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We propose a model for generating low-frequency synthetic aperture radar (SAR) clutter that relates model parameters to physical characteristics of the scene. The model includes both distributed scattering and large-amplitude discrete clutter responses. The model also incorporates the SAR imaging process, which introduces correlation among image pixels. The model may be used to generate synthetic clutter for a range of environmental operating conditions for use in target detection performance evaluation of the radar and automatic target detection/recognition algorithms. We derive a statistical representation of the proposed clutter model's pixel amplitudes and compare with measured data from the CARABAS-II SAR. Simulated clutter images capture the structure and amplitude responses seen in the measured data. A statistical analysis shows an order of magnitude improvement in model fit error compared with standard maximum-likelihood (ML) density fitting methods.

In this paper, a model for prediction of radar backscatter from coniferous forests in the VHF and UHF band is proposed. The model includes the double-bounce scattering originating from vertical stems standing on an undulating ground surface and is based on a physical-optics approach. The model can be used to assess the importance of ground topography in synthetic aperture radar (SAR) imagery of forests, and it is applicable to SAR systems using horizontally transmit and receive polarization (HH). The model was validated against data from the airborne SAR systems CARABAS-II and LORA. Precision measurements of ground topography and forest characterization at a single tree level were used as model input to simulate SAR images. The simulated images were compared to radar data in the frequency bands 22-82 and 225-470 MHz, and it was found that the model could predict much of the variation in backscatter observed in images ([R.sup.2] = 0.44 and 0.65 at best, for the lower and higher frequency band, respectively), which should be compared to [R.sup.2] [approximately equal to] 0.1 if the same model, but assuming a flat ground, was used. The results thus indicate that ground topography must be considered when predicting the variations in backscatter in the SAR images studied. The model did, however, fail to predict the absolute values of the backscattered intensity. The reason for the discrepancy is believed to be the value chosen for stem dielectric constant and unmodeled effects due to wave attenuation, tilting stems, and small-scale surface roughness. Index Terms--Forest, model, physical optics (PO), surface scattering, synthetic aperture radar (SAR), UHF radar, VHF radar.

The objective of this paper is to examine the link between the attenuation coefficients and the interferometric phase center heights, for several frequencies from P- to L-band, and to study the extent to which it depends on the canopy architecture and description. This paper relies on the use of a coherent and full polarimetric scattering model, which simulates the fields backscattered by a forested area. In the first part, we study the behavior with a frequency of the interferometric phase center heights, and in the second part, we focus on the attenuation coefficients. Then, we compare the behaviors of these two quantities, and we propose to empirically derive a relation between these two quantities and the mean forest height. Finally, we investigate if a change in the initial forest or radar configuration has an impact on the determination of this relation. Index Terms--Attenuation coefficients, forest, interferometry, modeling, P- and L-bands, synthetic aperture radar (SAR).

This study describes the use of a plant structure model from the field of macroecology to make generalized predictions about backscatter-biomass and height-biomass trends from synthetic aperture radar data over forests. A theoretical relationship between canopy height and biomass density is derived. The predicted values of the height exponent are comparable with those from the remote sensing literature. A radiative transfer model parameterized by the macroecology model is also used to predict trends in P-band backscatter. The results imply that backscatter can saturate even for low-opacity canopies and decreasing basal area results in backscatter saturating at lower biomass levels. The theoretical analysis is supported by reference to a range of published results. Index Terms--Biomass, forestry, synthetic aperture radar (SAR), vegetation.

This paper assesses the accuracy and reliability of tree height retrieval over coniferous plantations using X-band interferometry. Factors such as crown shape, density, tree height, incidence angle, and slope have been assessed and quantified using a simple polarimetric radar interferometry simulator to determine their impact on height retrieval. Results from model simulation show that the most important factors are: crown shape, plantation density, and tree height. Variation in viewing angle and small slopes (< 30[degrees]) appear to have only small effects. These results appear to be in reasonably good agreement with the retrieved tree height from airborne X-band Intermap data over coniferous plantations in the U.K. Index Terms--Forest inventory, signal coherence, synthetic aperture radar (SAR) interferometry, tree height retrieval.

Garlic mustard (Alliaria petiolata) is an invasive, spring-germinating, obligately biennial plant. In a central Pennsylvania forest, two distinctive plant patch types exist simultaneously: patches with mixed juveniles and adults and patches that are exclusively juvenile. We experimentally tested two hypotheses to explain this peculiar distribution. The habitat limitation hypothesis proposes that garlic mustard, like other biennials, is limited to rarely available habitats and can only reach maturity in a subset of the patches where its seeds germinate. The intraspecific competition hypothesis proposes that juveniles only survive to maturity in patches lacking adults. We mapped the natural distribution of ten juvenile and ten mature patches and used transects to assess juvenile and adult density in 2 y. We found that patches cycled annually from juvenile to mixed, supporting the intraspecific competition hypothesis. We looked for biotic (interspecific competition) and abiotic (shading) factors that might contribute to habitat limitation, but none affected juvenile performance. We also looked for evidence of intraspecific competition by monitoring juvenile growth in juvenile, mixed and mixed with adults-removed patches. We found that juveniles from juvenile patches have a higher probability of survival than juveniles from mixed patches and adult removal improved juvenile survival in mixed patches. We conclude that the spatial and temporal segregation of juvenile and adult age classes is maintained by intraspecific competition.

The sensitivity of radar backscatter to the amount of intercepted rain in temperate deciduous forests is analyzed to determine the feasibility of retrieval of this parameter from satellite radar data. A backscatter model is validated with X-band radar measurements of a single tree exposed to rain. A good agreement between simulation and measurements is observed and this demonstrates the ability of radar to measure the amount of intercepted rain. The backscatter model is next applied to simulate different satellite radar configurations. To account for forest variability, the backscatter difference between a wet and dry forest canopy is calculated for four deciduous tree species, above a wet and a dry soil. On average, the simulated backscatter of a wet forest canopy is 1 dB higher than the backscatter of a dry forest canopy at co-polarized L-band and 2 dB at co-polarized C and X-band. The simulated sensitivity is in agreement with observations. It is argued that current satellites can retrieve the amount of intercepted rain at best with a reliability of 50%, due to the variability in soil moisture, species composition and system noise. We expect that the reliability will improve with the launch of the next generation radar satellites. The results of this analysis may also be used to assess the influence of rain, fog or dew upon other radar applications for temperate deciduous forests. Index Terms--Dew, fog, forest, interception, radar, rain.

RADARSAT synthetic aperture radar (SAR) data acquired at C Band, HH polarization, and for the 20 [degrees]-27 [degrees] and 45 [degrees]-49 [degrees] incidence angle ranges were available over northern Quebec, Canada, (54 [degrees] N, 72 [degrees] 12'W), in the fall of 1996, the winter of 1997, and the spring of 1997. The main land occupation of this area is sparse black spruce (Picea mariana) forests. Vegetation characteristics are jointly used with snow and soil observations coinciding with the satellite overpasses to simulate the seasonal changes in the backscattering coefficient of the subarctic forest. The aim of this study is twofold. First to evaluate the effects of the seasonal changes in vegetation on the RADARSAT SAR data, and second to use backscattering models as a tool for a better interpretation and understanding of the RADARSAT SAR data over snow-covered forested areas. Simulations show the importance of the surface-vegetation interaction term and the wet snow surface roughness on the discrimination between open forest and denser forest, and on the contrast between wet snow and dry snow covers. When comparing the simulations to the RADARSAT SAR data, the poorest results are obtained in the spring for a rough wet snow. It is shown that they are mainly due to a crude evaluation of the vegetation dielectric constant rather than to uncertainties introduced by the spatial variability in the wet snow surface roughness. Index Terms--Dry snow, RADARSAT (synthetic aperture radar (SAR) data, seasonal changes, subarctic forest, surface-vegetation interaction term, wet snow, wet snow surface roughness.