Björn Hallberg

Measurements on individual trees using multiple VHF SAR images

Multiple VHF-band radar images from the airborne CARABAS-II system, retrieved with varying look direction to the imaged area, are coregistered and combined to improve spatial and radiometric resolution. Combined images over boreal forest in southern Sweden are used to identify and make backscatter measurements of individual trees. A coregistration scheme is proposed, and backscatter is compared to ground measurements of individual stem volume. It is found that the spatial resolution and radiometric precision could be significantly improved by combining the images coherently. A nearly linear relation between backscattered amplitude and individual stem volume for trees with a stem volume over 0.2 m/sup 3/ was found, which is in agreement with previous results at stand level.

A Physical-Optics Model for Double-Bounce Scattering From Tree Stems Standing on an Undulating Ground Surface

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 (R2 = 0.44 and 0.65 at best, for the lower and higher frequency band, respectively), which should be compared to R2 = 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.

Effects of Forest Biomass and Stand Consolidation on P-Band Backscatter

In previous studies, P-band synthetic aperture radar (SAR) has shown potential for biomass retrieval in forests. However, while measurements show a general agreement that backscatter increases with increasing biomass, different studies show that the backscatter from stands of similar biomass can significantly vary depending on forest structure, hence making biomass retrieval more challenging. In this letter, we show that, while biomass may be the single most important parameter determining the backscatter from a forest, the number density of trees has also a major impact. This can be explained using simple arguments, leading us to propose the use of the biomass-consolidation index to describe P-band HV-polarized backscatter. This is supported by electromagnetic-modeling studies and by a few measurements from boreal forest made with the AIRSAR system over the BOREAS test site in Canada.

Mapping of wind-thrown forests in Southern Sweden using space- and airborne SAR

Space- and airborne SAR have a potential for providing timely information of forest storm damage. In this paper, we compare Envisat, Radarsat and CARABAS with aerial photography and field observations after a severe storm event. The spaceborne C-band images were not able to detect the storm- damaged areas due to unfavorable frequency band and coarse resolution. The airborne CARABAS VHF SAR, on the other hand, detects most storm-damaged forest areas and even partly damaged which are often not detected in the aerial photography.

Performance simulation of spaceborne P-band SAR for global biomass retrieval

This paper evaluates the use of a spaceborne low-frequency synthetic aperture radar (SAR) for forest biomass retrieval. Airborne radar data are used as input to a SAR simulator in which SAR system parameters of the assumed spaceborne system and propagation effects in the ionosphere (primarily scintillation and Faraday rotation) are modelled. The simulations are performed for different ionospheric perturbation states. Some simulated spaceborne low-frequency SAR images over boreal forest are shown and their usefulness for forest biomass retrieval are studied and discussed The results indicate that it is possible to separate boreal forest into three classes assuming a moderate distorted ionosphere

ALOS PALSAR Calibration and Validation Results from Sweden

In 2006 calibration activities for ALOS PALSAR were conducted in Sweden. Four five-metre trihedral corner reflectors and three smaller dihedral reflectors were deployed and operated during eight months. 23 PALSAR scenes were acquired over the calibration site allowing an evaluation of the quality and temporal stability of the data. Results show that the co-polarized data have been stable during the whole calibration period with variations in the trihedral responses lower than 0.7 dB. The measured resolution in azimuth was 4.4 m and in slant range 4.7 m for single polarization images and 9.5 m for polarimetric data. For the cross-polarized data large variations in the dihedral responses were found. It is assumed that this is caused by a larger sensitivity to pointing errors. For the polarimetric data, estimation of Faraday rotation gave values ranging from 0.1deg to 3deg.

Forest stem volume estimation using high-resolution lidar and SAR data

Presents a comparison of remote sensing of forests using two complementary, high-resolution sensors; namely the TopEye lidar and CARABAS VHF SAR systems. The lidar data are from a small footprint configuration, allowing discrimination between pulses reflected from the tree crowns, and those penetrating through gaps in the canopy. From these data, measurements of tree height and crown size can be made for individual trees, and the tree volume calculated through empirical relationships. The CARABAS VHF SAR provides a more direct measurement of tree volume, since the long wavelengths penetrate the forest canopy and are scattered by the trunk-ground dihedral. Results of stem volume retrieval for individual trees using data from the two systems are presented, and the possibility of using the complementary measurements discussed.

Individual tree detection using CARABAS-II

This paper presents an example showing how VHF SAR images acquired over the same area, but with different look directions, can be combined in order to improve signal-to-noise ratio and spatial resolution. It is demonstrated that when combining images with 360o of aspect variation, the spatial resolution approaches the theoretical limit set by diffraction. Results are presented showing that it is possible to resolve individual trees in VHF SAR images over reasonably dense coniferous forests, and that there is good correlation between stem volume of individual trees and amplitude in combined SAR images.

Mapping of Wind-Thrown Forests Using the VHF-Band CARABAS-II SAR

A severe storm struck the southern part of Sweden in early January 2005 and caused much damage on the forests. Most of the large contiguous patches of wind-thrown timber were cleared during the same year. Numerous of smaller areas with fallen trees located in otherwise untouched forest stands were, however, still present and in many cases not even yet noticed. In an attempt to detect and position such areas, the airborne VHF SAR sensor CARABAS-II mapped 125 km by 125 km of mainly forested land in January 2006. The mission required 15 flights in total and all collected data are now processed and analyzed, with the detection results delivered to the end-user.

Automatic Detection of Wind-Thrown Forest in VHF SAR Images

A method for automatic detection of wind-thrown forest from VHF SAR images has been developed. It has been used to detect wind-thrown trees and estimate the volume of fallen timber within an area of 125 km × 125 km located in southern Sweden.