H. Israelsson’s research while affiliated with Chalmers University of Technology and other places

A coherent scattering model to determine the forest radar backscattering at VHF frequencies (20-90 MHz) has been developed. The motivation for studying this frequency band is the recent development of the CARABAS Synthetic Aperture Radar (SAR). In order to model the scattering from branches and trunks, homogeneous dielectric cylinders placed above a semi-infinite di-electric ground have been analyzed. An analytical approach, where the theoretically exact currents induced in an infinite cylinder are truncated, has been compared to a numerical solution using the finite difference time domain (FDTD) method. If the first-order coherent ray tracing is included in the analytical approach, the results match well with the numerically exact FDTD solution. The results show that, in order to determine the VHF-backscattering from a forest stand, the coherent ground interaction is an important part and has to be considered. In this paper, modeling results are in good agreement with CARABAS measurements

The CARABAS system is a wide band SAR, that operates at VHF frequencies (20-90 MHz). The image information includes a higher dynamic range in the radar backscattering from forested areas of varying stem volume, than what has been found using conventional microwave SAR. A modified MIMICS model has been used to model the radar backscattering at VHF. The various terms in the solution have been added coherently, taking the variation with height above ground and the phase shift of the ground reflection into account. The results show an improved agreement with measured data

The increasing interest for forest backscattering in the VHF band is stimulated by recent results from the CARABAS (20-90 MHz) SAR. Potential applications include forest biomass estimation and concealed target detection. For understanding the forest backscatter mechanisms it is important to develop scattering models and assess the limits of validity. Special consideration should be made to the fact that typical trunk and branch scattering is in the resonance region and that multiple scattering effects are important. A numerical method (FDTD) has been implemented which can be used to validate analytical scattering models. It features a five-layer PLM boundary condition, a far-field transformation method based on the reciprocity theorem, and an optional dielectric ground surface. It has been used to assess a coherent reflection model of a horizontal cylinder above dielectric ground. The results indicate an excellent agreement between the model and the FDTD method

The ability to retrieve forest stem volume using CARABAS (coherent all radio band sensing) SAR images (28-60 MHz) has been investigated. The test site is a deciduous mixed forest on the island of Oland in southern Sweden. The images have been radiometrically calibrated using an array of horizontal dipoles. The images exhibit a clear discrimination between the forest and open fields. The results show that the dynamic range of the backscattering coefficient among the forest stands is higher than what has been found with conventional SAR using microwave frequencies. The backscatter increases with increasing radar frequency. This work shows an advantage compared to higher frequencies for stem volume estimation in dense forests

MAESTRO I data from the Flevoland lest site in the Netherlands have been used for this study. From a complementary ground data collection, the bole volumes of a large number of stands of mainly poplar and ash have been estimated. The relationships between radar backscattering and bole volumes have been examined experimentally and theoretically. In the case studied, the radar backscattering sensitivity to bole volumes increases as the wavelengths increase. and is highest at P band. The sensitivity of the radar backscattering to variations of forest canopy components and moisture contents has been investigated theoretically at P band. It is important to obtain information on such variations before the inversion problem can be solved. The present study indicates a potential for bole volume determination by P-band SAR.

The radar backscattering from extraordinary poplar stands, where all the trunks are leaning in the same direction, has been modelled. Data acquired with the JPL/AIRSAR during the MAESTRO-I campaign have been used. At the C and L bands the back scattering is not influenced by the leaning of the trunks. At the P band the backscattering decreases at HH polarization particularly. This can be predicted theoretically and is explained by distortion of the trunk-ground dihedral reflection. A similar effect is obtained when topography affects the backscattering.

The radar backscattering from extraordinary poplar stands, where all the trunks are leaning towards the same direction, has been modelled. Data acquired with the JPL/AIRSAR during the MAESTRO-1 campaign have been used. At C- and L-band the backscattering is not influenced by the leaning of the trunks. At P-band the backscattering decreases at HH polarisation particularly. This can be predicted theoretically and is explained by distortion of the trunk-ground dihedral reflection. A similar effect is obtained when topography affects the backscattering

Discusses the information content in satellite SAR images of forested areas. As methodology, the authors study temporal changes in ERS-1 (C-band) and JERS-I (L-band) images. Three ERS-1 scenes and five JERS-1 scenes are used. Experience from the MAESTRO-1 data set demonstrates a higher sensitivity of L-band to biomass attributes, while C-band images are not that sensitive to such parameters. A multi-temporal C-band analysis over a year helps however to discriminate between deciduous and coniferous stands and to detect changes in moisture conditions. The test area is located 20 km north-west of Umea, a town in northern Sweden by the Gulf of Bothnia. The forest consists mainly of naturally generated Norway spruce and Scots pine with ages up to 150 years. Birch is the only prevalent broadleaved species. Maximum values of tree heights and volumes are 25 m and 300 m³/ha respectively. Most of the area is located in flat terrain. Stand sizes vary between 3 and 25 ha. Eleven stands within the scenes have been chosen to be test sites. Four stands of Norway spruce, five stands of Scots pine, one birch stand and one clear-cut were selected. Also, one bog has been examined

The analysis of polarimetric SAR observations acquired by the JPL polarimetric multi frequency (P-, L- and C-band) SAR during the MAESTRO-1 campaign has continued. We study the Flevoland test site, which extends over two scenes of mainly forested areas and one scene of agricultural areas. The data type studied is the high resolution complex one look product. Natural extended targets have been used to correct for cross-talk and phase errors, and co-channel imbalance is removed by using trihedral reflector responses. Additional field measurements have provided us with a larger amount of ground data. 'we have measured the basal area and the mean stand height of over one hundred stands of mainly poplar, ash, pine and spruce. The correlations between radar backscattering and the measured parameters generally increase with increasing wavelength. The correlation is however strongly species dependent.

Not Available