VHF-band SAR Research Articles

A “Matchstick Model” of microwave backscatter from a forest

Given that the majority of biomass is contained in the stems of trees within forests (as much as 90%), a new radar modelling approach is proposed here wherein the stems are the major biomass contributor in the context of Synthetic Aperture Radar backscatter sensitivity to forest biomass. The new model regards stems are as “matchsticks” consisting of constant radius, constant density, and constant dielectric properties. Furthermore, by considering only the larger constituents of the forest a clearer understanding of the correlation of biomass with backscatter from P and VHF Band SAR can be obtained.Using backscatter data, and specifically the trends, produced from the RT2 radiative transfer model this paper considers the effects of SAR scattering when interacting with forest stands consisting of identical vertical stems, or matchsticks. SAR frequencies of 50MHz (VHF) and 429MHz (P Band) are used to generate a comparative radar interaction data. These not only allow a comparison of the scattering of SAR signals of different wavelengths but also of how interactions with stems can reveal novel clues as to the source of the backscatter saturation phenomenon of forests. Removing the random volume scattering aspect of a forest, the canopy, would be expected to eliminate the saturation behaviour which occurs at increasing levels of biomass density, an effect believed to be due to extinction effects, but is shown here to not be the case. Results reveal that saturation behaviour can occur even for the low number density case where increasing the size of stems through the radii associated with Rayleigh, Mie and Optical scattering will result in backscatter saturation as the biomass density is increased. Consistent with this finding, a forest of solely vertical stems will exhibit saturation behaviour at lower biomass density values when lower planting densities of similar stems are used. In this study the backscatter model considers the ground contribution to be negligible but does include the double bounce as a result of interactions between the stems and ground.Also considered are Mie backscatter oscillations which are shown to average out when using both non-vertical stems and random radius values about a mean, both independently and collectively. These “Mie” effects can also be removed by altering the incident SAR angle. These findings allow a reduction of the backscattering scenario of forests to a two-region problem involving solely Rayleigh and Optical scattering. This serves to further provide an explanation as to the origin of saturation, whereby the balance between the Optical scattering increase and the matchstick number density decrease cancel each other out.The important conclusion is that backscatter does not measure biomass, but measures structural trends that are correlated with biomass in different ways, driven by stand level competition, resource use, etc. As a result different forest growth regimes should expect different backscatter–biomass trends.

Signal-to-clutter ratio enhancement in bistatic very high frequency (VHF)-band SAR images of truck vehicles in forested and urban terrain

Multiple scattering is often a dominating scattering mechanism in VHF-band SAR, in particular in scenes including forested and urban terrain. Most important in this context is multiple scattering generated by double- or triple-bounce scattering from structures with orthogonal corners which often dominate for monostatic SAR geometries. Examples include double-bounce scattering from vertical tree stems or building walls on horizontal ground. In such situations, bistatic SAR may offer a significant advantage compared to monostatic SAR by choosing a suitable bistatic imaging geometry. We give experimental results which show increased signal-to-clutter ratio in bistatic VHF-based SAR (28-73 MHz). Results show that the signal-to-clutter ratio of a vehicle truck in a background of forest and urban clutter increases by up to 10 dB when the broadside bistatic elevation angle increases from 4 to 20°. The results indicate that significant improvement in detection performance can be expected for vehicle-sized objects in forest and urban terrain by using bistatic geometries in VHF-band SAR.

Validating backscatter models for CARABAS SAR images of coniferous forests

This paper describes a quantitative comparison of previously developed models for the relationship between CARABAS VHF-band SAR measurements and coniferous forest parameters. On level ground, four models of the backscatter have been studied. For stem volume retrieval, we propose using the normalized backscatter amplitude or the square root of the backscattering coefficient rather than the backscattering coefficient. The normalized backscatter amplitude and the square root of the backscattering coefficient show high correlation with stem volume over the investigated range. In addition, the standard deviation of multiple images of these two measures was found to be independent of the stem volume. For single-image retrievals, the estimated standard deviation in stem volume varies from 32 to 74 m3·ha-1 for forest on level ground, with stem volume ranging from 100 to 600 m3·ha-1. The precision is improved when multiple images with different headings are averaged, since the noise is statistically random between images. On sloping ground, the observed variations in backscatter of images with different headings are larger than those on level ground because the backscatter is affected by the topography. In previous work a model has been derived to describe the effect of a sloping ground plane on backscatter but was not validated due to lack of data. In this paper, 10 CARABAS images acquired from different directions are used to show that the model provides an adequate description of the effects of topography, i.e., the variations in backscatter that remain after using the model are equal to those observed on level ground.