Passive Microwave Remote Sensing of Soil Moisture: The Effect of Tilled Row Structure

Abstract
The tilled row structure is known to be one of the important factors affecting the observations of the microwave emission from a natural surface. Measurements of this effect were carried out with both L-and X-band radiometers mounted on a mobile truck on a bare 40 m × 45 m row tilled field. The soil moisture content during the measurements ranged from ~10 to ~30 percent by dry weight. The results of these measurements showed that the variations of the antenna temperatures with incident angle θ changed with the azimuthal angle α measured from the row direction. In particular, at θ = 0° and α ≠ 45°, the observed horizontally and vertically polarized antenna temperatures, TBH(θ, α) and TBV(θ, α), were not equal. In general, TBH(ο°, α) > TBV(θ°, α) when 0° ⩽ α < 45° and TBH(Ο°, α) < TBV(0°, α) when 45° < α ⩽ 90°. The difference between TBH(0°, α) and TBV(0°, α) was observed to decrease with α approaching 45° and/or with soil moisture content. A numerical calculation based on a composite surface roughness-a small-scale RMS height variations superimposed on a large periodic row structure-was made and found to predict the observed features within the model's limit of accuracy. It was concluded that the difference between TBV(0°, α) and TBH(0°, α) was due to the change in the local angle of field emission within the antenna field of view caused by the large-scale row structure.