| Abstract: |
Passive microwave remote sensing has long been recognized as a promising technique for soil moisture estimation [1]. While L-band at 1.4 GHz (λ ≈ 21 cm) is considered as a favorable frequency choice for soil moisture monitoring due to negligible atmospheric influence and sufficient vegetation penetration capabilities, it is common practice to use solely horizontally and vertically polarized brightness temperature measurements for retrieval algorithms [1]. Hence, the fully polarimetric acquisition capabilities of radiometer instruments (e.g., [2]) are neglected.
However, it has been shown that besides H- and V-polarized also fully polarimetric observations (i.e., the third and fourth Stokes parameters) are sensitive to environmental properties like azimuthal orientation effects from periodic surface roughness [3], or plant row alignment [4, 5]. Moreover, model simulations in [6] indicate that also permittivity patterns of a smooth surface can induce azimuthal dependencies of H- and V-polarized microwave emissions as well as the third and fourth Stokes parameters.
It is the aim of this research study to investigate the effects of permittivity patterns on fully polarimetric low-frequency microwave radiometer observations. For this purpose, L-band polarimetric brightness temperature signatures of an artificial smooth surface with azimuthal oriented (striped) permittivity patterns have been obtained with a ground-based fully polarimetric radiometer, and compared with model simulations. The experiment was conducted in September 2017 at the TERENO test site in Selhausen (Germany) [7], using a man-made surface of alternating striped wood and Styrodur layers as target. Fully polarimetric L-band brightness temperature signatures were obtained with the EMIRAD radiometer that was developed at the Technical University of Denmark (DTU) [8]. The results clearly indicate that striped permittivity patterns induce a sinusoidal-like azimuthal dependency of H- and V-polarized brightness temperatures as well as of the third Stokes parameter, which is in very good agreement with model predictions of [6]. The amplitude of the sinusoidal signatures is dependent on the permittivity difference of the striped layers Δε', and is in the range of ~1 K for V-polarization, ~3.5 K for H-polarization and ~4 K for the third Stokes parameter for the wood/Styrodur setup (Δε'≅2.5). Furthermore, the described effects are fully observed for permittivity layer widths smaller than half the wavelength (≤ 8 cm), with solely H-polarization showing azimuthal dependencies also at layer widths being in the range of the wavelength (16 cm).
Further analyses and the full evaluation of the campaign data are still ongoing and will be presented at the conference.
[1] Njoku, E. G., & Entekhabi, D. (1996). Passive microwave remote sensing of soil moisture. Journal of Hydrology, 184(1), 101-129.
[2] Entekhabi, D. et al. (2010). The Soil Moisture Active Passive (SMAP) Mission. Proceedings of the IEEE, 98, 704-716.
[3] Tsang, L. (1991). Polarimetic Passive Microwave Remote Sensing of Random Discrete Scatterers and Rough Surfaces. Journal of Electromagnetic Waves and Applications, 5(1), 41-57.
[4] Schmidl Søbjærg, S., & Skou N. (2003). Polarimetric Signatures from a Crop Covered Land Surface Measured by an L-Band Polarimetric Radiometer. Proceedings of IGARSS 2003, 2626-2628.
[5] Macelloni, G., Pampaloni, P., Paloscia, S., & Ruisi, R. (1996). Effects of spatial inhomogeneities and microwave emission enhancement in random media: An experimental study. IEEE Transactions on Geoscience and Remote Sensing, 34(5), 1084-1089.
[6] Soellner, M. (1997). “Vollpolarimetrische Helligkeitstemperatur-Verteilungen von natürlichen und künstlichen Objekten bei einer Frequenz von 90 GHz”, PhD Thesis, Technical University of Munich, Munich.
[7] Jonard, F. et al. (2015). Estimation of hydraulic properties of a sandy soil using ground-based active and passive microwave remote sensing. IEEE Transactions on Geoscience and Remote Sensing, 53(6), 3095-3109.
[8] Søbjærg, S.S., Kristensen, S.S., Balling, J.E., & Skou, N. (2013). The Airborne EMIRAD L-Band Radiometer System. Proc. of IGARSS 2013, 1-4.
|
