N.S. Chauhan

A microwave scattering model for layered vegetation

A microwave scattering model was developed for layered vegetation based on an iterative solution of the radiative transfer equation up to the second order to account for multiple scattering within the canopy and between the ground and the canopy. The model is designed to operate over a wide frequency range for both deciduous and coniferous forest and to account for the branch size distribution, leaf orientation distribution, and branch orientation distribution for each size. The canopy is modeled as a two-layered medium above a rough interface. The upper layer is the crown, containing leaves, stems, and branches. The lower layer is the trunk region, modeled as randomly positioned cylinders with a preferred orientation distribution above an irregular soil surface. Comparisons of results obtained using this model with measurements from deciduous and coniferous forests show good agreement at several frequencies for both like and cross polarizations. >

Radar modeling of a boreal forest

The authors report on the use of microwave modeling, ground truth, and synthetic aperture radar (SAR) data to investigate the characteristics of forest stands. A mixed coniferous forest stand has been modeled at SAR frequencies (P-, L-, and C-bands). The extensive measurements of ground truth and canopy geometry parameters were performed in a 200 m-square hemlock-dominated plot inside a forest. Hemlock trees in the forest are modeled by characterizing tree trunks, branches, and needles (leaves) with randomly oriented, lossy dielectric cylinders whose area and orientation distributions are prescribed. The distorted Born approximation is used to compute the backscatter at P-, L-, and C-SAR frequencies. >

Mapping of boreal forest biomass from spaceborne synthetic aperture radar

As part of the Boreal-Ecosystem Atmosphere Study (BOREAS), an investigation is being made of the use of satellite data including shuttle imaging radar-C (SIR-C), X-band synthetic aperture radar (XSAR), and Landsat-Thematic Mapper data for estimating total and component aboveground woody biomass in boreal forest study sites in Canada. The goal of this paper is to present progress in mapping above ground woody biomass over portions of the BOREAS southern study area using spaceborne sensor data. Relationships of backscatter to total biomass and total biomass to foliage, branch, and bole biomass are used to estimate biomass across the landscape. The procedure involves image classification with SAR and Landsat data and development of simple mapping techniques using combinations of SAR channels. The analysis uses measurements from forest stands representing a range of biomass and structures. Field measurements included plot level mensuration (species, stem diameter, height, density, and basal area) and tree geometry measurements (leaf, branch, bole size, and angle distributions). The results indicate that aboveground biomass can be estimated to within about 1.6 kg/m2 and up to about 15 kg/m2 across the SIR-C image evaluated. A general method produced equivalent results with those obtained by treating forest type (pine, spruce, and aspen) separately. The biomass mapping was extended to bole, branch, and foliage components from relationships with total aboveground biomass developed from detailed tree measurements. Average biomass within the imaged area was estimated to be about 7.3 kg/m2 with biomass components of bole, branch, and foliage comprising 83, 12, and 5% of the total. Examination of the scaling of biomass estimates from remote sensing images of varying resolution shows that information at scales useful for ecosystem models can be obtained. In addition, the biomass estimation technique provides similar information at different image resolutions.

Discrete scatter model for microwave radar and radiometer response to corn: comparison of theory and data

As part of the Multisensor Aircraft Campaign, MACHYDRO, two microwave sensors, NASAs Airborne Synthetic Aperture Radar (AIRSAR) and Pushbroom Microwave Radiometer (PBMR) collected data over the same corn fields during the summer of 1990. During these flights, measurements were made on the ground of soil moisture and plant parameters. In this paper the measured canopy and soil parameters are used in a discrete scatter model to predict the response of both sensors (radar and radiometer). A distorted Born approximation is used to compute the scattering coefficient for the corn canopy. The backscatter coefficient gives the radar response and the radiometer response is obtained by integrating the bistatic coefficient over all scattering angles above ground. The objective of this analysis is to test the model and, in particular, to determine how well a single set of plant parameters and single model can yield agreement with both the radar and radiometer measurements. The model values are in reasonably good agreement with the measurements at horizontal polarization and reflect observed changes in soil moisture. >

L-band radiometer measurements of conifer forests

Airborne radiometer measurements were made at L-band over conifer forests in Virginia to study radiometer response to biomass and soil moisture. The horizontally polarized synthetic aperture radiometer, ESTAR, was deployed aboard a NASA-P3B Orion aircraft. Measurements were made in July, August and November of 1999 over relatively homogeneous conifer stands of varying biomass. Concurrently with the aircraft measurements, soil moisture measurements were made in several of the imaged stands. The images of the area show a strong correlation between forest stand biomass and radiometric brightness temperature. In addition, stands growing in soils with poor drainage were identifiable on the image.

Polarization utilization in the microwave inversion of leaf angle distributions

The inverse problem of deducting the inclination angle distribution of leafy vegetation has been investigated using L-band multipolarization backscattered data. The modeling procedure replaces canopy leaves with thin circular dielectric disks. The Born approximation is then used to establish a linear relationship between the radar backscattering coefficients and the leaf inclination angle distribution. The inversion of the leaf angle distribution is carried out for horizontal, vertical, and cross-polarized data. It is shown that the results of the inversion using vertical and cross-polarized data are comparable to the inversion results of horizontally polarized data obtained previously (R. Lang and H. Saleh, 1985). >

Effect of species structure and dielectric constant on C-band forest backscatter

A joint experiment between Canadian and USA research teams was conducted early in October, 1992 to determine the effect of species structure and dielectric variations on forest backscatter. Two stands, one red pine and one jack pine, in the Petawawa National Forestry Institute (PNFI) were utilized for the experiment. Extensive tree architecture measurements had been taken by the Canada Centre for Remote Sensing (CCRS) several months earlier by employing a total station surveying instrument which provides detailed information on branch structure. A second part of the experiment consisted of cutting down several trees and using dielectric probes to measure branch and needle permittivity values at both sites. The dielectric and the tree geometry data have been used in the George Washington University (GWU) Vegetation Model to determine the C-band backscattering coefficients of the individual stands for W polarization. The model results show that backscatter at C-band comes mainly from the needles and small branches and the upper portion of the trunks acts only as an attenuator. The paper concludes with a discussion of variation of backscatter with species structure and how dielectric variations in needles for both species may affect the total backscatter returns.<<ETX>>

L-Band Active and Passive Sensing of Soil Moisture through Forests

Remote sensing of soil moisture under a mature forest canopy by active and passive techniques is discussed. Emphasis is placed on examining the sensitivity of the backscattering coefficient and the brightness temperature to soil moisture variations. The effects of the underlying surface roughness of the forest floor on these sensitivities are addressed. The backscattering coefficient from the forest is modeled using the distorted Born approximation. Using this method the backscatter response is decomposed into a direct or volume scattering component, an interaction component between the vegetation and the average surface, an interaction component between the vegetation and the surface fluctuations and finally direct backscatter from the surface attenuated by the vegetation. Peaks method is used to determine the brightness temperature. The models are validated using ground truth and remote sensing data taken during the Forest Ecosystem Dynamics (FED) experiment conducted near Rowland, Maine, USA in 1990. During this period, the AIRSAR synthetic aperture radar and the Push Broom Microwave Radiometer overflew the Rowland area collecting data. Once the model predictions are compared with the passive and active data, the results of a sensitivity analysis involving soil moisture and surface roughness will be presented

Spectrum management for the NPOESS Conical-scanning Microwave Imager/Sounder (CMIS)

The Conical-scanning Microwave Imager/Sounder (CMIS) currently under development by the National Polar-orbiting Operational Environmental Satellite System (NPOESS) Integrated Program Office utilizes segments of microwave spectrum from /spl sim/6 GHz to above 190 GHz. During the development and optimization of the current CMIS baseline design, the Earth Exploration Satellite Service (EESS) (Passive) frequency allocations, and estimates of the expected RF environment played a substantial role in determining channelization. This will allow CMIS performance requirements to be met while minimizing the potential for radio frequency interference occurrences to CMIS.

Multi-stand radar modeling from a boreal forest: Results from the BOREAS Intensive Field Campaign-1993

A Boreal Ecosystem Atmosphere Study (BOREAS) Intensive Field Campaign was conducted in August 1993 to study portions of the Canadian boreal forest. The overall project has a strong remote sensing and modeling component to assess and monitor important ecosystem attributes as they relate to carbon, water and energy fluxes. The objective of the work is to assess the capabilities of multi-frequency and multi-polarization synthetic aperture radar (SAR) to remotely sense forest stands of different species and age. NASA/JPLs airborne SAR was flown over forested areas reasonably pure stands of old and young aspen (broad-leaf) and jack pine (needle-leaf) trees. Ground truth, stand characteristics and tree architecture data were collected over the period of the experiment from four stands (one stand for each species and age type). L and C-band portable dielectric probes were used to measure the dielectric properties of the trees. The model parameters of each stand have been derived from this data set. They have been used in the GWU vegetation model to compute the backscattering coefficients. At L-band, model calculations show that large HH backscatter from the old jack pine stand is due to double bounce scattering from tree trunks while VV backscatter arises from direct backscatter from the tree canopy. It is also observed that L-band like and cross polarization ratios tracked biomass for old and young jack pine stands. These findings have been confirmed by comparison with AIRSAR data.<<ETX>>