In radiometric sensing of soil moisture through a forest canopy, knowledge of canopy attenuation is required. Active sensors have the potential of providing this information since the backscatter signals are more sensitive to forest structure. In this dissertation, a new radar technique is presented for estimating canopy attenuation. The technique employs details found in a transient solution where the canopy (volume scattering) and the tree-ground (double interaction) effects appear at different times in the return signal. The influence that these effects have on the expected time-domain response of a forest stand is characterized through numerical simulations. A coherent forest scattering model, based on a Monte Carlo simulation, is developed to calculate transient response from distributed scatterers over a rough surface. The forest transient response model for linear co-polarized cases is validated with the microwave deciduous tree data acquired by the ComRAD (Combined Radar/Radiometer) system.
The attenuation algorithm is applicable when the forest height is sufficient to separate components of the radar backscatter transient response. The frequency correlation functions (FCF) of the volume scatter and the double interaction terms are computed by the distorted Born approximation. A ratio of these FCF's is formed and compared with data at a set of frequencies over the decorrelation bandwidth of the returns. The resulting system of equations only depends on the canopy thickness, the canopy attenuation and a combined parameter involving the forest scattering coefficients and the ground reflectance. A least square method is used to solve for the attenuation and the combined parameter by assuming the canopy thickness is estimated a priori from the transient response. Finally, the technique is used with ComRAD L-band stepped frequency data to evaluate its performance under various physical conditions.
|Advisor:||Lang, Roger H.|
|Commitee:||Kahn, Walter K., LeVine, David, Schneider, Allan, Wasylkiwskyj, Wasyl|
|School:||The George Washington University|
|School Location:||United States -- District of Columbia|
|Source:||DAI-B 70/05, Dissertation Abstracts International|
|Subjects:||Hydrologic sciences, Electrical engineering, Environmental science|
|Keywords:||Attenuation, Forest canopy, Frequency correlation, Radar, Radar backscatter, Soil moisture, Time-domain|
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