Engineering topics

Wang, James R.

Publications and source records attributed to Wang, James R..

22 records · Page 2

NASA NTRS1987

Evaluating roughness models of radar backscatter

Three radar backscatter roughness models were assessed using soil moisture data collected by the Space Shuttle flight 41G SIR-B SAR in an intensively farmed area. The SIR-B data swath included a large number of bare, dry fields with a large variety of surface roughnesses. The small perturbation model gives the best results, particularly when fields with a definite periodic row structure were omitted. The standard deviation of surface heights appears to be a good measure of relative roughness conditions, but the correlation length is not a good descriptor of the surface, and does not seem to be related in any way to the measured backscatter.

Engman, Edwin T.↗

NASA NTRS1987

The effects of soil moisture, surface roughness, and vegetation on L-band emission and backscatter

Measurements performed with SIR-B at 1.28 GHz and an airborne multiple-beam push-broom radiometer at 1.4 GHz over agricultural fields near Fresno, California are examined. A theoretical model (Kirchhoff approximation) was used to assess the effects of surface roughness and vegetation (alfalfa and lettuce) with respect to the responses of microwave emission and backscatter to soil-moisture variations. It is found that the surface roughness plays a dominant role compared to the vegetation cover in the microwave backscatter.

Wang, James R.↗

NASA NTRS1987

Microwave emission from smooth bare fields and soil moisture sampling depth

This paper studies the depth to which soil moisture can be directly estimated with microwave measurements over smooth bare fields. The analyses are based on both theoretical and experimental considerations at the frequencies of 1.4, 5.0, and 10.7 GHz. Radiative transfer calculations of microwave emissivities at these frequencies are performed with a number of moisture profiles measured for two soils. The calculated emissivities are compared with those derived from the Fresnel equation to deduce the microwave sampling depth in soils. The data acquired from the ground-level radiometric measurements during the summers of 1979-1981 are examined and compared with the theoretical analysis. Both theoretical and experimental analyses lead to the conclusion that the microwave samplinhg depth in soils is about one tenth of the wavelength of observation. It is shown that the moisture content at any depth near the surface of a smooth soil can be estimated, in principle, by a combination of a radiometric measurement and a curve generated by the Fresnel equation at an appropriate frequency, provided that the texture of the soil is known.

Wang, James R.↗

NASA NTRS1987

Retrieval of surface roughness parameters from dual-frequency measurements of radar backscattering coefficients

The use of dual-frequency data of backscattering coefficients at a fixed angle to estimate surface roughness parameters is evaluated. Radar backscattering coefficients at 1.5 and 4.25 GHz are calculated using a model based on Kirchhoff approximation of electromagnetic wave scatttering from a rough soil surface. Plots of the calculated backscattering coefficients for Kansas soil moisture contents at the C- and L-band frequencies and HH polarization are analyzed. The effects of changes in correlation length on the backscattering coefficients are investigated. The calculated backscattering coefficients are compared with scatterometer data collected at 1.5 and 4.25 GHz, and it is detected that the model and field data correlate well. The data reveal that it is possible to retrieve the surface roughness parameters from measured radar data.

Mo, Tsan↗