Darrel L. Williams

BOREAS in 1997: Experiment overview, scientific results, and future directions

The goal of the Boreal Ecosystem-Atmosphere Study (BOREAS) is to improve our understanding of the interactions between the boreal forest biome and the atmosphere in order to clarify their roles in global change. This overview paper describes the science background and motivations for BOREAS and the experimental design and operations of the BOREAS 1994 and BOREAS 1996 field years. The findings of the 83 papers in this journal special issue are reviewed. In section 7, important scientific results of the project to date are summarized and future research directions are identified.

Optical remote sensing of vegetation: Modeling, caveats, and algorithms

Abstract The state-of-the-art on radiative transfer modeling in vegetation canopies arul the application of such models to the interpretation and analysis of remotely sensed optical data is summarized. Modeling of top-of-the-atmosphere and top-of-the-canopy radiance field is developed as boundary value problems in radiative transfer. The parameterization of the constituent functions with simple models and/or empirical data is outlined together with numerical solution methods and examples of results of model validation. Caveats in the assignment of signal characteristics to surface properties are itemized and discussed with example results. Algorithms to estimate surface properties from remote observations are classified as spectral vegetation indices, model inversion, expert systems, neural networks, and genetic algorithms. Their applicability is also discussed.

Landsat sensor performance: history and current status

The current Thematic Mapper (TM) class of Landsat sensors began with Landsat-4, which was launched in 1982. This series continued with the nearly identical sensor on Landsat-5, launched in 1984. The final sensor in the series was the Landsat-7 Enhanced Thematic Mapper Plus (ETM+), which was carried into orbit in 1999. Varying degrees of effort have been devoted to the characterization of these instruments and data over the past 22 years. Extensive short-lived efforts early in the history, very limited efforts in the middle years, and now a systematic program for continuing characterization of all three systems are apparent. Currently, both the Landsat-5 TM and the Landsat-7 ETM+ are operational and providing data. Despite 20+ years of operation, the TM on Landsat-5 is fully functional, although downlinks for the data are limited. Landsat-7 ETM+ experienced a failure of its Scan Line Corrector mechanism in May 2003. Although there are gaps in the data coverage, the data remain of equivalent quality to prefailure data. Data products have been developed to fill these gaps using other ETM+ scenes.

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. >

A comparison of spectral reflectance properties at the needle, branch, and canopy level for selected Conifer species

Abstract The optical-reflective radiative transfer characteristics of three conifer species [Norway spruce (Picea abies), red pine (Pinus resinosa), and white pine (Pinus strobus)] and one broadleaf, deciduous species [sugar maple (Acer saccharum)] were measured and compared at the leaf, twig, branch and canopy level. The magnitude of reflectance throughout the visible and near infrared wavelength region was found to decrease dramatically for the conifer species as scene complexity increased from the needle, to the branch, to the canopy level. Comparison of the conifer data with equivalent data obtained for sugar maple served to validate quantitatively that conifer canopies, in general, are more absorptive than their deciduous counterparts, particularly in the near infrared region. These data emphasize the role of canopy constituents, such as needles, twigs, branches, bark, and understory material, in altering the reflectance characteristics of the overall “scene,” and may be valuable in developing improved radiative transfer models for forest canopies.

The effects of spatial resolution on the classification of Thematic Mapper data

Abstract Actual and degraded LANDSAT-4 Thematic Mapper (TM) data were analysed to examine the effect of spatial resolution on the performance of a per pixel, maximum-likelihood classification algorithm. Analysis of variance (ANOVA) and a balanced, three-factor, eight-treatment, fixed-effects model were used to investigate the interactions between spatial resolution and two other TM refinements, spectral band configuration and data quantization. The goal was to evaluate quantitatively the effects of these attributes on classification accuracies obtained with all pixels (pure pixels plus mixed pixels) and on accuracies obtained with pure pixels alone. A comparison of results from these separate analyses supported previous explanations of the effects of increasing spatial resolution. First, the difficulty in classifying mixed pixels was demonstrated by an average 21 per cent decrease in percentage accuracy from the pure-pixel case to the pure-plus-mixed-pixel case for the eight ANOVA treatments. In the pure-...

Historical Record of Landsat Global Coverage: Mission Operations, NSLRSDA, and International Cooperator Stations

The long-term, 34� year record of global Landsat remote sensing data is a critical resource to study the Earth system and human impacts on this system. The National Satellite Land Remote Sensing Data Archive (NSLRSDA) is charged by public law to: “maintain a permanent, comprehensive Government archive of global Landsat and other land remote sensing data for long-term monitoring and study of the changing global environment” (U.S. Congress, 1992). The advisory committee for NSLRSDA requested a detailed analysis of observation coverage within the U.S. Landsat holdings, as well as that acquired and held by International Cooperator (IC) stations. Our analyses, to date, have found gaps of varying magnitude in U.S. holdings of Landsat global coverage data, which appear to reflect technical or administrative variations in mission operations. In many cases it may be possible to partially fill these gaps in U.S. holdings through observations that were acquired and are now being held at International Cooperator stations.

The Landsat 7 mission: Terrestrial research and applications for the 21st century

The Landsat Earth observation approach introduced in 1972 created a new way of monitoring land cover and land use globally. The Landsat 7 mission, successfully launched on April 15, 1999, continues those observations and demonstrates significant progress in precise numerical radiometry, spectral differentiation, and seasonally repetitive monitoring. Substantial improvements in calibration procedures, both prior to launch and during normal operations, have also been made to ensure long-term stability in the acquired spectral radiometry. Landsat 7 data acquisitions are being driven by a long-term data acquisition plan that was designed to ensure that substantially cloud-free, seasonal coverage would be recorded and archived in the US for all land areas of the globe. NASA competitively selected a Landsat Science Team, consisting of representatives from US universities and government agencies, to exploit the Landsat 7 record for global change research. This team is addressing the technical and analytical means to process and analyze the core of this observation record, and for the first time in the history of the Landsat mission, the technical and operational aspects of the mission are being driven by the goals of the US science community. The expected outcome of these efforts is a rapid improvement in understanding the Earth system, as well as conceptual knowledge that will underpin significant advancements in the application of this technology for commercial, operational, educational, and research purposes. Pathways to achieve effective Landsat continuity in the early decades of the 21st century are also being given careful attention, and there is no question that the lessons learned from the Landsat 7 mission will strongly influence these next-generation sensor systems.

Landsat-7 Long-Term Acquisition Plan: Development and Validation

The long-term acquisition plan (LTAP) was developed to fulfill the Landsat-7 (L7) mission of building and seasonally refreshing an archive of global, essentially cloud-free, sunlit, land scenes. The LTAP is considered one of the primary successes of the mission. By incorporating seasonality and cloud avoidance into the decision making used to schedule image acquisitions, the L7 data in the U.S. Landsat archive is more complete and of higher quality than has ever been previously achieved in the Landsat program. Development of the LTAP system evolved over more than a decade, starting in 1995. From 2002 to 2004 most attention has been given to validation of LTAP elements. We find that the original expectations and goals for the LTAP were surpassed for Landsat 7. When the L7 scan line corrector mirror failed, we adjusted the LTAP operations, effectively demonstrating the flexibility of the LTAP concept to address unanticipated needs. During validation, we also identified some seasonal and geographic acquisition shortcomings of the implementation: including how the spectral vegetation index measurements were used and regional/seasonal cloud climatology concerns. Some of these issues have already been at least partially addressed in the L7 LTAP, while others will wait further attention in the development of the LTAP for the Landsat Data Continuity Mission (LDCM). The lessons learned from a decade of work on the L7 LTAP provide a solid foundation upon which to build future mission LTAPs including the LDCM.

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

Abstract 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 Papers 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 (FAPAR) 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 FAPAR 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 FAPAR and NDVI.