometric-optical forest canopy reflectance model and to demonstrate that both spectral bidirectional reflectance factors and hemispherical reflectance can be estimated with some success. With the geometric-optical model, a vegetated surface is treated as an assemblage of partially illuminated tree crowns of ellipsoidal shape, and through geometric-optics and Boolean set theory, the proportion of sunlit and shadowed canopy and background is modeled as a function of view angle. The model is driven by ground measurements of spectral reflectance and tree crown shape, size, and spacing. Atmospherically corrected multiangular radiance measurements of the FEDMAC spruce site from the Advanced Solid State Array Spectroradiometer (ASAS) were found to fit the shape of the modeled reflectance function quite well along the principal and cross-principal planes. Furthermore, integration of the modeled reflectance functions yielded spectral surface albedos (hemispherical reflectance), which, when extended to the full solar spectrum, were found to agree closely with pyranometer measurements obtained at the spruce site.

Modeling bidirectional radiance measurements collected by the advanced Solid-State Array Spectroradiometer (ASAS) over oregon transect conifer forests

A geometric-optical model of the bidirectional reflectance of a forest canopy, developed by Li and Strahler, fits observed directional radiance measurements with good accuracy. This model treats the forest cover as a scene of discrete, three-dimensional objects (trees) that are illuminated and viewed from different positions in the hemisphere. The shapes of the objects, their count densities and patterns of placement, are the driving variables, and they condition the mixture of sunlit and shaded objects and background that are observed from a particular viewing direction, given a direction of illumination. This mixture, in turn, controls the brightness apparent to an observer or a radiometric instrument. The Advanced Solid-State Array Spectroradiometer (ASAS) was used to validate this model. This aircraft sensor presently acquires images in 29 spectral bands in the range (465–871 nm) and is pointable fore-and-aft, allowing directional measurements of radiance as a target is approached and imaged at view angles ranging ± 45° from nadir. Through atmospheric correction, ASAS radiances were reduced to bidirectional reflectance factors (BRFs). These were compared to corresponding BRF values computed from the Li-Strahler model using, wherever possible, ground measured component BRFs for calibration. The comparisons showed a good match between the modeled and measured reflectance factors for four of the five Oregon Transect Sites. Thus, the geometric-optical approach provides a realistic model for the bidirectional reflectance distribution function of such natural vegetation canopies. Further modifications are suggested to improve the predicted BRFs and yield still better results.

Multispectral bidirectional reflectance of northern forest canopies with the advanced solid-state array spectroradiometer (ASAS)

Understanding the bidirectional reflectance characteristics of forest canopies is important for relating remote sensing observations to biomass, species, stand structure, and albedo. Studies of the directional scattering of forest canopies have been limited in the past because of difficulties in suspending instruments over tall canopies or by being restricted to airborne scanning systems. In this article we discuss the use of the Advanced Solid-State Array Spectroradiometer (ASAS) to measure the bidirectional reflectance of several diverse forest canopies. ASAS data were acquired in 1989 and 1990 as part of an extensive field measurement campaign at International Paper's Northern Experimental Forest (NEF) located near Howland, Maine. Multiangle data sets were acquired for several sites under clear sky conditions and at view azimuths parallel, oblique, and perpendicular to the principal plane of the sun. The sensor radiance data for several surface types including forest stands and openings were converted to bidirectional reflectance using an atmospheric correction algorithm and optical thickness measurements. In addition, photosynthetically active radiation (PAR) hemispherical reflectance (albedo) was calculated from the corrected ASAS data and compared to a vegetation index. Highest observed reflectance factors were recorded in or near the solar principal plane at viewing geometries approaching the antisolar direction. Minimum reflectances were also observed in the solar principal plane, but in the forward scattered direction. Bidirectional reflectance factors (BRFs) acquired in the backscatter direction at visible wavelengths showed larger percent differences between viewing angles of 45° in the forward and backscattered direction (up to 95%) than near-IR data (up to 60%). The normalized vegetation index (NDVI) also varied with view angle but to a lesser degree than single-band BRFs. Also in contrast to single-band BRFs, maximum NDVI was recorded in the forward scattered direction and minimum NDVI was observed in the backscattered direction. Estimated fraction of absorbed photosynthetically active radiation ( F APAR) was determined from the hemispherical PAR reflectance for several canopy types within the forest area. Examining the relationships with NDVI revealed a strong dependence of NDVI on F APAR for nadir and 45° forward scattered data. A poor relationship was observed for data acquired at 45° in the backscattered direction and for NDVI derived from hemispherical reflectance. From these results, it is apparent that the sensor viewing geometry that minimizes the contribution of reflectance from branches and the ground will yield higher relationships between F APAR and NDVI.

Specular scattering on a terrestrial playa and implications for planetary surface studies

Multiangle Advanced Solid State Array Spectroradiometer and Portable Apparatus for Rapid Acquisition of Bidirectional Observations of Land and Atmosphere visible and near infrared reflectance data covering the Lunar Lake playa, Nevada, were modeled using specular and volume scattering theory. The volume component used to model the data was based on the Hapke (1986) model and the specular scattering component was based on Fresnel reflection from surface facets with a distribution of tilts. Specular scattering was needed to explain the several‐fold increase in reflectance observed at high phase angles in the solar principal plane. Results imply single scattering albedos of approximately 0.93 to 0.95, a real index of refraction value of approximately 1.52 (at a wavelength of 0.66 μm), a small imaginary index of refraction and an exponential facet tilt probability function for the playa. Electron micrographs showed the playa surface to consist of ensembles of smooth, micrometer‐scale ellipsoidal particles on gently undulating topography at the millimeter to centimeter scale. The exponential probability function is consistent with this observation. Mars and the icy satellites are discussed as possible places to look for a significant specular scattering component.

Airborne bidirectional radiances of snow-covered surfaces in Montana, U.S.A.

Bidirectional radiance measurements of various snow-covered surfaces have been acquired using the Advanced Solid-State Array Spectro-radiometer (ASAS) which was flown on the NASA C-130 aricraft over Glacier National Park, Montana on 14 March 1991. A frozen lake, a snow-covered meadow, a mixed coniferous/deciduous forest and a dense coniferous forest were studied. Knowledge of the bidirectional reflectance characteristics of various snow-covered surfaces is key to calculation of albedo from satellite sensors in the future. While the reflectance characteristics of pure snow targets have been measured from the ground, aircraft and satellites, very few bidirectional reflectance data exist for surface covers of mixed snow and vegetation. Analysis of the 14 March 1991 ASAS data show that the anisotropic reflectance of snow was strong over the snow-covered frozen lake but slightly dampened over the meadow because of the presence of low vegetation. Over the dense coniferous forest, strong backscatter was observed while higher radiances in both the forward and aftward sensor view zenith directions was measured over the mixed forest. Snow that was known to be beneath the coniferous forest canopy was not detected in our data.

Airborne bidirectional radiances of snow-covered surfaces in Montana, U.S.A.

Bidirectional radiance measurements of various snow-covered surfaces have been acquired using the Advanced Solid-State Array Spectro-radiometer (ASAS) which was flown on the NASA C-130 aricraft over Glacier National Park, Montana on 14 March 1991. A frozen lake, a snow-covered meadow, a mixed coniferous/deciduous forest and a dense coniferous forest were studied. Knowledge of the bidirectional reflectance characteristics of various snow-covered surfaces is key to calculation of albedo from satellite sensors in the future. While the reflectance characteristics of pure snow targets have been measured from the ground, aircraft and satellites, very few bidirectional reflectance data exist for surface covers of mixed snow and vegetation. Analysis of the 14 March 1991 ASAS data show that the anisotropic reflectance of snow was strong over the snow-covered frozen lake but slightly dampened over the meadow because of the presence of low vegetation. Over the dense coniferous forest, strong backscatter was observed while higher radiances in both the forward and aftward sensor view zenith directions was measured over the mixed forest. Snow that was known to be beneath the coniferous forest canopy was not detected in our data.

An off-nadir-pointing imaging spectroradiometer for terrestrial ecosystem studies

The Advanced Solid-state Array Spectroradiometer (ASAS), an airborne, off-nadir-pointing imaging spectroradiometer used to acquire bidirectional radiance data for terrestrial targets, is described. As its platform aircraft flies over a target, the sensor can image the target through a sequence of at least seven fore-to-aft view directions ranging up to 45 degrees on either side of nadir. ASAS acquires data for 29 spectral bands in the visible and near-infrared portions of the spectrum (465 to 871 nm) with a resolution of 15 nm. The basic ASAS data product is a sequence of digital images acquired from multiple view directions and consisting of calibrated spectral radiance values. Examples of ASAS data from field experiments are presented. The data demonstrate the combined effects of reflectance anisotropy and increased atmospheric path length on off-nadir observations. One result of these effects is a variation in vegetation indices as a function of view direction. >