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General formulation of resonant backscattering from vegetation, mean field and Green's function in three media, and electromagnetic backscattering coefficients from a layer of vegetation are discussed.
Electromagnetic backscattering from a layer of vegetation over a flat lossy ground is studied. The vegetated region is composed of leaves which are modeled by lossy circular dielectric discs with prescribed orientation statistics. The mean field in the layer is computed by using the Foldy approximation and a two-variable technique which is valid for small fractional volume. Simple expressions are given for the skin depth of the mean wave. Following this, the distorted Born approximation is used to calculate the co-polarized and cross polarized backscattering coefficients. These are then computed as a function of angle of incidence for different layer depths and leaf configurations.
The wind-scale relationships for L band radar wavelengths near 25 cm and 20 deg angle of incidence and HH polarization are reviewed using a number of aircraft and Seasat-1 SAR observations. The dependence of the L band backscatter coefficient from the ocean upon surface wind speed and direction is stated. The wind speed coefficient is 0.5 + or - 0.1 for a wide range of wind speeds. The wind direction coefficient is near zero for lower winds and stable marine boundary layers, but may be 0.20 + or - 0.05 for moderate wind speeds and an unstable marine boundary layer. These results are interpreted in terms of existing theoretical models for radar scattering from the ocean.
In connection with studies concerning the use of an orbiting optical radar (lidar) to conduct aerosol and cloud measurements, attention has been given to the accuracy with which lidar return signals could be measured. However, signal-measurement error is not the only source of error which can affect the accuracy of the derived information. Other error sources are the assumed molecular-density and atmospheric-transmission profiles, and the lidar calibration factor (which relates signal to backscatter coefficient). The present investigation has the objective to account for the effects of all these errors sources for several realistic combinations of lidar parameters, model atmospheres, and background lighting conditions. In addition, a procedure is tested and developed for measuring density and temperature profiles with the lidar, and for using the lidar-derived density profiles to improve aerosol retrievals.
Experimental measurements of canopy attenuation at 10.2 GHz (X-band) for canopies of wheat and soybeans, experimental observations of the effect upon the microwave backscattering coefficient (sigma) of free water in a vegetation canopy, and experimental measurements of sigma (10.2 GHz, 50 deg, VV and VH polarization) of 30 agricultural fields over the growing season of each crop are discussed. The measurements of the canopy attenuation through wheat independently determined the attenuation resulting from the wheat heads and that from the stalks. An experiment conducted to simulate the effects of rain or dew on sigma showed that sigma increases by about 3 dB as a result of spraying a vegetation canopy with water. The temporal observations of sigma for the 30 agricultural fields (10 each of wheat, corn, and soybeans) indicated fields of the same crop type exhibits similar temporal patterns. Models previously reported were tested using these multitemporal sigma data, and a new model for each crop type was developed and tested. The new models proved to be superior to the previous ones.
Reflecting metal screens were placed beneath vegetation of three types to allow measurement of the brightness temperature of the vegetation cover alone. A two-frequency radiometer (wavelengths: 11.1 and 5.88 cm) was used to measure the emission from the vegetation, and a theoretical model involving a loss term and a scattering albedo was fitted to the measured temperatures. Passive and active microwave measurements were also made on fields immediately adjacent to the 'screened' fields. Emission and backscattering were measured for these fields under two conditions: with full vegetation cover, and with vegetation removed. Using the same theoretical model and the calculated values for absorption and scattering derived from the screened fields, the effect of the vegetation cover on the soil emission was predicted. The predicted full-canopy temperatures were then compared to those measured. The values for loss and scattering in the vegetation canopy were used in another model to predict the effect of vegetation on backscattering from the soil surface. Again, the predicted backscattering from the vegetation canopy was compared with the measured values of the backscattering coefficient.
Radar provides a valuable tool in the study of sea-ice conditions and the solution of sea-ice operational problems. For this reason, the U.S. and Canada have conducted studies to define a bilateral synthetic aperture radar (SAR) satellite program. The present paper is concerned with work which has been performed to explore the needs associated with the study of sea-ice-covered waters. The design of a suitable research or operational spaceborne SAR or real aperture radar must be based on an adequate knowledge of the backscatter coefficients of the ice features which are of interest. In order to obtain the needed information, studies involving the use of a helicopter were conducted. In these studies L-C-X-Ku-band calibrated radar data were acquired over areas of Arctic first-year and multiyear ice during the first half of the summer of 1982. The results show that the microwave response in the case of sea ice is greatly influenced by summer melt, which produces significant changes in the properties of the snowpack and ice sheet.
Active and passive microwave remote sensing of earth terrains is studied. Electromagnetic wave scattering and emission from stratified media and rough surfaces are considered with particular application to the remote sensing of soil moisture. Radiative transfer theory for both the random and discrete scatterer models is examined. Vector radiative transfer equations for nonspherical particles are developed for both active and passive remote sensing. Single and multiple scattering solutions are illustrated with applications to remote sensing problems. Analytical wave theory using the Dyson and Bethe-Salpeter equations is employed to treat scattering by random media. The backscattering enhancement effects, strong permittivity fluctuation theory, and modified radiative transfer equations are addressed. The electromagnetic wave scattering from a dense distribution of discrete scatterers is studied. The effective propagation constants and backscattering coefficients are calculated and illustrated for dense media.
The airborne L-band synthetic aperture radar (SAR) collected multipolarization calibrated image data over an irrigated agricultural test site near Fresno, CA, on March 6, 1984. The conclusions of the study are as follows: (1) the effects of incidence angle on the measured backscattering coefficients could be removed by using a correction factor equal to the secant of the angle raised to the 1.4 power, (2) for this scene and time of year, the various polarization channels were highly correlated such that the use of more than one polarization added little to the ability of the radar to discriminate vegetation type or condition; the exception was barley which separated from vineyards only when a combination of like and cross polarization data were used (polarization was very useful for corn identification in fall crops), (3) an excellent separation between herbaceous vegetation (alfalfa, barley, and oats) or bare fields and trees in orchards existed in brightness was well correlated to alfalfa height or biomass, especially for the HH polarization combination, (5) vineyards exhibited a narrow range of brightnesses with no systematic effects of type or number of stakes nor of number of wires in the trellises nor of the size of the vines, (6) within the orchard classes, areal biomass characterized by basal area differences caused radar image brightness differences for small to medium trees but not for medium to large trees.
Potential users of radar imagery need a better fundamental understanding of the capabilities of radar systems for vegetation studies than past studies provide. One approach is the use of theoretical models to predict observable active microwave properties of vegetation. This in turn requires accurate observations of backscattering coefficients and other active microwave properties in field research studies. The background document for the SRAEC program emphasizes the need to relate electromagnetic parameters to classical biophysical descriptors and to understand the role of polarization, especially cross-polarization. The broad goal of this study is to increase the understanding of the effects of canopy structure on the active microwave properties of vegetation canopies, with particular attention to polarization.
Two series of carefully controlled experiments were conducted. First, plots of important crops (corn, soybeans, and sorghum), prairie grasses (big bluestem, switchgrass, tal fescue, orchardgrass, bromegrass), and forage legumes (alfalfa, red clover, and crown vetch) were manipulated to produce wide ranges of phytomass, leaf area index, and canopy architecture. Second, coniferous forest canopies were simulated using small balsam fir trees grown in large pots of soil and arranged systematically on a large (5 m) platform. Rotating the platform produced many new canopies for frequency and spatial averaging of the backscatter signal. In both series of experiments, backscatter of 5.0 GHz (C-Band) was measured as a function of view angle and polarization. Biophysical measurements included leaf area index, fresh and dry phytomass, water content of canopy elements, canopy height, and soil roughness and moisture content. For a subset of the above plots, additional measurements were acquired to exercise microwave backscatter models. These measurements included size and shape of leaves, stems, and fruit and the probability density function of leaf and stem angles. The relationships of the backscattering coefficients and the biophysical properties of the canopies were evaluated using statistical correlations, analysis of variance, and regression analysis. Results from the corn density and balsam fir experiments are discussed and analyses of data from the other experiments are summarized.
Aerosol concentration profiles were measured by an aerosol spectrometer above the landing sites of the Vega 1 and Vega 2 landers. Approximately the same altitude zones were found as in previous experiments: a three-layered basic cloud cover, an intermediate zone and subcloud haze. There were significant quantitative differences in the concentrations of particles, however, and especially in the spectra of their dimensions. Nightglow was found in the troposphere of Venus at a wavelength of about 1 micron. The backscatter coefficient and the extinction coefficient change very little between 32 and 63 km. Large numbers of submicron particles apparently exist in the atmosphere above the landing sites.
The stratospheric volcanic cloud from the eruption of El Chichon, Mexico, on April 4, 1982 was observed routinely by a Nd:YAG lidar system from April 18, 1982 at Kyushu University. There were two phenomena detected by lidar observation of the El Chichon volcanic cloud: (1) The main enhanced layer was formed in the summer easterly wind region over Fukuoka (33.5 deg N, 130.4 deg E). The layer had two high gradient top and bottom boundaries. The layer width slowly broadened until September 1982, when the easterly wind changed to the westerly wind. A similar phenomenon was observed after the eruption of Mt. St. Helens in May 1980. (2) The stratospheric integrated backscattering coefficient (IBC) reached a peak value on May 3, 1982. It then gradually decreased until August, but reincreased significantly from September to December 1982. After January 1983, IBC decreased slowly. This remarkable reincrease had not been seen in past increases which had been observed by lidar at Kyushu University. These phenomena are briefly discussed.
This paper presents preliminary results of C-band radar scatterometer measurements of forest canopies of southeastern forests in the vicinity of NASA/NSTL. The results are as follows: (1) the radar backscattering coefficients (BSC) of deciduous forests such as oak, maple, blackgum, and cypress are higher than those of coniferous forests such as slash pine plantation and natural pine; (2) at a large incidence angle, where polarization effect is significant, and by ranging measurement, the VV polarization BSC obtain peak value at the first few meters from the canopy top and decrease rather quickly, while the HH polarization BSC obtain peak value at longer distances from the canopy top and decrease rather slowly through the canopy; and (3) using the active radar calibrator for tree canopy attenuation measurement of a dense and a sparse live oak, it is found that the tree canopies with higher attenuations have higher BSC for all three polarizations, with VV polarization containing the largest differential (2.2 dB).
Models for scattering from a vegetation layer treated as a collection of discrete scatterers usually assume far field interaction among scatterers. In a real vegetation medium such as a deciduous forest it is not always true that each leaf is in the far field of the others. This paper examines the additional effect when scatterers are permitted to be in the Fresnel zone. It is found that this in general causes the backscattering coefficient to be higher and also to have a larger rate of increase with frequency than the case of purely far field interaction.
Data collected by the Shuttle Imaging Radar-B (SIR-B) on two ascending orbits over a site in west-central Illinois were used to establish end-to-end transfer functions for conversion of the digital numbers on the eight-bit image to values of the radar backscattering coefficient, sigma(0). The transfer function for each data take was defined by the SIR response to an array of six calibrated point targets of known radar cross section and to a large number of area-extended targets, also with a known cross section, as measured by externally calibrated, truck-mounted scatterometers. In spite of variance in the SIR transmit power, end-to-end transfer functions generated from either point targets or area-extended targets were found to yield close agreement for a given pass, indicating that targets of known radar cross section can be used to provide transfer functions for orbital SAR. Although the use of area-extended targets yielded the most satisfactory results, the use of point targets is far simpler and may be the preferred alternative for many situations.
Two predawn ascending data-takes by the Shuttle Imaging Radar-B (SIR-B) were used to evaluate the effects of surface roughness, crop canopy, and soil moisture on radar backscatter. The two images, separated by three days, were both obtained at 30-deg local angle of incidence, but with opposite azimuth viewing directions. The imagery was externally calibrated with respect to the radar backscattering coefficient sigma(0) via response to arrays of point and area-extended targets of known radar cross section. Three land-cover classes: (1) corn, (2) corn stubble and plowed bare soil, and (3) disked bare soil, soybeans, soybean stubble, alfalfa, and clover could be readily separated for either observation date on the basis of image tone alone. The dependence of sigma(0) on the surface roughness and canopy brightness inhibits the capability of SIR to globally estimate the near-surface soil moisture from the value of sigma(0) for single date observations, unless the surface roughness or canopy cover conditions are accounted for. However, within given ranges of these conditions, the sigma(0) was found to be highly correlated with the soil moisture.
Lidar measurement on Antarctic aerosols were made during the Antarctic Middle Atmosphere (AMA) period, 1983 to 1985, at Syowa Station. Topics measured are winter enhancement aerosol layer and volcanic effect of El Chichon on the Antarctic stratosphere aerosols. The large depolarization ratio (maximum value was about 0.8) seems to support sublimation growth of ice crystals. The lidar measurements showed a meaningful time lag between aerosol content increase and depolarization ratio increase. Considering the balloon observations made in early winter, it was speculated that an increase in large particle number concentration also contributed to the winter enhancement. The El Chichon cloud spread to the Antarctic region by the beginning of 1983. The temporal change of integrated backscatter coefficient shows a clear decay pattern, although strong winter enhancement superposes.