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To investigate the possibility of detecting clear-cutting of boreal forests with ERS interferometry, 17 interferograms from 1992 to 1995 of forested and clear-cut areas in northern Sweden have been studied. Using the measurements, and the known statistics of coherence estimation, we show that in typical cases 95% of clear-cuts larger than 0.4 ha should be detectable. The detection accuracy depends mainly on the interferometric coherence and thus varies significantly depending on weather conditions at and between the image acquisitions. The results indicate that the main benefit of using backscatter intensity to supplement the interferometric coherence is to increase the robustness of clear-cut detection using ERS images. [ABSTRACT FROM AUTHOR]

Examines the energy relations for surface waves. Use of auxiliary driving term in the determination of energy flow and stored energy for wave types; Mathematical representations of particle velocity; Transposition of tensors and vectors.

By inclusion of an external driving force, wave motion of any kind can be characterized by a dispersion function. This function is closely related to the energetic properties of wave motion, and then also to the averaged Lagrangian density. Linear and non-linear wave interaction can be analysed by inclusion of internal driving forces. Normalization procedures for the amplitudes can be avoided and time and space perturbations studied simultaneously. This analysis is further connected to slowly varying amplitudes and quasi-monochromatic waves. This paper presents the above-mentioned method and applies it to linear two-wave coupling, and non-linear three-wave coupling between positive and negative energy waves, and finally to amplitude modulation. The general equations obtained by this procedure are useful for general discussions. The simplicity of the method may prove useful in different applications.
The aim of this paper is to present a new and a simple method for analysis of wave interaction in dispersive media and to apply it on some illustrative problems. Further applications, which are found in plasma physics, microwave components and so on, will be presented later. [ABSTRACT FROM AUTHOR]

A plane wave is incident upon a bounded, cold plasma with a density varying periodically in time and space due to a high power pump wave. The modulation of the medium will produce a spectrum of reflected and transmitted components in different directions and with different frequencies. Inside the medium parametric effects will occur. The analysis takes into account all the time and space harmonics inside the medium and the reflection and transmission coefficients outside the medium. Four different types of wave interaction are obtained. Particularly the case of travelling-wave amplification is extensively studied by means of approximate analysis as well as computer analysis. It is found that when the width of the medium exceeds a critical value, a gain saturation is obtained and degenerate oscillations may occur. If the pump wave propagates obliquely, the maximum angular separation will be between the amplified components. [ABSTRACT FROM AUTHOR]

Presents a study which developed a model to understand the mechanisms of decorrelation in interferometric SAR images of bare soil. Theoretical development; Results and discussion; Conclusions.

JERS‐1 L‐band SAR backscatter from test sites in Sweden, Finland and Siberia has been investigated to determine the accuracy level achievable in the boreal zone for stand‐wise forest stem volume retrieval using a model‐based approach. The extensive ground‐data and SAR imagery datasets available allowed analysis of the backscatter temporal dynamics. In dense forests the backscatter primarily depended on the frozen/unfrozen state of the canopy, showing a ∼⃒4 dB difference. In sparse forests, the backscatter depended primarily on the dielectric properties of the forest floor, showing smaller differences throughout the year. Backscatter modelling as a function of stem volume was carried out by means of a simple L‐band Water Cloud related scattering model. At each test site, the model fitted the measurements used for training irrespective of the weather conditions. Of the three a priori unknown model parameters, the forest transmissivity coefficient was most affected by seasonal conditions and test site specific features (stand structure, forest management, etc.). Several factors determined the coefficient's estimate, namely weather conditions at acquisition, structural heterogeneities of the forest stands within a test site, forest management practice and ground data accuracy. Stem volume retrieval was strongly influenced by these factors. It performed best under unfrozen conditions and results were temporally consistent. Multi‐temporal combination of single‐image estimates eliminated outliers and slightly decreased the estimation error. Retrieved and measured stem volumes were in good agreement up to maximum levels in Sweden and Finland. For the intensively managed test site in Sweden a 25% relative rms error was obtained. Higher errors were achieved in the larger and more heterogeneous forest test sites in Siberia. Hence, L‐band backscatter can be considered a good candidate for stand‐wise stem volume retrieval in boreal forest, although the forest site conditions play a fundamental role for the final accuracy. [ABSTRACT FROM AUTHOR]

The use of spaceborne synthetic aperture radar (SAR) systems to estimate stem volume and biomass in boreal forests has shown some promising results, but with saturation of the radar backscatter at relatively low stem volumes and limited accuracy of stem volume estimation. These limitations have motivated evaluation of more advanced methods, such as interferometry. The results presented in this study show that ERS interferometry, under favourable conditions, may be used to estimate stem volume at stand level with saturation level and accuracy useful for operational forestry management planning in boreal forests. Five interferograms were analysed, covering a test site located in the central part of Sweden with stem volume in the range of 0-305 m[sup 3] ha[sup -1]. The best interferogram showed a linear relationship between stem volume and coherence with a root mean square error (RMSE) of approximately 26 m[sup 3] ha[sup -1], corresponding to 20% of the average stem volume, throughout the range of stem volume. No saturation was observed up to the maximum stem volume. However, the sensitivity of coherence to stem volume varied considerably between the interferograms. Finally, four SPOT XS images were evaluated and compared with the stem volume estimations obtained from the interferograms, resulting in a relative RMSE of about 24% of the stem volume, for the best case. The estimation of stem volume using coherence data was found to be better than optical data for stem volumes exceeding about 110 m[sup 3] ha[sup -1]. The statistical analysis was performed using linear regression models with cross-validation. [ABSTRACT FROM AUTHOR]

As part of the European Multi-Sensor Airborne Campaign on Snow and Ice 1995 (EMAC-95), the radar signatures of smooth low-salinity level ice in the Gulf of Bothnia were measured by means of a ship mounted scatterometer. The scatterometer was operated at 1.25, 2.6, 5.4 and 10GHz ( L -, S -, C -, and X -band) at incidence angles from 20degrees to 60degress. Various in situ snow and ice parameters were acquired during the field work. Utilising the ranging capabilities of the scatterometer system, the travel times of the radar returns are analysed at the different frequencies in order to locate the dominant scattering contributions as a function of depth in the ice. Model predictions which are based on the in situ snow and ice properties are compared with the measured backscattering coefficients in order to determine the scattering mechanisms of the dominant radar returns. Experimental and model results show that the backscattering responses at X -band are mainly caused by volume inhomogeneities in the uppermost part of the ice, and at C - and S -band by the roughness of the snow-ice interface. At L -band, however, dominant radar returns can originate also from the interior of the ice, probably from the ice-water interface, which is substantiated at smaller incidence angles. Implications of the presented results are discussed. [ABSTRACT FROM AUTHOR]

SAR interferometry (InSAR) offers new interesting possibilities for research in sea ice radar scattering and sea ice mechanics. A case study of this is presented from the Baltic Sea in late March 1992. Interferometric coherence is mainly dependent of the temporal characteristics of the scattering sources in sea ice. Different areas with different scattering properties were examined and the present data indicates that more field data is necessary to fully understand the InSAR coherence over sea ice. However, some interesting features were noted. Over low-salinity ice, backscattering and coherence seems to be related, high backscatter areas are more unstable than low backscatter areas. Over areas with surface roughness scattering, the scattering is relatively stable and also that a snow cover seems to retain the coherence over such areas. Interferometric phase measurements are dependent on small deformations of the ice pack. Fast ice which is (nearly) stationary experiences small discontinuous slips and deformations. Interferometric phase measurements are very sensitive to these slips, displacements and deformations and will provide new insight into the rheology for fast ice and how the fast ice starts to move. How the fast ice starts to move is one of the major problems in sea ice mechanics research and there is not much earlier data on the subject. In the present case, the ice was nearly stationary as the stresses were below the yield limit under the low forcing conditions. Two ice floe compressions have been observed and the strains are believed to be viscous with a viscosity value at approximately 1013-1014kg (ms). Both the interferometric phase and the coherence measurements over ice are believed to be of great value in future backscattering models and sea ice mechanics models. [ABSTRACT FROM AUTHOR]