[1] Non-standard radio wave propagation in the atmosphere is caused by anomalous changes of the atmospheric refractivity index. In recent years, refractivity from clutter (RFC) has been an active field of research to complement traditional ways of measuring the refractivity profile in maritime environments which rely on direct sensing of the environmental parameters. Higher temporal and spatial resolution of the refractivity profile, together with a lower cost and convenience of operations have been the promising factors that brought RFC under consideration. Presented is an overview of the basic concepts, research and achievements in the field of RFC. Topics that require more attention in future studies also are discussed.
OBJECTIVE Investigate the validity of a parabolic equation (PE) model for predicting radio field strengths in horizontally inhomogeneous environments by performing comparisons between the model and experimental data. RESULTS Excellent agreements were found at VHF and UHF frequencies with good agreement in Sand X-bands. In some cases, the predicted curves for the S-band comparisons underestimated that of the measured data at large ranges. This may be the result of phenomena such as surface roughness, backscatter, etc., not accounted for in the model. Discrepancies may also result from the presence of evaporation ducts not included in the environmental inputs to the model because of a lack of detailed measurements. This would account for lower predicted signal levels at higher frequencies.
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[1] This paper describes a technique to estimate, from measured surface clutter, parameters defining a surface-based duct using a simple ray trace method. The approach is novel in that it incorporates a rank correlation scheme between the observed surface clutter and the surface ray density for a given propagation path. While more sophisticated methods for duct parameter estimation have been developed with successful results, these have concentrated on using observations entirely from sea surface clutter. The technique described here looks at using land clutter also as an added source of information to adequately estimate surface-based duct parameters.
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