Sensitivity of C-Band SAR Polarimetric Variables to the Directionality of Surface Roughness Parameters

Zohreh Alijani,Melanie Chabot,Tracy Rowlandson,John Lindsay,Aaron Berg

doi:10.3390/rs13112210

Abstract

Surface roughness is an important factor in many soil moisture retrieval models. Therefore, any mischaracterization of surface roughness parameters (root mean square height, RMSH, and correlation length, ʅ) may result in unreliable predictions and soil moisture estimations. In many environments, but particularly in agricultural settings, surface roughness parameters may show different behaviours with respect to the orientation or azimuth. Consequently, the relationship between SAR polarimetric variables and surface roughness parameters may vary depending on measurement orientation. Generally, roughness obtained for many SAR-based studies is estimated using pin profilers that may, or may not, be collected with careful attention to orientation to the satellite look angle. In this study, we characterized surface roughness parameters in multi-azimuth mode using a terrestrial laser scanner (TLS). We characterized the surface roughness parameters in different orientations and then examined the sensitivity between polarimetric variables and surface roughness parameters; further, we compared these results to roughness profiles obtained using traditional pin profilers. The results showed that the polarimetric variables were more sensitive to the surface roughness parameters at higher incidence angles (θ). Moreover, when surface roughness measurements were conducted at the look angle of RADARSAT-2, more significant correlations were observed between polarimetric variables and surface roughness parameters. Our results also indicated that TLS can represent more reliable results than pin profiler in the measurement of the surface roughness parameters.

Highlights

The characterization of soil surface roughness is a significant issue for the modelling of surface and near-surface water flow, sediment and nutrient transport, and hydrological erosion, in agricultural systems
In November, several fields were rougher with respect to the absolute value of mean root mean square height (RMSH); there was a lower rougher with respect to the absolute value of mean range in roughness variations observed among fields.RMSH; there was a lower rangeThree in roughness variations among fields. situations were selected to demonrepresentative fieldsobserved with different roughness strate the sensitivity of surface roughness characteristics with respect to the orientation Table 2
Since the surface roughness measurements using the pin profiler were not conducted in the RADARSAT-2 look angles, the sensitivity analysis of radar parameters to the RMSH at the look angle presented in this study only applies to the terrestrial laser scanner (TLS)

Summary

Introduction

The characterization of soil surface roughness is a significant issue for the modelling of surface and near-surface water flow, sediment and nutrient transport, and hydrological erosion, in agricultural systems. The parameterization of surface roughness plays an important role in soil moisture retrieval using synthetic aperture radar (SAR). Interpretation of SAR backscatter, from longer wavelengths in the C or L band, permits high-resolution retrieval of soil moisture through characterization of the transmitted energy, or backscatter, from the surface. It is well known that the SAR backscattered signal primarily depends on the radar parameters (wavelength, polarization, and incidence angle [θ]) and surface parameters (dielectric constant or soil moisture, vegetation, and roughness) [3]. Surface roughness has a stronger impact on radar backscatter than that of soil moisture [4] and it is often the most important factor contributing to SAR backscatter response at higher incidence angles and in bare soils [4,5]

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Discussion

Conclusion