A coherent scattering model to determine forest backscattering inthe VHF-band

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 effect of inhomogeneous roughness on radar backscattering fromslightly deformed sea ice

This paper focuses on the spatially varying backscattering signature of an area of refrozen brash ice observed by a ship based scatterometer. The measurements were carried out as part of the Baltic Experiment for ERS-1 in 1994. The scatterometer was operated at 5.4 GHz at different incidence angles and polarizations. By analysing the scatterometer data over azimuth scans, it was found that the backscattering variabilities are not only due to fading but also contain a textural component. Surface height profiles were measured using a laser. The observed ice surface roughness was nonstationary over the measurement area. The ice surface can be approximated by adjacent patches of stationary roughness with patch dimensions of about 4.5 m. From the roughness spectra of different stationary patches, two roughness classes can be distinguished. The implications of estimating the roughness parameters from relatively short profile lengths is discussed and the effect on theoretical predictions of the backscattering coefficient is investigated. The texture variance is evaluated theoretically on the basis of the simulated backscattering coefficients of the two observed roughness classes and is found to compare with the scatterometer data     

Low VHF-band backscatter from coniferous forests on sloping terrain

Low-frequency synthetic aperture radar (SAR) is a promising technique for stem volume retrieval, particularly for dense forests, due to the good penetration of forest canopies. However, it is well known that the dominant scattering mechanism, the trunk-ground dihedral interaction, decreases rapidly on sloping terrain. In this paper, we use low VHF-band SAR data, collected with CARABAS over dense coniferous forests in Sweden, to examine the effect of topography. Using flight passes with different headings, the effect of slope and aspect angle on backscatter is characterized. For tall trees (/spl sim/30 m), on the steepest slopes in the test-site (up to /spl sim/12/spl deg/), differences of up to 8 dB are observed between images acquired with different look directions relative to the slope. A physical model is developed to investigate the different scattering mechanisms and their sensitivity to terrain slopes. The model shows that the trunk-ground scattering still dominates the response for large trees on moderate slopes, and a semiempirical model for the effect of topography on backscatter is proposed. The model shows good agreement with measurements, indicating the possibility of using it to compensate for the effects of sloping terrain when retrieving stem volume in coniferous forest.     

Fe-Cr-C hardfacing alloys for high-temperature applications

The microstructure and phase chemistry of a Fe-34Cr-4.5C wt% hardfacing alloy has been investigated using transmission electron microscopy and microanalytical techniques. The microstructure is found to consist of large primary M₇C₃. carbides in a eutectic mixture of austenite and more M₇C₃. The results indicate that the microstructure of the undiluted alloy becomes configurationally frozen at a temperature of about 1150° C during deposition by the manual metal arc welding technique. This allows the metastable austenite phase to contain a large chromium concentration ( 16 to 17 wt %), thus imparting good corrosion and oxidation resistance. Experimental data on the partitioning of chromium, manganese and silicon between the carbide phases are discussed in the context of the high-temperature stability of the alloy.     

L- and P-band backscatter intensity for biomass retrieval in hemiboreal forest

At present, the greatest source of uncertainty in the global carbon cycle is in the terrestrial ecosystems. In order to reduce these uncertainties it is necessary to provide consistent and accurate global estimates of the world forest biomass. One of the most promising methods for obtaining such estimates is through polarimetric SAR backscatter measurements at low frequencies. In this paper, the relation between polarimetric SAR backscatter at L- and P-bands and forest biomass is investigated using data acquired within the BioSAR-I campaign in southern Sweden during 2007. Methods for estimating biomass on stand level using these data are developed and evaluated, and the results for the two frequency bands are compared. For L-band data, the best results were obtained using HV-polarized backscatter only, giving estimation errors in terms of root mean square errors (RMSE) between 31% and 46% of the mean biomass for stands with biomass ranging from 10 to 290 t/ha, and an (adjusted) coefficient of determination (R²) between 0.4 and 0.6. For P-band data, the results are better than for L-band. Models using HV- or HH-polarized P-band backscatter give similar results, as does a model including both HV and HH. The RMSEs were between 18 and 27%, and the R² values were between 0.7 and 0.8.     

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.     

C-band backscatter signatures of old sea ice in thecentral Arctic during freeze-up

Radar backscatter signatures of old sea ice in the central Arctic have been measured and analyzed. A ship-mounted scatterometer was used to acquire backscattering coefficients at 5.4 GHz in the four linear polarization states and at incidence angles between 20° and 60°. Detailed in situ characterizations of the snow and ice were also made to enable comparison with theoretical backscatter models. Freeze-up conditions were prevalent during the experiment. The average backscattering coefficient was found to increase when the temperature of the ice surface layer decreased. The semi-empirical backscatter model is used to evaluate the measurements and shows that the backscatter increase is due to an increasing penetration depth, causing the volume scattering to increase. Model predictions also show that both surface and volume scattering contribute significantly at incidence angles of 20° to 26°. At these incidence angles, the dominating scattering mechanism changes from surface to volume scattering as the ice surface temperature decreases     

Digital elevation map generation using VHF-band SAR data in forested areas

The paper investigates digital elevation model (DEM) generation based on data from the ultra wideband coherent all radio band sensing (CARABAS) very high frequency (VRF)-band synthetic aperture radar (SAR). The results show excellent capability to penetrate forest areas, i.e., the generated DEMs are found to be close to the true ground height. A conventional DEM, based on stereo photography and surveying, and additional phase differential Global Positioning System (GPS) measurements have been used for comparison. The results in heavily vegetated areas (stem volume up to 600 m/sup 3//ha) show a mean height difference of less than 1.5 m and a root-mean-square (rms) error of less than 1.0 in compared to the conventional DEM. Stable backscattering properties allows us to use large baselines in order to obtain high height sensitivity. However, the amount of poor data due to low coherence increases with the increase of the baseline. The optimum baseline which balances these two effects is found to correspond to an incidence angle difference of 4/spl deg/-8/spl deg/.     

Repeat-pass SAR interferometry over forested terrain

Repeat-pass synthetic aperture radar (SAR) interferometry provides the possibility of producing topographic maps and geocoded as well as radiometrically calibrated radar images. However, the usefulness of such maps and images depends on our understanding of how different types of terrain affect the radar measurements. It is essential that the scene coherence between passes is sufficient. In this paper, the authors derive a general system model including both radar system and scene scattering properties. The model is used to interpret measurements over a forested area where the scene coherence varies between 0.2 and 0.5. The coherence is found to be sensitive to temperature changes around 0°C but surprisingly insensitive to wind speed. The interferometric height discontinuity at the forest to open-field boundary shows good agreement with in situ tree height measurements. For a dense boreal forest, but is observed to decrease for a less dense forest. This suggests the possibility of estimating bole volume from the interferometric tree height and a ground DEM. The decrease of scene coherence over a dense forest with increasing baseline is also used to estimate the effective scattering layer thickness     

Quality of life and independence in activities of daily living preoperatively and at follow-up in patients with colorectal cancer

The immune capacity of young and adult axolotls (Ambystoma mexicanum) was evaluated by examining the combinatorial and junctional diversity of the VH chain. A large number of VDJ rearrangements isolated from 2.5-, 3.5-, 10-, and 24-month-old animals were sequenced. Six JH segments were identified with the canonical structure of all known vertebrate JHs, including the conserved Trp103-Gly104-X-Gly106 motif. Four core DH-like sequences were used by most (80%) of the VDJ junctions. These G-rich sequences had structures reminiscent of the TCRB DB sequences, and were equally used in their three reading frames. About 25% of the Igh, VDJ junctions from 3.5-month-old axolotls were out of frame, but most rearrangements were in frame at 10 and 24 months, suggesting that there is active selection of the productively rearranged Igh chains in the developing animals. There was no significant difference between the size of CDR3 in young (3.5 months) and subadult (10 months) axolotls (mean: 8.5 amino acids). However, the CDR3 loop was 1 amino acid longer in 2-year-old adult animals (mean: 9.5 residues). Several pairs of identical VDJ/CDR3 sequences were shared between 3.5-month-old individually analyzed axolotls, or between groups of axolotl of different ages. These identical rearrangements might be provided by the selection of some B-cell clones important for species survival, although the probability that different 3.5-month-old axolotl larvae would produce identical junctions seems very low, considering their limited number of B cells (less than 10(5)). The high frequency of tyrosine residues and the paucity of charged residues in the axolotl CDR3 loops may explain the polyreactivity of natural antibodies, and also clarify why it is so difficult to raise specific antibodies against soluble antigens.