Double-bounce Mechanism Research Articles

Insights Into Polarimetric Processing for Wetlands From Backscatter Modeling and Multi-Incidence Radarsat-2 Data

We have observed unexpected results using the Freeman–Durden (FD) and other polarimetric decompositions in Radarsat-2 quad-pol data from many swamps in Eastern Ontario. In particular, the decompositions reported minimal backscatter from the double-bounce mechanism in a situation where there was compelling evidence that double-bounce backscatter contributed substantially to the return. This led to a hypothesis that the FD and similar models give erroneous results because of the physics of Fresnel reflection of wood, a lossy dielectric material, that makes up the vertical reflecting surfaces in swamps. We found some support for this hypothesis in the literature, and now report on an extensive theoretical and observational investigation. This work has shown that the Freeman-Durden decomposition, and other decompositions that use the same logic, will often mistake double-bounce backscatter as single-bounce backscatter in wetlands. This is a consequence of the fundamental physics of Fresnel reflection. It is important for users to be aware of this pitfall. Double-bounce backscatter from natural surfaces can be identified without recourse to polarimetric decomposition. The simplest, and most reliable, indicator of double-bounce backscatter is a high return in HH polarization. Double-bounce backscatter will generally produce higher return in HH than any other scattering mechanism. If both HH and VV polarizations are available, a high HH/VV intensity ratio is also a strong indicator of double-bounce backscatter. Additional modeling efforts are expected to provide further insights that can lead to improved applications of polarimetric data.

Modelling temporal variations in microwave backscattering from reed marshes

ABSTRACTA full understanding of radar backscattering characteristics and their seasonal variations is one of the important ways to analyse the growth conditions in wetlands. This research simulated seasonal C-band and L-band synthetic aperture radar (SAR) backscattering from reed marshes using a refined version of the electromagnetic (REM) model, which was first validated by time series of multimode SAR observations at the experimental site used. Then, two factors including sensor parameters and vegetation structure, which influence the temporal evolution of the radar response from reed marshes, were assessed. The results demonstrate that the radar response is closely related to growth processes in the reed marsh. At the early growth stage when reed marshes are sparse, the double-bounce mechanism was dominant at all the incidence angles of C-band radar, but for L-band radar, strong specular reflection was produced from the smooth water if the incidence angle is lower than 25°. It was also found that the sensitivity to the density and height of reed marshes is greater for L-band radar than at the C-band, indicating that L-band backscattering may be useful for reed marsh biomass retrieval.

Effects of Inundated Vegetation on X-Band HH–VV Backscatter and Phase Difference

TerraSAR-X HH and HH-VV polarization data of a wetland in Estonia are used to study the effects of HH-VV phase difference on detecting inundated vegetation. Patches with different tree species composition are selected from two images representing flooded and unflooded conditions. Contribution of the double bounce mechanism is studied via phase difference and backscatter analysis. An increase in both HH and HH-VV backscatter was observed for all study cases in flooded conditions, as compared to unflooded periods. Over the different patches, HH backscatter increased from 2.3 dB to 8.2 dB, while for HH-VV - from 3 dB to 9.8 dB. The HH-VV channel allowed for a better separation between flooded and un flooded forest than HH, with the difference between the HH-VV and HH backscatter increase in inundated conditions ranging from 0.2 dB for coniferous stands with a tree height of less than 10 m to 1.6 dB for deciduous stands with a tree height of less than 10 m. A larger separation on both channels was in general observed in deciduous stands as the leaf-off season allowed for a better penetration depth. A considerable phase shift between HH and VV channels ranging from 10° to 28° was observed due to inundation, suggesting a strong contribution from the double bounce mechanism. For unflooded patches, the difference in backscatter between the two acquisition dates ranged from -0.8 dB to 1 dB for HH, and from 0 dB to 2 dB for HH-VV channel. Phase shift for unflooded patches varied from 5° to 8°.

A MODIFIED POLARIMETRIC DECOMPOSTION FOR APPLICABILTY IN COMPLEX AGRICULTURAL ENVIRONMENT

Abstract. A new model based decomposition on coherency matrix is demonstrated for the analysis of polarimetric SAR data. In this method a first order Bragg scattering model was fitted to explain the Bragg scatter from a moderately rough surface; randomly oriented dipole for explaining the volume scattering from a canopy layer; and a rotated dihedral model for pair of orthogonal scatters in Copol and Crosspol channels. The distinctive nature of this model is: fitting a rotated dihedral model to the coherency matrix which provides an ability to distinguish the same double-bounce mechanism occurs in small size bunded cropland and in orthogonal manmade structures. The model is applied to Radarsat-2 full polarimetric data which was acquired in a complex agricultural landscape. The result from decomposition was quite effective in reducing the scattered double-bounce pollutions from bunded trees surrounding croplands with extra enhancement in decomposition of orthogonal as well as oblique manmade structures.

High-resolution measurements of scattering in wheat canopies-implications for crop parameter retrieval

Polarimetric X- and C-band measurements by the University of Sheffield ground-based synthetic aperture radar (GB-SAR) indoor system provide three-dimensional images of the scattering processes in wheat canopies, at resolutions of around a wavelength (3-6 cm). The scattering shows a pronounced layered structure, with strong returns from the soil and the flag leaves, and in some cases a second leaf layer. Differential attenuation at horizontal (H) and vertical (V) polarization, due to the predominantly vertical structure of the wheat stems, gives rise to marked effects. At both C and X bands, direct return from the canopy exceeds the soil return at large incidence angles for VV polarization, but is comparable to or less than the soil return in all other cases. At HV, the apparent ground return is probably due to a double-bounce mechanism, and volume scattering is never the dominant term. Direct sensing of the crop canopy is most effective at X band, VV, and large incidence angles, under which conditions the return is dominated by the flag leaf layer. Field measurements with the outdoor GB-SAR system suggest, however, that for sensitivity to biomass and reduced susceptibility to disturbances by rainfall, a two-channel C-band system operating at a medium range of incidence angles is preferred.

A physical optics approximation to the range profile signature of a dihedral corner reflector

The frequency-domain field scattered from a perfect electrically conducting (PEC) 90/spl deg/ dihedral corner reflector is summarized. The inverse Fourier transform is used in conjunction with the frequency-domain description to obtain a high-frequency analytical approximation to the impulse response of the dihedral reflector. An amplitude modulated (AM) pulse is convolved with the impulse response of the target to yield the range profile signature. The range profile provides a one-dimensional image of the scatterer. This geometry was chosen because it is the most basic canonical shape that manifests the double-bounce mechanism. Numerical results presented show the relative importance of the single-bounce and double-bounce scattering mechanisms in the time-domain. Comparisons are made with range profiles computed using spectral windows. >