Jerrell R. Ballard

Effect of three-dimensional canopy architecture on thermal infrared exitance

We present a theoretical study of the effects of three- dimensional canopy structure on directional thermal infrared exitance. A physics-based model employing steady-state energy budget formula- tions is used to compute scene element temperatures. Two approaches are then used to combine soil and vegetation contributions to the com- posite scene response. One method uses a plane-parallel abstraction of canopy architecture to estimate canopy view factors for weighting of soil and vegetation emission terms. The second approach employs computer graphics and rendering techniques to estimate 3-D canopy view factors and scene shadows. Both approaches are applied to a test agricultural scene and compared with available measurements. The models cor- rectly estimate hemispherically averaged thermal infrared exitance to within experimental error with root-mean-square errors of 15.3 W m 22 for the 1-D model and 12.5 W m 22 for the 3-D model. However, the 1-D model systematically underestimates exitance at high sun angles. Ex- plicit modeling of canopy 3-D row structure indicates potential directional anisotropy in brightness temperature of up to 14°C.

Effect of spatial resolution on thermal and near-infrared sensing of canopies

Satellite observations of agricultural and other plant canopies in the thermal and near iR regime have generally been at spatial scales of tens to hundreds of meters. Advances in sensor technology will extend our capabilities for IR measurements from space to yield improved spatial resolutions. We explore the variability in brightness temperature and the covariation of the normalized difference vegetation index (NDVI) with brightness temperature as a function of viewing geometry and changing spatial resolution. Using 3-D models for both canopy reflectance and thermal infrared exitance, we employ a theoretical analysis for an agricultural scene. The directional viewing effects and correlation between the NDVI and brightness temperature are found to be scale independent and in agreement with experiment. Directional anisotropy in brightness temperature and NDVI are calculated to be less than 7 to 12% respectively for zenith view angles less than 30 deg, but range up to 22 to 40% for zenith view angles of 60 deg. Analysis of variation in local standard deviations with spatial resolution shows a maximum peak corresponding to crop row spacing with rapid fall-off at larger scales.

Thermal infrared hot spot and dependence on canopy geometry

We perform theoretical calculations of the canopy thermal infrared (TIR) hot spot using a first principles 3-D model described earlier. Various theoretical canopies of varying leaf size and for differing canopy height are used to illustrate the magnitude of the TIR effect. Our results are similar to predicted behavior in the reflective hot spot as a function of canopy geometry and comparable to TIR measurements from the literature and our own simple ground experiments. We apply the MODTRAN atmospheric code to estimate the at-sensor variation in brightness temperature with view direction in the solar principal plane. For simple homogeneous canopies, we predict canopy thermal infrared hot spot variations of 2 degrees C at the surface with respect to nadir viewing. Dependence on leaf size is weak as long as the ratio of leaf size to canopy height is maintained. However, the angular width of the hot spot increases as the ratio of leaf diameter to canopy height increases. Atmospheric effects minimize but do not eliminate the TIR hot spot at satellite altitudes.

Omicron: Rapid Mesh Generation on HPC Platforms for the Study of Near Surface Phenomena with Remote Sensing

A technique is presented for rapidly producing unstructured finite element meshes in support of large-scale remote sensing simulations. These tetrahedral meshes typically have more than one million elements and more than 250 thousand nodes, and allow for arbitrary placement of objects into the domain. Open-source mesh generation packages are used in conjunction with a tetrahedra element smoothing operation to achieve the desired final meshes. Meshes can be reproduced in less than 30 minutes on a CrayXT3 architecture.

Towards a High Temporal Frequency Grass Canopy Thermal IR Model for Background Signatures

In this paper, we present our first results towards understanding high temporal frequency thermal infrared response from a dense grass canopy. The model is driven by slowly varying, time-averaged meteorological conditions and by high frequency measurements of local and within canopy profiles of relative humidity and wind speed, and compared to high frequency thermal infrared observations. Previously, we have employed three-dimensional ray tracing to compute the intercepted and scattered solar and IR radiation fluxes and for final scene rendering. For the turbulent fluxes, simple resistance models for latent and sensible heat with one-dimensional profiles of relative humidity and wind speed are used. Our modeling approach has proven successful in capturing the directional and diurnal variation in background thermal infrared signatures. We hypothesize that at these scales, where the model is typically driven by time-averaged, local meteorological conditions, the primary source of thermal variance arises from the spatial distribution of sunlit and shaded foliage elements within the canopy and the associated radiative interactions. In recent experiments, we have begun to focus on the high temporal frequency response of plant canopies in the thermal infrared at 1 sec to 5 min intervals. At these scales, we hypothesize turbulent mixing plays a more dominant role. Our results indicate that in the high frequency domain, the vertical profile of temperature change is tightly coupled to the within canopy wind speed. In the results reported here, the canopy cools from the top down with increased wind velocities and heats from the bottom up at low wind velocities.

High-resolution soil moisture mapping using operational optical satellite imagery

Soil moisture conditions have an impact upon virtually all aspects of Army activities and are increasingly affecting its systems and operations. Soil moisture conditions affect operational mobility, detection of landmines and unexploded ordinance, military engineering activities, blowing dust and sand, watershed responses, and flooding. This study explores a novel method for high-resolution (2.7 m) soil moisture mapping using remote satellite optical imagery that is readily available from Landsat and QuickBird. The soil moisture estimations are needed for the evaluation of sensors for Improvised Explosive Devices (IEDs) using the Countermine Simulation Test Bed in regions where access is denied. The method has been tested in Helmand Province, Afghanistan, using a Landsat7 and a QuickBird image of April 23 and 24, 2009, respectively. The first implementation of the method yielded promising results.

Hyperspectral canopy reflectance modeling and EO-1 Hyperion

This paper describes our hyperspectral reflectance modeling of a forest canopy based on measured input parameters and comparison with Earth Observing - 1 (EO-1) Advanced Land Imager (ALI) and Hyperion data. The model uses a high resolution, three-dimensional (3D) ray-tracing approach to estimate the intercepted and scattered solar radiation at multiple narrow wavelength bands. We present the comparisons of the effects of woody biomass, leaf litter, and clumping on reflectance signatures. The experimental data used for the model were collected in a hardwood forest canopy in Rochester, New York. Model calculations also are compared to a more simplified, low-resolution 3D model and a simple, multi-layer differential equation model.

Sensor-based validation of synthetic thermal scenes: how close is good enough?

Using synthetic background scenes in the modeling of thermal infrared sensor-based smart munitions offers tremendous flexibility in exploring the performance envelope of these systems. However, to reach this goal, the synthetic background generation process must undergo the scrutiny of verification and validation to be accredited for use with a specific sensor system. Traditional approaches to validating synthetic scenes range from low-level subjective comparison to absolute pixel-to-pixel agreement between the two scenes. Neither of these approaches considers the specific smart munition sensor and processor which ultimately use the scene. In this paper we present an alternate validation approach based on comparison between end performance of a thermal infrared sensor-based smart munition system using synthetic/real scene pairs. Paired synthetic/real thermal scenes, including a low and a high-clutter level, are compared with conventional validation metrics and with the performance-based metric, using various smart munition sensor targeting algorithms. The degree of scene fidelity (absolute agreement between scene pairs) required to replicate performance varies with clutter level and processor algorithm. Under high clutter conditions, greater synthetic scene fidelity is required to match performance.

Integrated High-Fidelity Geoscience Simulations for Enhanced Terrain-Related Target Detection

A loosely coupled suite of physics-based geotechnical, geospatial, hydrogeologic, and thermal simulations are integrated into a virtual testing facility aimed at resolving terrain-related warfighter problems. This article describes the key aspects of the research conducted by the US Institute for Maneuverability and Terrain Physics Simulation and provides two examples of the virtual testing facility applied to subsurface threat detection.

High temporal and spatial thermal infrared characterization of dense grass during high-humidity conditions

This paper describes the high temporal (1 sec to 5 min) and spatial thermal infrared directional characterization of low dense grass canopy during high humidity conditions to study the diurnal and spatial variation of simple vegetation background signatures. The instruments used in the characterization effort consisted of two infrared cameras (8-14 μm) set at nadir and 45 degrees, four sets of radiometers (3-5 μm and 8-12 μm), micrometeorological instruments, and thermocouples placed within the grass. Micrometeorological measurements included wind speed, air temperature, and relative humidity observed at several heights above the canopy sampling occurred at 1 sec and 5 min intervals. These measurements were used to calculate wind speed, air temperature, and relative humidity profiles down to the top of the grass canopy. Analysis of the measured thermal images consists of quantifying the diurnal thermal differences in the directional background signatures, directional thermal variance, and thermal variance differences related to observation angle, solar radiation, and wind speed. These preliminary analyses indicate that for this environment, measurements at large temporal scales, the thermal variance is primarily affected by solar radiation, but at small temporal scales turbulent mixing of fluxes becomes the more dominant cause of the variance.